AN INTERVIEW WITH

Terry l. Orr-WeaverHow the daughter cells resulting from celldivision end up with equal numbers of chro­mosomes is the focus of much of Terry Orr­Weaver's research; she also studies how cellscontrol the DNA replication that precedescell division. Her research group has identi·fied a number of proteins involved in theseprocesses, A professor of biology at MIT andthe first woman to become a member of theWhitehead Institute for Biomedical Research,Dr. Orr-Weaver has an undergraduate de­gree in chemistry from UC San Diego and aPh.D. in biological chemistry from Harvard.She is a member of the U.S. National Acad­emy of SCiences and a past president of theGenetics Society of America.

After receiving your Ph.D., you switchedfrom using single-celled yeast as your modl'lorganism to thl' multicelled fruit flyDrosophila melanogaster. Why?[ was excited by the possibility of working atthe interface of two fields. At that time, the cellcycle field was exploding from work with sin­gle cells-mainly yeasts and mammalian cellsgrown in culture-and biochemical experi­ments using extracts. Meanwhile, develop­mental biology was entering a new era withthe discovery of pattern formation genes. [ re­alized there was a key question at the interfaceof these two fields that was being ignored: If anorganism starts as a single cell and ends up as amulticellular entity, how are pattern and celldivisions coordinated? And I realized therehad to be intrinsic regulation from the cell cy­cle components but also extrinsic develop­mental control feeding into that. This wasclearly going to be an important area of study.

In choosing a multicellular model organism,why use Drosophila?A lot of fundamental discoveries in biologyhave been made with fruit nil's. For example, itwas research with DroS<Jph.i1a that establishedin 1916 that chromosomes were the physical

,%

basis of inheritance, the structures that carriedthe genes. Decades later, the master regulatorygenes that set up the animal body plan in em­bryonic development were discovered inDrosoph.ila. These genes determine, for exam­ple, where the fly's head is and where its legsare. Then, to the amazement of biologists. itturned out that exactly the same genes controlhow the human body plan is set up! $0Drosophila has a really rich heritage. When Istarted working on Drosophila, it had a 70­

year-long history of use in genetics, prOVidingan incredible array of knowledge and methods.

How do geneticists approach biologicalquestions?Geneticists want to discover the genes that areinvolved in a biological process and then figureout what the genes do. First. you decide whatprocess you're interested in-in our case, howchromosomes gel partitioned during cell divi­sion. Next, you figure out what you would seeif you had a mutant in which that process wasperturbed-how would you recognize it? Thenyou generate mutations, usually with the helpof high-energy radiation or a chemical, hopingthat one or more of the resulting mutants areaffected in the process of interest. \vt1at you'redoing by making a mutation is generating adisease state in your organism. \vt1en you'vefound a mutant you're interested in. you canfind out what gene has been made defective,and you know that that gene has to play an im­portant role in the process you're interested in.

Now that we have the genome sequences ofmany organisms, including the fruit fly and thehuman, you might think that all their genes areknown-so what's to discover? But it turned outthat we didn't know the functions of many of thegenes that showed up in genome sequences.Even in an organism as simple and well-studiedas yeast, at the publication of the genome se­quence we didn't know what 70% ofthe genes did.And we still don't have a clue about halfofthehuman genes. So having genome sequences pro­vides a foundation, but we must still find outwhat all the genes do, and we geneticists feelthe best way to go about this is 10 use genetics.

A big and useful surprise in this gene­discovery enterprise has been how many geneshave been 'conserved" through evolutionaryhistory and are still verysimiJar in organisms asdistantly related as fruit flies and humans. It'svery hard to figure out directly what humangenes do. But when we discover a new gene inDrosophila, invariably it turns out thai there's asimilar gene in humans that does the samething. So Drosophila turns out to be an evenbetter model organism than we had guessed.

Besides genetics, what other approachesand methods do you use?A combination ofgenetics, biochemistry, and cellbiology turns out to be incredibly powerful forour research. For instance, with the microscope,a tool ofcell biology. we can literally watch thechromosomes as they undergo mitosis or meio­sis.Ifwe have a mutation causing a defect in oneofthose processes, we can look directly at howthe chromosomes behave in mutant cells. And inour research on DNA replication, we can look di­

rectly at proteins that attach to the DNA duringDNA replication. In this lab, we try to do thingsas directly as possible!

Here's an example from our work on DNAreplication in Drosophila. Using a genetic ap­proach. we discovered a mutant that couldn'tcarry out DNA replication-although such amutant can live and grow for a while by usingproteins its mother stockpiled in the egg. Itlooked like the gene affected might code for areally important protein. But we wanted to findout if it was a protein that played a direct role inDNA replication, and we couldn't tell that fromthe genetics alone. However, by labeling thenormal version of the protein with a fluores­cent tag, we could use the microscope to seewhere the protein was located in cells, and wesaw that the protein got on the DNA right at aplace where DNA synthesis starts. That estab­lished that this protein was directly involved.

How is meiosis different from mitosis?Let me first review mitosis. After a cell dupli­cates its DNA, mitosis ensures that each dupli­cated chromosome gets partitioned to the

daughter cells so that you end up with twocells that have exactly the same DNA contentand chromosome number. That's what hap­pens in normal cell division.

But what about making a sperm or an egg?In fertilization, a sperm and egg are going tofuse and give rise to a new progeny. [fthe or­ganism is diploid-that is, has two similarcopies of each chromosome, as nil'S and hu­mans do-and the progeny is going to bediploid, then you've got to make sperm andeggs that have only one copy of each chromo­some, so that when those sperm and egg cometogether you restore the right chromosomenumber. And to produce sperm and eggs withonly half the diploid chromosome number, youneed a special kind of cell division-meiosis.There has to be a way to bring together thetwo similar copies of each chromosome andthen pull them apart, with each going to a dif­ferent daughter cell. [n meiosis, there is an

extra round of chromosome partitioningwhere what we call the homologous pairs ofchromosomes-the chromosome that initially

came from dad and the similar chromosomethat came from mom-get separated fromeach other. To sum up, in mitosis you're sepa­rating identical copies of each chromosome,whereas in meiosis you have an extra round ofdivision stuck in there, where the copy ofeachchromosome from dad and the copy frommom get separated from each other.

What imporlanl questions about meiosisstill need to be answeredlWhat we don't understand at all is how the

chromosomes of a homologous pair find eachother. That pairing is unique to meiosis; itdoesn't happen in mitosis. The two strands ofthe DNA don't come apart, so although thehomologous chromosomes have very similarDNA sequences, it's not base-pairing that

brings the homologous chromosomes to­gether. Given the relatively gigantic volume ofthe nucleus and the huge mass of chromatin ina eukaryotic cell, how do the right chromo­

somes find each other? That's the number onemystery about meiosis. And [would say thesecond big mystery is why humans are so un­believably bad at carrying out meiosis.

What makes you say thaB Are we worsethan fruit flies?We're about a thousand times worse. Here are

some amazing statistics: Twenty percent ofrecognized pregnancies end in spontaneousmiscarriage, and out of those at least half aredue to a mistake during meiosis. And for everypregnancy that proceeds far enough to be rec­ognized, there have been many that endedwithout being recognized. So about 10-20% ofthe time meiosis doesn't work properly in hu­mans. In fruit flies, meiosis seems to occur in­accurately only 0.01-0.05% ofthe time. Evenmice, which are mammals like ourselves, do

much better than humans. So I would call thisthe second big mystery regarding meiosis:Why are humans so bad at it?

