Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies –...

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Biology 122 Biology 122 Meiosis Meiosis

Transcript of Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies –...

Page 1: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

Biology 122Biology 122

MeiosisMeiosis

Page 2: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

# of Chromosomes# of ChromosomesHumans – 46Humans – 46Fruit Flies – 4Fruit Flies – 4Pigeons – 80Pigeons – 80Cats – 38Cats – 38Elephants – 56Elephants – 56Guinea Pigs – 64Guinea Pigs – 64Horses - 64Horses - 64

Page 3: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.
Page 4: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

Why don’t all of our cells have Why don’t all of our cells have 92 chromosomes?92 chromosomes?

46 from male + 46 from female = 46 from male + 46 from female = 92?92?

Page 5: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

Gametes – sex cellsGametes – sex cellssome cells some cells

(namely the (namely the sex cells, or sex cells, or gametes - gametes - sperm and sperm and egg cells) egg cells) have half that have half that number, 23 number, 23 chromosomes. chromosomes.

Page 6: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

Where do these cells come Where do these cells come from? from?

If mitosis always produces If mitosis always produces two identical daughter cells two identical daughter cells from a mother cell, how do from a mother cell, how do the sex cells develop?the sex cells develop?

Page 7: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

MEIOSIS is the answer!MEIOSIS is the answer! It occurs only in the gonadsIt occurs only in the gonads Testes for menTestes for men Ovaries for womenOvaries for women It allows the chromosomes to It allows the chromosomes to

reduce their number by half so that reduce their number by half so that when the egg and sperm unite, they when the egg and sperm unite, they create a life-form with the same create a life-form with the same number of chromosomes as the number of chromosomes as the parent.parent.

Page 8: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.
Page 9: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

Cells that possess Cells that possess the full 23 pairs of the full 23 pairs of chromosomes are chromosomes are called called diploid cells cells

They have They have chromosome sets chromosome sets from both mother from both mother and fatherand father

Diploid cells are also Diploid cells are also referred to as referred to as 2n2n because they because they contain two sets of contain two sets of chromosomes chromosomes

Page 10: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

Sperm cells and egg cells, the sex Sperm cells and egg cells, the sex cells or gametes, are cells or gametes, are nn or or haploid cells because they only cells because they only contain one set of chromosomescontain one set of chromosomes

A sex cell is set to join with another A sex cell is set to join with another haploid sex cell to produce a 2n or haploid sex cell to produce a 2n or diploid celldiploid cell

Page 11: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.
Page 12: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

In SummaryIn SummaryMitosis takes a diploid (2n) cell Mitosis takes a diploid (2n) cell

and produces two identical and produces two identical diploid (2n) daughter cells. diploid (2n) daughter cells.

Meiosis takes a diploid (2n) cell Meiosis takes a diploid (2n) cell and produces four haploid (n) and produces four haploid (n) sex cells.sex cells.

Page 13: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.
Page 14: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

Meiosis is quite similar to Meiosis is quite similar to mitosis in that they are both mitosis in that they are both divided into the same phases. divided into the same phases.

The first phase of meiosis I, like The first phase of meiosis I, like mitosis, is always preceded by mitosis, is always preceded by an interphase. an interphase.

During this interphase, both During this interphase, both chromosomes of each pair (1 chromosomes of each pair (1 through 23) make an exact through 23) make an exact copy of themselves.copy of themselves.

Page 15: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

Like the interphase that occurred Like the interphase that occurred before mitosis, the one in meiosis is before mitosis, the one in meiosis is also not visible to the eye. also not visible to the eye.

If we could see something, it would If we could see something, it would possibly resemble the pictures possibly resemble the pictures below:below:

Page 16: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

Each chromosome is now Each chromosome is now double-stranded, and each double-stranded, and each chromosome consists of a chromosome consists of a pair of pair of sister chromatids

The two homologous The two homologous chromosomes now consist chromosomes now consist of four chromatids, an of four chromatids, an identical pair from your identical pair from your mother and an identical mother and an identical pair from your father. We pair from your father. We are now ready for meiosis are now ready for meiosis to begin! to begin!

Page 17: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

Prophase 1Prophase 1 The meiotic prophase The meiotic prophase

I begins much like the I begins much like the prophase of mitosis prophase of mitosis

centrioles migrate to centrioles migrate to the poles of the cell the poles of the cell

spindle fibres become spindle fibres become visible visible

the nuclear the nuclear membrane breaks membrane breaks down down

Page 18: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

But… there is a big But… there is a big differencedifference

The two homologous chromosomes The two homologous chromosomes move together and pair up. move together and pair up.