Tell us more about what happens whenmeiosis doesn't work correctly.Most of the time the result is early death oftheembryo. In humans, if any chromosome otherthan X or Y is present in only one copy ormore than two copies, the embryo dies veryearly. The only exceptions are for chromo­somes number 13, 18, and 21: When oneofthese is present in three copies, the embyo usu­ally dies at an early stage, but it may survive.Even in the case of Down syndrome, a rela­tively common condition where a person hasthree copies of chromosome 21, only a minor­ity of fetuses survive to term, And when the in­dividual does survive, there are serious effects,

In fact, errors in meiosis are the leading causeof menial retardation in the United States.Some scientists argue that, in evolutionaryterms, the human species is able to cope with

such a high rate of meiotic errors because mostof the pregnancies are lost very early,

Whal do you like besl about research?What's so great about research is that you getto unwrap presents all the time! \'ifhen youfind an interesting mutant, it's like a beautifullywrapped present. And then you unwrap it, and

it can be a big surprise to learn which gene isaffected and what its protein product is. Some­times you unwrap it and it's beautiful and sat­isfying and completely makes sense; othertimes you don't at first understand what thegift is, and then you have to figure that out, [want to tell students how wonderful it is get­ting these presents to unwrap. But they alsoneed to realize that it might take three years toget the ribbon and all the tape off. It can takesome real patience to unwrap the present!

In your view, how important is il forscientists to reach out beyond the scientificcommunity?I think scientists do have an obligation to edu­cate the public. It's especially critical for peo­ple to understand the importance of basicresearch and the study of model organisms,Because if I'll' shortchange basic research, inthe long run I'll' won't be able to benefit frommedical applications. The reason we've doneso well in this country is our willingness to in­vest in basic research science.

The challenge is figuring out how to convey amessage like this in an era ofSQundbites. The

media and the public want to hear that a piece ofresearch is directly going to cure a disease. Sohow can we get a message across that requires anexplanation more complex than a catchphrase?There are many stories I'll' can tell where discov­eries in basic research have led to very importantapplications, but these stories take at lea;,t acouple of paragraphs to communicate. Also,there's a real need for scientists to try to build a

bridge between science and the public.It's especially important to educate our rep­

resentatives in government. A coalition of sev­eral biology societies, including the GeneticsSociety of America, has a Joint Steering Com­mittee for Public Policy, which has been ter­rific in educating members of Congress andtheir staffs. They bring in scientists to explaintheir research and why investing in research isimportant. Maybe that's a good starting point.

Learn about an experiment by Terry Orr­Weaver in Inquiry Figure 16,22 on page 322,

Jane Reece and Terry Orr-Weaver

247

Mei"",,-,SexualCycles

KEY CONCEPTS

anife

.... Figure 13.1 What accounts for family resemblance?

13.1 Offspring acquire genes from parents byinheriting chromosomes

13.2 Fertilization and meiosis alternate in sexual lifecycles

13.3 Meiosis reduces the number of chromosomesets from diploid to haploid

13.4 Genetic variation produced in sexual life cyclescontributes to evolution

Most people who send out birth announcementsmention the sex of the baby, but they don't feel theneed to specify that their offspring is a human be­

ing! One of the characteristics of life is the ability of organ­isms to reproduce their own kind-elephants produce littleelephants, and oak trees generate oak saplings. Exceptionsto this rule show up only as sensational but highly suspectstories in tabloid newspapers.

Another rule often taken for granted is that offspring resem­ble their parents more than they do Wlrelated individuals. If youexamine the famUy members shown in Figure 13.1-SissySpacek and Jack Fisk with daughters Madison and SchuylerFisk-you can pick out some similar features among them. Thetransmission oftraits from one generation to the next is called in­heritance, or heredity (from the Latin heres, heir). However, sonsand daughters are not identical copies ofeither parent or oftheirsiblings. Along with inherited similarity, there is also variation.Farmers have exploited the principles of heredity and variationfor thousands of years, breeding plants and animals for desiredtraits. But what are the biological mechanisms leading to thehereditary similarity and variation that we call a "family resem­blance"? This question eluded biologists until the development ofgenetics in the 20th century.

248

Genetics is the scientific study of heredity and hereditaryvariation. In this unit, you will learn about genetics at multiplelevels, from organisms to cells to molecules. On the practicalside, you will see how genetics continues to revolutionize med­icineand agriculture, and you will be asked to consider someso­cial and ethical questions raised by our ability to manipulateDNA, the genetic material. At the end of the unit, you will beable to stand back and consider the whole genome, an organ­ism's entire complement of DNA. The rapid accumulation ofthe genome sequences of many species, including our own, hastaught us a great deal about evolution on the molecular level­in other words, evolution of the genome itself. In fact, geneticmethods and discoveries are catalyzing progress in all areas ofbiology, from cell biology to physiology, developmental biology,behavior, and even ecology.

We begin our study of genetics in this chapter by examin­ing how chromosomes pass from parents to offspring in sexu­ally reproducing organisms. The processes of meiosis (aspecial type of cell division) and fertilization (the fusion ofsperm and egg) maintain a species' chromosome count duringthe sexual life cycle. We will describe the cellular mechanics ofmeiosis and how this process differs from mitosis. Finally, wewill consider how both meiosis and fertilization contribute togenetic variation, such as the variation obvious in the familyshown in Figure 13.1.

~7;::~~g~'~ire genesfrom parents by inheritingchromosomes

Family friends may tell you that you have your mother's freck­les or your father's eyes. However, parents do not, in any lit­eral sense, give their children freckles, eyes, hair, or any othertraits. What, then, is actually inherited?

... Figure 13.2 Asexual reproduction in two multicellularorganisms. (a) This relatively simple animal. a hydra. reproduces bybudding. The bud. a localized mass of mitotically dividing cells. developsinto a small hydra. whICh detaches from the parent (LM). (b) Each tree inthis circle of redwoods grew from a single parent tree. whose stump is inthe center of the circle.

I. How are the traits of parents (such as hair color)

transmitted to their offspring?2. Explain how asexually reproducing organisms pro­

duce offspring that are genetically identical to eachother and to their parents.

3.•i,'!lfU1jM A horticulturalist breeds orchids, trying

to obtain a plant with a unique combination of desir­able traits. After many years, she finally succeeds. Toproduce more plants like this one, should she breed itor clone it? Why?

For suggested answers, see Appendix A.

capable of reproducing asexually (Figure 13.2). Because thecells of the offspring are derived by mitosis in the parent, the~chip off the old block" is usually genetically identical to itsparent. An individual that reproduces asexually gives rise to adone, a group ofgenetically identical individuals. Genetic dif­ferences occasionally arise in asexually reproducing organ­isms as a result of changes in the DNA called mutations,which we will discuss in Chapter 17.

In sexual reproduction, two parents give rise to offspringthat have unique combinations ofgenes inherited from the m'oparents. In contrast to a clone, offspring ofsexual reproductionvary genetically from their siblings and both parents: They arevariations on a common theme of family resemblance, not ex­act replicas. Genetic variation like that shown in Figure 13.1 isan important consequence of sexual reproduction. Whatmechanisms generate this genetic variation? The key is the be­havior of chromosomes during the sexual life cycle.

(b) Redwoods

13.1CONCEPT CHECK

(a) Hydra

Inheritance of Genes

Only organisms that reproduce asexually produce offspringthat are exact copies ofthemselves. In asexual reproduction,a single individual is the sole parent and passes copies of all itsgenes to its offspring. For example, single-celled eukaryoticorganisms can reproduce asexually by mitotic cell division, inwhich DNA is copied and allocated equally to two daughtercells. The genomes of the offspring are virtually exact copiesofthe parent's genome. Some multicellular organisms are also

Parents endow their offspring with coded information in theform of hereditary units called genes. The genes we inherit

from our mothers and fathers are our genetic link to our par­ents, and they account for family resemblances such as sharedeye color or freckles. OUf genes program the specific traitsthat emerge as we develop from fertilized eggs into adults.

The genetic program is written in the language of DNA,the polymer of four different nuc1eotides you learned aboutin Chapters 1 and 5. Inherited information is passed on inthe form of each gene's specific sequence of DNA nu­deotides, much as printed information is communicated inthe form of meaningful sequences of letters. In both cases,the language is symbolic. Just as your brain translates theword apple into a mental image of the fruit, cells translategenes into freckles and other features. Most genes programcells to synthesize specific enzymes and other proteins,whose cumulative action produces an organism's inherited

traits. The programming of these traits in the form of DNAis one of the unifying themes of biology.

The transmission of hereditary traits has its molecular ba­sis in the precise replication of DNA, which produces copiesofgenes that can be passed along from parents to offspring. Inanimals and plants, reproductive cells called gametes are thevehicles that transmit genes from one generation to the next.During fertilization, male and female gametes (sperm andeggs) unite, thereby passing on genes of both parents to theiroffspring.

Except for small amounts of DNA in mitochondria andchloroplasts, the DNA of a eukaryotic cell is packaged intochromosomes within the nucleus. Every living species has acharacteristic number of chromosomes. For example, hu­

mans have 46 chromosomes in almost all of their cells. Eachchromosome consists of a single long DNA molecule elab­orately coiled in association with various proteins. Onechromosome includes several hundred to a few thousandgenes, each of which is a specific sequence of nuc1eotideswithin the DNA molecule. A gene's specific location alongthe length of a chromosome is called the gene's locus (fromthe Latin, meaning "placen

; plural, loci). Our genetic en­dowment consists of the genes carried on the chromosomeswe inherited from our parents.

Comparison of Asexualand Sexual Reproduction

(HAPTE~ THIRTEEN Meiosis and Sexual Life Cycles 249

Centromere

[illk I ,,' Of "~ '. • • .'

• •• , .-,,, Ii", ' ,

'-" i i H I ; l(, \ ,,~ #i.. .. ,·, E ii. '.

" II ~\·, ••," "

, "0' H II ! I

" D•• " • •". II " H.,• • " ..

•

RESULTS This karyotype shows the chromosomes froma normal human male. The size of the chromosome, position ofthe centromere, and pattern of stained bands help identify spe­cific chromosomes. Although difficult to discern in the karyotype,each metaphase chromosome consists of two closely attachedsister chromatids (see the diagram of a pair of homologousreplicated chromosomes).

13.3

Preparing a karyotype

T Sisterchromatids

Metaphasechromosome

TECH Nt QUE Karyotypes are prepared from isolated somaticcells, which are treated with adrug to stimulate mitosis and thengrown in culture for several days. Cells arrested in metaphase arestained and then viewed with a microscope equipped with a digi­tal camera. A photograph of the chromosomes is displayed on acomputer monitor, and the images of the chromosomes arearranged Into pairs according to size and shape.

Pair of homologousreplicated chromosomes~

APPLICATION A karyotype is adisplay of condensed chromo·somes arranged in pairs. Karyotyping can be used to screen forabnormal numbers of chromosomes or defective chromosomesassociated with certain congenital disorders, such as Downsyndrome.

In humans, each somatic cell-any cell other than those in­

volved in gamete formation-has 46 chromosomes. Duringmitosis, the chromosomes become condensed enough to bevisible in a light microscope. Because chromosomes differ insize, in the positions of their centromeres, and in the patternof colored bands produced by certain stains, they can be dis~

tinguished from one another by microscopic examinationwhen sufficiently condensed.

Careful examination of a micrograph of the 46 humanchromosomes from a single cell in mitosis reveals that thereare two chromosomes of each of 23 types. This becomesdear when images of the chromosomes are arranged inpairs, starting with the longest chromosomes. The resultingordered display is called a karyotype (Figure 13.3). The twochromosomes composing a pair have the same length, cen­tromere position, and staining pattern: These are calledhomologous chromosomes, or homologs. Both chromo­somes of each pair carry genes controlling the same inher­ited characters. For example, if a gene for eye color issituated at a particular locus on a certain chromosome, thenthe homolog of that chromosome will also have a gene spec­

ifying eye color at the equivalent locus.The two distinct chromosomes referred to as X and Yare

an important exception to the general pattern of homolo­gous chromosomes in human somatic cells. Human femaleshave a homologous pair of X chromosomes (XX), but maleshave one Xand one Ychromosome (XY). Only small parts ofthe X and Yare homologous. Most of the genes carried onthe X chromosome do not have counterparts on the tiny Y,and the Y chromosome has genes lacking on the X. Becausethey determine an individual's sex, the X and Y chromo­somes are called sex chromosomes. The other chromo­somes are called autosomcs.

The occurrence of homologous pairs of chromosomes ineach human somatic cell is a consequence ofour sexual origins.We inherit one chromosome of each pair from each parentThus, the 46 chromosomes in our somatic cells are achlally twosets of 23 chromosomes-a maternal set (from our mother)

Sets of Chromosomes in Human Cells

A life cycle is the generation-ta-generation sequence ofstages in the reproductive history of an organism, from con­ception to production of its own offspring. In this section, weuse humans as an example to track the behavior of chromo­somes through sexual life cycles. We begin by considering thechromosome count in human somatic cells and gametes; wewill then explore how the behavior ofchromosomes relates tothe human life cycle and other types ofsexual life cycles.

r;:~~~I~::i~~'~nd meiosisalternate in sexual life cycles

250 UNIT THREE Genetics

Behavior of Chromosome Setsin the Human Life Cycle

The human life cycle begins when a haploid sperm from thefather fuses with a haploid egg from the mother. This unionof gametes, culminating in fusion of their nuclei, is calledfertilization. The resulting fertilized egg, or zygote, isdiploid because it contains two haploid sets of chromosomesbearing genes representing the maternal and paternal familylines. As a human develops into a sexually mature adult, mitosisofthe zygote and its descendants generates all the somatic ceUsof the body. Both chromosome sets in the zygote and all thegenes they carry are passed with precision to the somatic cells.

The only cells of the human body not produced by mitosisare the gametes, which de\'elop from specialized reUs calledgerm cells in the gonads-ovaries in females and testes inmales (Figure 13.5). Imagine what would happen if humangametes were made by mitosis: They would be diploid likethe somatic cells. At the next round offertilization, when twogametes fused, the normal chromosome number of46 woulddouble to 92, and each subsequentgeneration would double the

Key Haploid gametes (n " 23)

and a paternal set (from our father). The number of chromo­somes in a single set is represented by n. Any cell with twochromosome sets is called a diploid cell and has a diploidnumber of chromosomes, abbreviated 2n. For humans, thediploid number is 46 (2n = 46), the number ofchromosomesin our somatic cells. In a cell in which DNA synthesis has oc­curred, all the chromosomes are replicated, and therefore eachconsists oftwo identical sister chromatids, associated closelyatthe centromere and along the arms. Figure 13.4 helps clarifythe various terms that we use in describing replicated chromo­somes in a diploid ceiL Study this figure so that you understandthe differences between homologous chromosomes, sisterchromatids, nonsister chromatids, and chromosome sets.