This is known as This is known as synapsis . . The pairs intertwine their sister The pairs intertwine their sister

chromatids and become one chromatids and become one tangled unit known as a tangled unit known as a tetrad . .

Page 19: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

These intertwined chromatids swap These intertwined chromatids swap genetic information. genetic information.

It is at this point in prophase I that each It is at this point in prophase I that each chromatid is no longer either from your chromatid is no longer either from your mother or from your father.mother or from your father.

Each chromatid, after exchanging with its homologue, is a blend of information from both your parents. The actual exchange of genetic information is referred to as crossing-over.

Page 20: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

These blended These blended homologous pairs are homologous pairs are now uniquely yours. now uniquely yours. When you have When you have children of your own, children of your own, you will not simply you will not simply pass along a pass along a chromosome from chromosome from either your mother's either your mother's side or your father's side or your father's side of the family. The side of the family. The genetic information genetic information you pass along will be you pass along will be a unique blend from a unique blend from both sides. both sides.

Page 21: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.
Page 22: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

The diagram below illustrates two The diagram below illustrates two crossovers. In reality, each pair of crossovers. In reality, each pair of homologues crossover in at least homologues crossover in at least two or three different locations.two or three different locations.

Page 23: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

Metaphase 1Metaphase 1 The chromosomes line The chromosomes line

up along the equatorial up along the equatorial plane. plane.

They are still in They are still in homologous pairs and homologous pairs and may well still be may well still be intertwined into a tetrad. intertwined into a tetrad.

The spindle fibres are The spindle fibres are attached to the attached to the kinetochores of each kinetochores of each homologous pair. homologous pair.

Page 24: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

This is quite This is quite different from different from what happened what happened in the same in the same phase of mitosis. phase of mitosis. In mitosis In mitosis metaphase, the metaphase, the spindle fibres spindle fibres attached to each attached to each individual sister individual sister chromatid.chromatid.

Page 25: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

Anaphase 1Anaphase 1 Anaphase I is about Anaphase I is about

the same as the same as anaphase in anaphase in mitosis. However, mitosis. However, the sister the sister chromatids remain chromatids remain attached together. attached together.

Page 26: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

The homologous The homologous pairs are separated pairs are separated from one another. from one another. Each homologue is Each homologue is drawn to an drawn to an opposite pole using opposite pole using the spindle fibres the spindle fibres as guide wires.as guide wires.

Each pole will Each pole will receive one of the receive one of the newly jumbled newly jumbled (crossed-over) (crossed-over) homologues. homologues.

Page 27: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

Telophase 1 & Telophase 1 & CytokinesisCytokinesis

Each pole has now received one of Each pole has now received one of the homologues. the homologues.

The number of chromosomes has The number of chromosomes has been reduced to half, or the been reduced to half, or the haploid (n) number. haploid (n) number.

Each homologue is double-Each homologue is double-stranded. The cells have gone from stranded. The cells have gone from 2n2n to to nn, from diploid to haploid., from diploid to haploid.

Page 28: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.
Page 29: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

In animal cells, a cleavage furrow In animal cells, a cleavage furrow will now form. In plant species, an will now form. In plant species, an endplate grows between the two endplate grows between the two new nuclei.new nuclei.

Page 30: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

Meiosis IIMeiosis II Meiosis I ended with two new cells Meiosis I ended with two new cells

containing half the chromosome number containing half the chromosome number of the original. of the original.

Each of those chromosomes is double-Each of those chromosomes is double-stranded. stranded.

In meiosis II the sister chromatids will be In meiosis II the sister chromatids will be separated. separated.

The resulting cells at the end of this The resulting cells at the end of this phase will also be haploid, however, all phase will also be haploid, however, all the chromosomes will be single-stranded the chromosomes will be single-stranded and ready for fertilization. and ready for fertilization.

Page 31: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

Interphase IIInterphase II There is no need for an interphase There is no need for an interphase

between meiosis I and meiosis II. The between meiosis I and meiosis II. The chromosome strands are already doubled chromosome strands are already doubled so there is no need to make another copy. so there is no need to make another copy. Some organisms, however, will go through Some organisms, however, will go through a stage that can only be called interphase a stage that can only be called interphase II. The chromosomes unwind, the nuclear II. The chromosomes unwind, the nuclear membrane forms, and the cell produces membrane forms, and the cell produces proteins just as in any other interphase. proteins just as in any other interphase. However, this is the exception as most However, this is the exception as most cells go from telophase I and cytokinesis cells go from telophase I and cytokinesis directly into prophase II. directly into prophase II.