Unlike somatic cells, gametes (sperm and eggs) contain asingle chromosome set. Such cells are called haploid cells,and each has a haploid number of chromosomes (n). For hu­mans, the haploid number is 23 (n = 23). The set of 23 con­sists of the 22 autosomes plus a single sex chromosome. Anunfertilized egg contains an X chromosome, but a sperm maycontain an X or a Ychromosome.

Note that each sexually reproducing species has a charac­teristic diploid number and haploid number. For example, thefruit ny, DrosoplJila melanogaster, has a diploid number of 8and a haploid number of 4, while dogs have a diploid numberof78 and a haploid numberof39.

Now that you have learned the concepts ofdiploid and hap­loid numbers of chromosomes, let's consider chromosomebehavior during sexual life cycles. We11 use the human life cy­cle as an example.

Haploid (n)

DiplOId (2n)Egg (n)

• Figure 13.4 Desaibing chromosomes. Acell with a dipbdnumber of 6 (2n = 6) is depicted here followang chromosome replicatJOnand condensa1JOfl. Each of the SIX replicated chromosomes conSISts oftwo SiSler chromatids assoaated dosefy along their lengths. Eachhomologous pair IS composed of one chromosome from the maternalset (~ and one from the paternal set (blue). Each set is made up ofthr~ chromosomes Ir'I thIS example. NonSlSter chromatids are any twochromatids In a pa... of homologous chromosomes that a~ not SISter

"""""""n What is the haploid number of this celf? ~ a ~set~ of.. chromosomes haploid Of diploid?

Diploidzygote(2n = 46)

Mitosis anddevelopment

Mulllcellular diploidadults (2n = 46)

• Figure 13.5 The human life cycle. In each generation, thenumber of chromosome sets doubles at fertilization. but IS halveddunnq meIOSIS. For humans, the number of chromosomes in a haplOIdcell is 23. COOSlSllnq of ooe set (n = 23); the number of chromosomesIn the diploid lY90te and all somatIC cells ansmg from It is 46,Consrstlnq of two sets (2n = 46).

ThIS figure IntrodllCe5 a color code that wia be IlSf'C:f for other lifecydes later In thIS book. The teal arrows highlight haploid stages of atife~, and the betge arrows highlight dIploid stages.

Centromere

.., ,L-_7'_palf of homologous!!I chromosomes

(one from each set)

Koy

{

• Maternal set of2n = 6 chromosomes (n = 3)

Paternal set of• chromosomes (n = 3)

Two sister chromatidsof one replicatedchromosome

Two nonSlSter ~""::.l,,--.tlduomalldsma homologous pair

CHAHII THIRTUN Meiosis il.nd Sexua.l Life Cycles 251

number of chromosomes yet again. This does not happen,however, because in sexually reproducing organisms, the gam­etes are formed by a modified type of cell division calledmeiosis. This type ofcell division reduces the number ofsets of

chromosomes from two to one in the gametes, counterbalanc­

ing the doubling that occurs at fertilization. In animals, meiosisoccurs only in the ovaries or testes. As a result of meiosis, eachhuman sperm and egg is haploid (n = 23). Fertilization restores

the diploid condition by combining two haploid sets of chro­mosomes, and the human life cycle is repeated, generation af­ter generation (see Figure 13.5). You will learn more about theproduction of sperm and eggs in Chapter 46.

In general, the steps of the human life cycle are typical ofmany sexually reproducing animals. Indeed, the processesof fertilization and meiosis are the unique trademarks ofsexual reproduction, in plants as well as animals. Fertiliza­tion and meiosis alternate in sexual life cycles, maintaininga constant number of chromosomes in each species from

one generation to the next.

The Variety of Sexual Life Cycles

Although the alternation of meiosis and fertilization is com­mon to all organisms that reproduce sexually, the timing ofthese two events in the life cycle varies, depending on thespecies. These variations can be grouped into three maintypes of life cycles. In the type that occurs in humans and mostother animals, gametes are the only haploid cells. Meiosis oc­curs in germ cells during the production of gametes, whichundergo no further cell division prior to fertilization. After

fertilization, the diploid zygote divides by mitosis, producinga multicellular organism that is diploid (Figure 13.6a).

Plants and some spe<ies of algae exhibit a second type oflifecycle called alternation of generations. This type includesboth diploid and haploid stages that are multicellular. The mul­ticellular diploid stage is called the sporophyte. Meiosis in the

sporophyte produces haploid cells called spores. Unlike a gam­ete, a haploid spore doesn't fuse with another cell but divides

mitotically, generating a multicellular haploid stage called thegametophyte. Cells of the gametophyte give rise to gametes bymitosis. Fusion of two haploid gametes at fertilization resultsin a diploid zygote, which develops into the next sporophytegeneration. Therefore, in this type ofHfe cycle, the sporophytegeneration produces a gametophyte as its offspring, and thegametophyte generation produces the next sporophyte gener­ation (Figure 13.6b). Clearly, the term alternation ofgenerations is a fitting name for this type of life cycle.

A third type oflife cycle occurs in most fungi and some pro­tists, including some algae. After gametes fuse and form adiploid zygote, meiosis occurs without a multicellular diploid

offspring developing. Meiosis produces not gametes but hap­loid cells that then divide by mitosis and give rise to either uni­cellular descendants or a haploid multicellular adult organism.Subsequently, the haploid organism carries out further mi­toses, producing the cells that develop into gametes. The onlydiploid stage found in these species is the single-celled zygote(Figure 13.6c).

Note that either haploid or diploid cells can divide by mito­sis, depending on the type oftife cycle. Only diploid cells, how­ever, can undergo meiosis because haploid cells have a singleset of chromosomes that cannot be further reduced. Though

the three types ofsexual life cycles differ in the timing of meio­sis and fertilization, they share a fundamental result: genetic

K.y

Zygote

Haploid unicellular ormultICellular organism

n

n

Gametes

20

n

Diploidmulticellular J.~ Mitosisorganism / ~

(sporophyte)

n

FERTILIZATION

o

Gametes

Haploid (n)

• Diploid (2n)

DiPIOid~~-::~::;::::::multicellularorganism

(a) Animals (b) Plants and some algae (c) Most fungi and some protists

.. Figure 13.6 Three types of sexual life cycles. The common feature of all three cycles is the alternation ofmeiosis and fertilization, key events that contribute to genetic variation among offspring. The cycles differ in the limingof these two key events.

252 UNIT THREE Genetics

variation among offspring. A closer look at meiosis will revealthe sources of this variation.

r:·~~::~s·r::~~es the numberof chromosome sets fromdiploid to haploid

Many of the steps of meiosis closely resemble correspondingsteps in mitosis. Meiosis, like mitosis, is preceded by the repli­cation of chromosomes. However, this single replication isfollowed by not one but two consecutive cell divisions, calledmeiosis I and meiosis II. These two divisions result in fourdaughter cells (rather than the two daughter cells of mitosis),each with only half as many chromosomes as the parent cell.

)Diploid cell withreplicatedchromosomes

Chromosomesreplicate

8 Sister chromatids

D'P'"'' c~

1JHomologous pair of replicated chromosomes

/\Si"" mchrom~

;\X 0 Homolog,", X

chromosomesseparate

Haploid cells withreplicated chromosomes

Homologous pair of chromosomes

io diploid P'A'

~ ~ )

13.ZCONCEPT CHECK

I. How does the karyotype ofa human female differfrom that ofa human male?

2. How does the alternation of meiosis and fertilizationin the life cycles of sexually reproducing organismsmaintain the normal chromosome count for eachspecies?

3. Each sperm ofa pea plant contains seven chromo­somes. What are the haploid and diploid numbers forpeas?

4. • i,'llfu1iM A certain eukaryote lives as a unicellu­lar organism, but during environmental stress, its cellsproduce gametes. The gametes fuse, and the resultingzygote undergoes meiosis, generating new single cells.What type of organism could this be?

For suggested answers, see Appendix A.

The Stages of Meiosis Haploid cells with unrepllCated chromosomes

The overview of meiosis in Figure 13.7 shows that both mem­bers ofa single homologous pair of chromosomes in a diploidcell are replicated and that the copies are then sorted into fourhaploid daughter cells. Recall that sister chromatids are m'O

copies of one chromosome, closely associated all along theirlengths; this association is called sisterchromatid cohesion. To­gether, the sister chromatids make up one replicated chromo­some (see Figure 13.4). In contrast, the two chromosomes ofahomologous pair are individual chromosomes that were inher­ited from different parents. Homologs appear alike in themicroscope. but they may have different versions of genes,called alleles, at corresponding loci (for example, an allele forfreckles on one chromosome and an allele for the absence offreckles at the same locus on the homolog). Homologs are notassociated with each other except during meiosis, as you willsoon see.

... Figure 13.7 Overview of meiosis: how meiosis reduceschromosome number. After the chromosomes replicate ininterphase. the diploid cell divides twice. yielding four haploiddaughter cells This overview tracks just one pair of homologouschromosomes, which for the sake of simplicity are drawn in thecondensed state throughout (they would not normally be condensedduring interphase), The red chromosome was inherited from thefemale parent, the blue chromosome from the male parent.••l.f.WIII Redraw the cells in this figure using a simple DNAdouble helix (0 represent each DNA molecule

Figure 13.8, on the next rn'o pages, describes in detail thestages of the two divisions of meiosis for an animal cell whosediploid number is 6. Meiosis halves the total number of chro­mosomes in a very specific way, reducing the number of setsfrom two to one. with each daughter cell receiving one set ofchromosomes. Study Figure 13.8 thoroughly before going on.

CHAPTE~ THI~HEN Meiosis and Sexual Life Cycles 253

• FIguro 13.1

Telophase I andCytokinesis

Anaphase IMetaphase I

..........MEIOSiS I: Separates homologous chromosomes"

Prophase I

@:j[;mmim·TheMeioticDivisionofanAnimaICell

"omologoo,chromosomes

Two haploid cellsform; each chromosomestill consists of twosister chromatids

SISter chromatidsremdlll attached

Homologouschromosomesseparate

Each pair of homologouschromosomes separates

Metaphasep"~

Centromere(WIth kinetochore)

Microtubuleattached tokinetochore

Chromosomes line upby homologous pairs

Fragmentsof nuclearenvelope

Replkated homologouschromosomes (red and blue)pair and exchange segments;2n '" 6 in this example

Centrosome(WIth centnole pair)

Sister ChlilSlTlillachromattds

Prophase I• Chromosomes begin to

condense, and homologs looselypair along their lengths. alignedgene by gene.

• Crossing over (the exchange ofcorresponding seglTl(!nts of DNAmole<ules by nonsisler chromatids)is completed while homologs alein synapsis, held tightly togetherby proteins along their lengths(before the stage shown).

• Synapsis ends in mid·prophase.and the chromosomes in eachpair move apart slightly, asshown above.

• Each homologous pair has one 01more chiasmata, points whereGossing over has occuued andthe homologs are still associateddue 10 cohesion betweef1 sisterdllomalids (sister chromatidcohesion).

• Centrosome movement, spindleformation, and nuclear envelopeb/eakdown occur as in mitosis.

• In late prophase I(after the stageshown), microtubules from onepole or the other attach to thetwo kinetochores, proteinstructures at the centromeres ofthe two homologs. Thehomologous pairs then movetoward the metaphase plate.

Metaphase I• Pairs of homologouschromoso~ are now arrangedon the metaphase plate, withone chromosome in each pairfacing each pole.

• Bolh dwomatids ofall! tonoIog areattad'led 10 kinetochoreiOOouDJIes frrm all! jXIIe; those ofthe other hlmolog are attathed 10

iOOoniluIes frrm the oppo5ite pole.

Anaphase I• Breakdown 01 proteins

responsible for sister chromatidcohesion along chromatid armsallows homologs to separate.

• The homologs move towardopposite poles. guided by thespindle apparatus.

• Sister chromatid cohesion persistsat the centromere. causingchromatids to move as a unittoward the same pole.

Telophase I andCytokinesis• At the beginning of telophase I,

each half of the cell has acomplete haploid set ofreplicated chromosomes. Eachchromosome is composed of twosister chromatids; one or bothchromatids include regions ofnonsister chromatid DNA.

• Cytokinesis (division of thecytoplasm) usually occurssimultaneously with telophase I.forming two haploid daughter ceI~

• In animal cells. a cleavage furrowIOlms.. (In plant cells. a cell plateIOlms..)

• In some spe<ies. chromoso~deconr:lense aoo the nudearenvtlope re-forms..

• No replication occurs betweenmeiosis Iand meiosis II.

254 UNIT TUff Genetics

Telophase II andCytokinesis

Anaphase 11Metaphase n-l,.--1111.................. MEIOSIS II: Separates sister chromatids .....

Prophase 11

During another round of (ell division, the sister dlromatids finally separate:four haploid daughter cells result., containing unrepficated chromosomes

Haploid daughtef cellsfOfml"9

SISler chromatidsseparate

Prophase II• Aspindle apparatus forms.

• In late prophase II (nOI shownhere), chromosomes, each stillcomposed of two chromatidsasso<iated <lIthe centromere,move toward the metaphase IIplale.

Metaphase II• The chromosomes are positioned

on the metaphase plate as inmitosis.

• Because of crossing over inmeiosis I, the two sisterchromatids of each chromosomeare not genetically identical.

• The kinetochores of sislerchromatids afe attached tomicrotubules extl'nding fromopposite poles,

Anaphase II• Breakdown of proteins holding

the sister chromatids togethel atthe centromere allows thechromatids to separate, Thechromatids move towardopposite poles as individualchromosomes.

Telophase IIand Cytokinesis• Nuclei form, the chlOmosomes

begin de(Ondensing, andcytokinesis occurs.

• The meiotic division of oneparent cell produces fourdaughter cells, each with ahaploid set of (unreplicatedlchlOmosomes.

• Each of the fOUl daughter cells isgeflelically distinct hom theolhel daughter cells and flOmthe parent cell.