Page 32: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

Prophase IIProphase II There is no synapsis, There is no synapsis,

and there are no and there are no tetrads, nor crossing-tetrads, nor crossing-over in prophase II. over in prophase II.

There is simply a There is simply a forming of a set of forming of a set of spindle fibres. It is spindle fibres. It is interesting to note that interesting to note that the spindle fibres in this the spindle fibres in this phase is set up at 90 phase is set up at 90 degrees to where they degrees to where they did during prophase I. did during prophase I.

Page 33: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

Metaphase IIMetaphase II The chromosomes The chromosomes

are aligned along are aligned along the equatorial the equatorial plane, just as in plane, just as in mitosis and meiosis mitosis and meiosis I. I.

Here, the spindle Here, the spindle fibres are attached fibres are attached to each kinetochore to each kinetochore of each chromatid.of each chromatid.

Page 34: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

Anaphase IIAnaphase II The centromere The centromere

region between region between the sister the sister chromatids now chromatids now widens and widens and each chromatid each chromatid is drawn is drawn towards an towards an opposite pole opposite pole of the cell.of the cell.

Page 35: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

Telophase II & CytokinesisTelophase II & Cytokinesis Nuclear Nuclear

membranes form membranes form around the around the haploid, single-haploid, single-stranded nuclei. stranded nuclei. There are now four There are now four cells that were cells that were derived from the derived from the original single cell.original single cell.

Page 36: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.
Page 37: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.
Page 38: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

NondisjunctionNondisjunction We know a little about what happens We know a little about what happens

when mitosis goes wrong. Does meiosis when mitosis goes wrong. Does meiosis take a wrong turn sometimes too? Yes, take a wrong turn sometimes too? Yes, it does. it does.

Imagine what might happen if two Imagine what might happen if two sister chromatids got "stuck" together sister chromatids got "stuck" together and did not separate during anaphase I. and did not separate during anaphase I.

Page 39: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

The spindle fibers The spindle fibers would eventually break would eventually break under the strain and under the strain and both of the homologues both of the homologues would be pulled would be pulled towards the same pole towards the same pole of the same "new" cell. of the same "new" cell.

There would be an There would be an extra chromosome in extra chromosome in one of the cells and one one of the cells and one less chromosome in the less chromosome in the other cell. other cell.

This occurrence is This occurrence is known as known as nondisjunction.nondisjunction.

Page 40: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

Nondisjunction can occur with any Nondisjunction can occur with any chromosome. chromosome.

The first 22 pairs of human The first 22 pairs of human chromosomes are called autosomes. chromosomes are called autosomes.

The 23rd pair of chromosomes is called The 23rd pair of chromosomes is called the sex chromosomes, since they the sex chromosomes, since they determine the gender of the person. determine the gender of the person.

Women possess 2 Women possess 2 XX chromosomes for chromosomes for their 23rd pair, while men have an their 23rd pair, while men have an XX chromosome and a chromosome and a YY chromosome. chromosome.

Nondisjunction can occur with Nondisjunction can occur with autosomes as well as with the sex autosomes as well as with the sex chromosomes. chromosomes.

Page 41: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

Edward’s Edward’s SyndromeSyndrome

One extra 18th chromosomesOne extra 18th chromosomes Severe birth defects with a life Severe birth defects with a life

expectancy less than three expectancy less than three months months

Page 42: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

Down SyndromeDown SyndromeOne extra 21One extra 21stst

chromosomechromosome Individuals have Individuals have

rounded faces with rounded faces with well spaced eyes and well spaced eyes and an enlarged tongue. an enlarged tongue. Often these Often these individuals have individuals have lower academic lower academic abilities. abilities.

Page 43: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

Turner’s Turner’s SyndromeSyndrome

One missing 23One missing 23rdrd chromosomechromosome

These women have These women have only one X only one X chromosome for chromosome for their sex their sex chromosomes. They chromosomes. They are often small in are often small in size and do not fully size and do not fully develop sexually. develop sexually.

Page 44: Biology 122 Meiosis. # of Chromosomes Humans – 46 Humans – 46 Fruit Flies – 4 Fruit Flies – 4 Pigeons – 80 Pigeons – 80 Cats – 38 Cats – 38 Elephants.

Patau SyndromePatau Syndrome

One extra 13One extra 13thth chromosomechromosome

Severe defects, very Severe defects, very short life expectancy. short life expectancy.


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