BioFlix Vlsn the Study Areaat www.masteringbio.(omfOf the 8IoAix 3-D Ammation on

"""'"

(HAHUI THIRTUH Meiosis and Sexual Life C)"des 255

MITOSIS MEIOSIS

Parent cell(before chromosome replication) MEIOSIS I

Metaphase I

Prophase I

Homologous chromosomepair held together bychiasma and sisterchromatid cohesion

Chiasma (site ofcrossing over)

.Ai\,LcOh-,-,m-,,-,-m-,-( =- t

replication

Chromosomes lineup by homologouspairs at themetaphase plate

2n= 6

Chromosomesline up Individu­ally at themetaphase plate

Chromosomereplication

ReplICated chromosome(two sister chromatids)

Metaphase

Prophase

AnaphaseTelophase

®-..... --" 20

Daughter cellsof mitosis

Sister chromatidsseparate duringanaphase

@-.... -­......20

Homologsseparateduringanaphase I;sisterchromatidsremainattached atcentromere

Sisterchroma­tidsseparatedunnganaphase II

oDaughter cells of meiosis II

Anaphase ITelophase I

Haploidn=3

MEIOSIS II

SUMMARY

Property MitOSIS MeIOSIS

DNAreplication

Number ofdivisions

Occurs during interphase before mitosis begins

One, including prophase, metaphase,anaphase, and telophase

Occurs during interphase before meiosis I begins

Two, each including prophase, metaphase, anaphase, andtelophase

Synapsis ofhomologouschromosomes

Does not occur Occurs during prophase I along with crOSSing overbetween nonsister chromatids; resulting chiasmatahold pairs together due to sister chromatid cohesion

Number ofdaughter cellsand geneticcomposition

Role in theanimal body

Two, each diploid (2n) and geneticallyidentical to the parent cell

Enables multicellular adult to arise fromzygote; produces cells for growth, repair, and,in some species, asexual reproduction

Four, each haploid (n), containing half as many chromosomesas the parent cell; genetically different from the parentcell and from each other

Produces gametes; reduces number of chromosomes by halfand introduces genetic variability among the gametes

... Figure 13.9 A comparison of mitosis and meiosis in diploid cells.••I;t-W'I' Could any other combinations of chromosomes be generated during meiosis II from the specific cellsshown in telophase P Explain. (Hint: Draw fhe cells as they would appear in metaphase II.)

256 UNIT THREE Genetics

What prevents the separation of sisterchromatids at anaphase I of meiosis?

N'mu". Draw agraph showing what you expect happened tothe chromatids of the unlabeled chromosome in both strains of cells,

# 10080

~ 60~ 40&. 20~ 0 .t,2~::-;;c

Shugoshin+ Shugoshin-

SOURCE

CONCLUSION The researchers concluded that shugoshin pro·tects cohesins at the centromere at anaphase I. thus maintainingthe attachment between sister chromatids and ensuring that theyseparate properly during meiosis II,

Shugoshin+ (normal) Shugoshin-

Spore case Fluorescent label

~tL - -Metaphase I

-)('-- t -1/ ~.--Anaphase I

kt

II l.Metaphase II

t OR

-I • ,II' ,II'-.... Anaphase II ""- ""-

~t

Maturespores

Two of three possible arrange-Sporements of labeled chromosomes

T. S, KitaJ'III<l, S. A. KiJWa'>hlma, and Y. Watanabe, Theconserved konetochore proteon shugoshin protects centromeric coheSIon during

meIOsis. N~lure 427510-517 aOO4)

Yoshinori Watanabe and colleagues knew thatduring anaphase I. the protein shugoshin is present only aroundcentromeres, They wondered whether it protects cohesins therefrom degradation in meiosis I. ensuring that chromatids stay to­gether while homologs separate. To test this hypothesis. they useda species of yeast in which meiosis produces haploid spores linedup in aspecific order inSide a spore case To follow the movementof chromosomes. they fluorescent/y labeled a region near the cen­tromere of both chromatids in one homolog, leaving the other ho­molog unlabeled. They then disabled the gene coding forshugoshin and compared this yeast strain (shugoshin ) with nor­mal yeast cells (shugoshin +), The researchers expected that the twolabeled chromosomes ariSing from the labeled chromatids in nor­mal cells would end up in separate spores at one end of the sporecase. They further predicted that if shugoshin does protect co­hesins from cleavage at the centromere at anaphase I, then the la­beled chromosomes in shugoshin- cells would separate randomlyin meiosis II, sometimes ending up in the same spore.

EXPERIMENT

RESULTS In shug­oshin +cells, the two labeledchromosomes ended up indifferent spores in almost allcases, In shugoshin- cells.they were in the same sporein about half the cases.

AComparison of Mitosis and Meiosis

Figure 13.9 summarizes the key differences between meiosis

and mitosis in diploid cells. Basically, meiosis reduces thenumber of chromosome sets from two (diploid) to one (hap­[oid), whereas mitosis conserves the number ofchromosomesets. Therefore, meiosis produces cells that differ geneticallyfrom their parent cell and from each other, whereas mitosis

produces daughter cells that are genetically identical to their

parent cell and to each other.Three events unique to meiosis occur during meiosis I:

1. Synapsis and crossing over. During prophase I, repli­cated homologs pair up and be<ome physically connectedalong their lengths by a zipper-like protein structure, thesynaptonemal complex; this process is called synapsis.

Genetic rearrangement between nonsister chromatids,

known as crossing over, is completed during this stage.

Following disassembly of the synaptonemal complex in

late prophase, the hvo homologs pull apart slightly but re­

main connected by at least one X-shaped region called a

chiasma (plural, chiasmata). A chiasma is the physical

manifestation of crossing over; it appears as a cross be­

cause sister chromatid cohesion still holds the m'o origi­

nal sister chromatids together, even in regions where one

of them is now part of the other homolog. Synapsis and

crossing over normally do not occur during mitosis.

2, Homologs on the metaphase plate, At metaphase I of

meiosis, chromosomes are positioned on the metaphase

plate as pairs of homologs, rather than individual chro­

mosomes, as in metaphase of mitosis.

3. Separation of homologs. At anaphase I of meiosis, the

replicated chromosomes ofeach homologous pair move

toward opposite poles, but the sister chromatids of each

replicated chromosome remain attached. In anaphase of

mitosis, by contrast, sister chromatids separate.

How do sister chromatids stay together through meiosis I

but separate from each other in meiosis II and mitosis? Sister

chromatids are attached along their lengths by protein com­

plexes called cohesins. In mitosis, this attachment lasts until the

end of metaphase, when enzymes cleave the cohesins, freeing

the sister chromatids to move to opposite poles of the cell. In

meiosis, sister chromatid cohesion is released in two steps. In

metaphase I, homologs are held together by cohesion between

sister chromatid arms in regions where DNA has been ex­

changed. At anaphase I, cohesins are cleaved along the arms,

allowing homologs to separate. At anaphase II, cohesins are

cleaved at the centromeres, allowing chromatids to separate.

Figure 13.10 shows one of a series of experiments carried

out by Yoshinori Watanabe and colleagues at the University of

Tokyo. They knew that similar proteins were present in co­

hesin complexes during mitosis and meiosis, and they won­

dered what was responsible for preventing cohesin cleavage at

the centromere while it was occurring along sister chromatid

CHAPTER THIRTEEN Meiosis and Sexual Life Cycles 257

~:~':t~;v~~i~~on producedin sexual life cycles contributesto evolution

I. How are the chromosomes in a cell at metaphase ofmitosis similar to and different from the chromo­

somes in a cell at metaphase of meiosis II?

2. -'M"'I. Given that the synaptonemal complexdisappears by the end of prophase, how would thetwo homologs be associated if crossing over did notoccur? What effect might this ultimately have ongamete formation?

For suggested answers, see Appendi~ A.

arms at the end of metaphase I. They found a protein theynamed shugoshin (Japanese for "guardian spirit") that protectscohesins from cleavage at the centromere during meiosis I.Shugoshin is similar to a fruit fly protein identified lOyears ear~

lier by Terry Orr-Weaver, this unit's interviewee.

Meiosis I is called the reductional division because ithalves the number of chromosome sets per cell-a reduc·tion from two sets (the diploid state) to one set (the haploidstate). During the second meiotic division, meiosis II (some­

times called the equational division), the sister chromatidsseparate, producing haploid daughter cells. The mechanismfor separating sister chromatids is virtually identical inmeiosis II and mitosis. The molecular basis of chromosomebehavior during meiosis continues to be a focus of intenseresearch interest.

CONCEPT CHECI( 13.3

Origins of Genetic Variation Among Offspring

In species that reproduce sexually, the behavior of chromo­somes during meiosis and fertilization is responsible for most

of the variation that arises each generation. Let's examinethree mechanisms that contribute to the genetic variationarising from sexual reproduction: independent assortment ofchromosomes, crossing over, and random fertilization.

Independent Assortment of Chromosomes

One aspect ofsexual reproduction that generates genetic vari­ation is the random orientation of homologous pairs of chro­mosomes at metaphase of meiosis L At metaphase I, thehomologous pairs, each consisting of one maternal and onepaternal chromosome, are situated on the metaphase plate.(Note that the terms maternal and paternal refer, respectively,

to the mother and father of the individual whose cells are un·dergoing meiosis.) Each pair may orient with either its mater·nal or paternal homolog closer to a given pole-its orientationis as random as the flip of a coin. Thus, there is a 50% chancethat a particular daughter cell of meiosis I will get the mater­nal chromosome of a certain homologous pair and a 50%chance that it will get the paternal chromosome.

Because each homologous pair of chromosomes is posi­tioned independently ofthe other pairs at metaphase I, the firstmeiotic division results in each pair sorting its maternal andpaternal homologs into daughter cells independently of every

other pair. This is called independent assortment. Each daugh·ter cell represents one outcome of all possible combina·tions of maternal and paternal chromosomes. As shown inFigure 13.11, the number of combinations possible fordaughter cells formed by meiosis of a diploid cell with twohomologous pairs of chromosomes is four (two possiblearrangements for the first pair times two possible arrange­ments for the second pair). Note that only two of the four

Possibility 2How do we account for the genetic varia­tion illustrated in Figure 13.1? As you willlearn in more detail in later chapters, mu­tations are the original source of geneticdiversity. These changes in an organism'sDNA create the different versions ofgenes known as alleles. Once these differ~

ences arise, reshuffling of the alleles dur­

ing sexual reproduction produces thevariation that results in each member of aspecies having its own unique combina­tion of traits.

.... Figure 13.11 The independentassortment of homologouschromosomes in meiosis.

258 UNIT THREE Genetics

Possibility 1

Two equally probablearrangements ofchromosomes at

metaphase I

Metaphase II

Daughtercells

Combination 3 Combination 4

... Figure 13.12 The results of crossing over during meiosis.

Nonsist~r chromatidsh~ld tog~ther

during synapsis

f.) Chiasmata andatlachm~nts betweensister chromatids holdhomologs together;they move to themetaphase I plate,

o In prophas~ t synapsisand crossing overoccur; then homologsmov~ apart slightly.

oBreakdown of prot~ins

holding sister chroma·tid arms togetherallows homologs withrecombinant chroma­tids to separate,

Pair of --''-'';:0-/homologs

Recombinantchromosomes

Chiasma.site ofcrossingover

Centromere

IEM

Anaphase [

Anaphase II

Prophas~ Iof m~iosis

Daughtercells

ing meiosis II further increases the number of genetic types of

daughter cells that can result from meiosis.

You will learn more about crossing over in Chapter 15. The

important point for now is that crossing over, by combining

DNA inherited from two parents into asinglechromosome, is

an important source of genetic variation in sexual life cycles.

The random nature of fertilization adds to the genetic vari·

ation arising from meiosis. In humans, each male and fe­

male gamete represents one of about 8.4 million (223)

possible chromosome combinations due to independent as­

sortment. The fusion of a male gamete with a female gamete

during fertilization will produce a zygote with any of about70 trillion (223 x 223

) diploid combinations. If we factor in

the variation brought about by crossing over, the number of

possibilities is truly astronomical. It may sound trite, but

you really are unique.

Random Fertilization

combinations of daughter cells shown in the figure would result

from meiosis of a single diploid cell, because a single parent cell

would have one or the other possible chromosomal arrangement

at metaphase I, but not both. However, the population of daugh­

ter cells resulting from meiosis of a large number of diploid cells

contains aU four types in approximatelyequal numbers. In the caseof n = 3, eight combinations of chromosomes are possible for

daughter cells. More generally, the number of possible combina­

tions when chromosomes sort independently during meiosis is 2n,

where n is the haploid number of the organism.

In the case of humans (n = 23), the number of possible

combinations of maternal and paternal chromosomes in the

resulting gametes is 223, or about 8.4 million. Each gamete that

you produce in your lifetime contains one of roughly 8.4 mil­

lion possible combinations of chromosomes.

Crossing Over

As a consequence of the independent assortment of chro­

mosomes during meiosis, each of us produces a collection of

gametes differing greatly in their combinations of the chro­

mosomes we inherited from our two parents. Figure 13.11

suggests that each individual chromosome in a gamete is ex­

clusively maternal or paternal in origin. In fact, this is notthe case, because crossing over produces recombinant

chromosomes, individual chromosomes that carry genes

(DNA) derived from two different parents (Figure 13.12).In meiosis in humans, an average of one to three crossover

events occur per chromosome pair, depending on the size of

the chromosomes and the position oftheir centromeres.

Crossing over begins very early in prophase I, as homolo­

gous chromosomes pair loosely along their lengths. Each gene

on one homolog is aligned precisely with the corresponding

gene on the other homolog. In a single crossover event, spe­

cific proteins orchestrate an exchange of corresponding seg­

ments of two nonsister chromatids-one maternal and one

paternal chromatid ofa homologous pair. In this way, crossing

over produces chromosomes with new combinations of ma­

ternal and paternal alleles (see Figure 13. I2).

In humans and most other organisms studied so far, cross­

ing over also plays an essential role in the lining up ofhomolo­

gous chromosomes during metaphase I. As seen in Figure 13.8,

a chiasma forms as the result of a crossover occurring while

sister chromatid cohesion is present along the arms. Chias­

mata hold homologs together as the spindle forms for the first

meiotic division. During anaphase I, the release of cohesion

along sister chromatid arms allows homologs to separate.

During anaphase II, the release ofsister chromatid cohesion at

the centromeres allows the sister chromatids to separate.

At metaphase II, chromosomes that contain one or more re­

combinantchromatids can be oriented in two alternative, non­

equivalent ways with respect to other chromosomes, because

their sister chromatids are no longer identical. The different

possible arrangements of nonidentical sister chromatids dur-

CHAPTE~ THIRTEEN Meiosis and Sexual Life Cycles 259

CONCEPT CHECK

The Evolutionary Significance of GeneticVariation Within Populations

Now that you've learned how new combinations of genesarise among offspring in a sexually reproducing popula­tion, let's see how the genetic variation in a population re­lates to evolution. Darwin recognized that a populationevolves through the differential reproductive success of itsvariant members. On average, those individuals bestsuited to the local environment leave the most offspring,thus transmitting their genes. This natural selection re·suits in the accumulation of those genetic variations fa­vored by the environment. As the environment changes,the population may survive if, in each generation, at leastsome of its members can cope effectively with the newconditions. Different combinations of alleles may work

better than those that previously prevailed. Mutations are

the original source of different alleles, which are then

mixed and matched during meiosis. In this chapter, we

have seen how sexual reproduction greatly increases the

genetic variation present in a population. In fact, the abil­

ity of sexual reproduction to generate genetic variation is

one of the most commonly proposed explanations for the

persistence of sexual reproduction.

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SUMMARY OF KEY CONCEPTS

.i.III1I'_ 13.1Offspring acquire genes from parents by inheritingchromosomes (pp. 248-249).. Inheritance of Genes Each gene in an organism's DNA exists

at a specific locus on a certain chromosome. We inherit one setof chromosomes from our mother and one set from our father.

.. Comparison of Asexual and Sexual Reproduction Inasexual reproduction, a single parent produces geneticallyidentical offspring by mitosis. Sexual reproduction combinessets of genes from two different parents. forming geneticallydiverse offspring.

Acti\ity Asexual and Sexual Life Cycles

.i.IIIII'-13.2Fertilization and meiosis alternate in sexual life cycles(pp.250-253)

.. Sets of Chromosomes in Human Cells Normal human so­matic cells are diploid. They have 46 chromosomes made up

260 UNIT THREE Genetics

Although Darwin realized that heritable variation is what

makes evolution possible, he could not explain why offspring

resemble-but are not identical to-their parents. Ironically,

Gregor Mendel, a contemporary ofDarwin, published a theory

of inheritance that helps explain genetic variation, but his dis­

coveries had no impact on biologists until 1900, more than 15years after Darwin (1809-1882) and Mendel (l822-1884) had

died. In the next chapter, you will learn how Mendel discovered

the basic rules governing the inheritance of specific traits.

13.41. What is the original source of all the different aneles

of a gene?

2. The diploid number for fruit flies is 8, while that for

grasshoppers is 46. If no crossing over took place,

would the genetic variation among offspring from a

given pair of parents be greater in fruit flies or

grasshoppers? Explain.

3. -Q@i1IM Under what circumstances would

crossing over during meiosis not contribute to genetic

variation among daughter cells?

For $ugg~sted an$w~rs, see Appendix A.

of two sets of23-one set from each parent. In human diploidcells, there are 22 homologous pairs of autosomes, each witha maternal and a paternal homolog. The 23rd pair, the sexchromosomes, determines whether the person is female (XX)or male (XY).

.. Behavior of Chromosome Sets in the Human Life CycleAt sexual maturity, ovaries and testes (the gonads) producehaploid gametes by meiosis, each gamete containing a Singleset of 23 chromosomes (n == 23). During fertilization, anegg and sperm unite, forming a diploid (2n = 46) single­celled zygote, which develops into a multicellular organismby mitosis.

.. The Variety of Sexual life Cycles Sexual life cycles differin the timing of meiosis relative to fertilization and in thepoint(s) of the cycle at which a multicellular organism isproduced by mitosis.

.ill"I'-13.3Meiosis reduces the number of chromosome sets fromdiploid 10 haploid (pp. 253-258)

.. The Stages of Meiosis The two cell divisions of meiosis pro­duce four haploid daughter cells. The number of chromosomesets is reduced from two (diploid) to one (haploid) duringmeiosis I, the reductional division.

.. A Comparison of Mitosis and Meiosis Meiosis is distin­guished from mitosis by three events of meiosis I, as shownon the next page:

Prophase [: EilCh homologous pa,r undergoes sYT1~psis

~nd crossmg over l>elween nClnSJster chrom~1lds

c. the daughter cells are diploid.

d. homologous chromosomes synapse.

e. the chromosome number is reduced.

Metaphase I: Chromosomes line up ~s homologouspairs o<l1he mel~pnase pl~1e.

Anaphase I: Homologs separate from eilCh other; sislerchroma1ids fem~ln joined at lhe centromere.

5. If the DNA content of a diploid cell in the G1 phase of the cell

cycle is x, then the DNA content ofthe same cell at metaphase

of meiosis I would be

a.0.25x. b. D.5.\:. c. x. d. 2x. e. 4x.

6. Ifwe continued to follow the cell lineage from question 5, then the

DNA content ofa single cell at metaphase of meiosis II would be

a. 0,25x. b. D.5.\:. c. x. d. 2x. e. 4x.

d. fertilization.

e. binary fission.

Meiosis II separates the sister chromatids.

-MNt.•RioFlix J.D Animation Meiosis

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MPJ Tutor Mito,is-Meiosi, Comparison

Acthity Meiosis Animation

_',11'''''-13.4Genetic variation produced in sexual life cyclescontributes to evolution (pp. 258-260).. Origins of Genetic Variation Among Offspring Three

events in sexual reproduction contribute to genetic variationin a population: independent assortment of chromosomesduring meiosis, crossing over during meiosis I, and randomfertilization of egg cells by sperm. Due to sister chromatid co­hesion. crossing over leads to chiasmata. which hold ho­mologs together until anaphase I.

.. The Evolutionary Significance of Genetic Variation WithinPopulations Genetic variation is the raw material for evolutionby natural selection. Mutations are the original source of thisvariation; the production of new combinations ofvariant genesin sexual reproduction generates additional genetic diversity.

-tiN',·Acthity Origins ofGenetk Variation

In,'estigation How Can the FrequencyofCrossing Over Be Estimated?

TESTING YOUR KNOWLEDGE

SELF·QUIZ

I. A human cell containing 22 autosomes and a Y chromosome is

a. a sperm, d. a somatic cell of a male.

b. an egg. e. a somatic cell of a female.

c. a zygote.

2. \Vhich life cycle stage is found in plants but not animals?

a. gamete d. multicellular haploid

b. zygote e. unicellular diploid

c. multicellular diploid

3. Homologous chromosomes move toward opposite poles of a

dividing cell during

a. mitosis.

b. meiosis I.c. meiosis II.

4. Meiosis 11 is similar to mitosis in that

a. sister chromatids separate during anaphase.b. DNA replicates before the division.

7. How many different combinations of maternal and paternal

chromosomes can be packaged in gametes made by an organ­ism with a diploid number of 8 (2n = 8)?

a. 2 b. 4 c. 8 d. 16 e. 32

Use the diagram of a cell below to answer questions 8-10.

8. How can you tell this cell is under­

going meiosis, not mitosis?

9. Identify the stage of meiosis

shown.

to. ••i;flWIl. Copy the

drawing to a sepamte sheet

of paper and label appropri.

ate structures with these

terms, drawing lines or brack-

ets as needed: chromosome (label

as replicated or unreplicated), cen-

tromere, kinetochore, sister chromatids, nonsister chromatids,

homologous pair, homologs, chiasma, sister chromatid cohesion.

Describe the makeup of a haploid set and a diploid set.

For Self-Qui~ answers, see Appendix A.

em If·. Visit the Study Area at www.masteringbio.com for aPractice Test.

EVOLUTION CONNECTION

II. Many species can reproduce either asexually or sexually. What

might be the evolutionary significance of the switch from asex­

ual to sexual reproduction that occurs in some organisms when

the environment becomes unfavorable?

SCIENTIFIC INQUIRY

12. The diagram accompanying questions 8-10 represents a meiotic

cell in a certain individual. A previous study has shown that the

freckles gene is located at the locus marked F, and the hair color

gene is located at the locus marked H, both on the long chromo·

some. The individual from whom this cell was taken has inher­ited different alleles for each gene ("freckles" and "black hair"

from one parent, and "no freddes" and "blond hair" from the

other). Predict allele combinations in the gametes resulting from

this meiotic event. (It will help ifyou draw out the rest of meiosis,

labeling alleles by name.) List other possible combinations of

these alleles in this individual's gametes.

CHAPTE~ THIRTEEN Meiosis and Sexual Life Cycles 261