Blueprint of Life ANSWERS

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Copyright © 2008 McGraw-Hill Australia. Permission is granted to reproduce for classroom use.

BLUEPRINT OF LIFE

CHAPTER

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Answers to end of chapter revision questions

1. Distinguish between the terms selective ‘pressure’ and ‘competition’.Answer: Selective pressure refers to changes in the environment which impact onthe evolution of organisms, whereas competition refers to the organisms’ vyingagainst one another for limited resources, with the successful organism surviving topass on their DNA to offspring.

2. OutlineOutline the role of heredity and variation in the process of evolution by naturalselection.

Answer: Heredity is the passing on of genetic characteristics from one generationto the next; this means that species with characteristics favourable for survival areable to pass on these traits to their offspring and subsequent generations.

Variation in a population leads to organisms within a species having different

characteristics. If this variation has a genetic basis, it will be passed on to thenext generation during reproduction (heredity). Variation therefore leads to theevolution (by natural selection) of organisms that are best equipped to survive intheir respective environments.

3. DescribeDescribe one example of a transitional form and explain how it supports thetheory of evolution.

Answer: The Archaeopteryx shows features common to birds, such as a featheredtail and wings and a ‘wishbone’ extending to a keel bone for attachment to flightmuscles. It also had reptilian features such as teeth, a bony tail and claws onthree digits of its forelimbs. The presence of both bird and reptile characteristicssuggests that the Archaeopteryx is a transitional form between reptiles and birds.

This supports the theory of evolution by natural selection as it provides links in thefossil record between reptiles and birds, backing the idea that living things arosefrom a common ancestor and that living things change over time.

4. CompareCompare convergent and divergent evolution, using one example of each.Answer: Convergent and divergent evolution are both indicative of living thingschanging over time as a result of natural selection favouring those variations thatbest suit the organism to its habitat.

Convergent evolution occurs when distantly related species show similarcharacteristics, for example the hedgehog and echidna. This has occurred due totheir occupying a similar niche in their environment and therefore evolving similartraits to suit that niche.

‘ ’ ‘ ’

Evolution

Please note that the following answers are sample answers only. There may be many alternative answers to the same question that are also correct. These are examples of correct answers.


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Divergent evolution refers to animals which have basic similarities due totheir recent evolutionary divergence from a common ancestor. For example, theostrich, emu and kiwi are all thought to have evolved from a common ancestor onGondwanaland, evolving different characteristics once the continents split whichenabled them to survive in their different habitats. Divergent evolution is evolvingand becoming different as a result of adaptive radiation.

5. DistinguishDistinguish between the terms ‘analogous’ and ‘homologous’ in relation tolimb structure.

Answer: Homologous structures are similar in anatomy and development, eventhough they may have different functions. For example, the wing of a bat, theburrowing foot of a mole and the fin of a manatee may look very different fromone another at first glance, but studies of structural anatomy reveal that they shareunderlying similarities—each has five digits connected by hand bones to small

wrist bones, which in turn connect to two long bones that meet at the elbow witha single long upper arm bone. Homologous structures suggest that these organismsshared a common ancestor and any differences are due to divergent evolution bynatural selection resulting from pressures imposed by the environment.

Analogous structures carry out a similar function, but have a differentstructural basis, for example the wings of a bat and those of a grasshopper.

Analogous structures suggest that organisms are distantly related and have evolvedto become similar (convergent evolution) as a result of selective pressures insimilar environments.

6. DiscussDiscuss the advantages and limitations of using biochemical technology todetermine evolutionary relatedness between living organisms.

Answer: Advantages of biochemical technology evidence:■ It allows comparisons of organisms where homologous structures are not

available, for example organisms that are very distantly related (such as aeucalypt and a human) or organisms whose complete body parts are notavailable for comparison. This comparison of evolutionary relatedness was notpossible in the past.

■ It allows the relatedness of organisms to be quantitatively determined— similarities and differences in biochemical make up (such as amino acids inproteins or nucleotides in DNA) can actually be measured. Results are objectiveand more accurate than results that were based on observation (such ascomparative anatomy).

Limitations of biochemical evidence:■ Some changes in DNA/amino acid sequences may not be identified if a

particular change that occurred in the past has reverted back to its original formin a more recent organism, which would lead to false evidence.

■ Sometimes biochemical evidence gives conflicting results.


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7. CompareCompare the concept of punctuated equilibrium in evolution with Darwinianevolution. Present your answer in the form of a table.

Answer:

8. Chimpanzees are more closely related to humans than to orangutans. Explainhow advances in biochemical technology have changed scientific thinking aboutthe relationships between the organisms described above and use evidence to justify the statement.

Answer: As a result of increased understanding of the structure of proteins

and DNA , new technologies have been developed which allow us to objectivelymeasure similarities and differences between biochemical components of livingorganisms, such as proteins or DNA genetic material, in order to determine theirevolutionary relatedness. The quantitative results obtained make it possible toreconstruct the evolutionary history of organisms, both living and extinct.

Recent advances in technology, such as DNA sequencing, DNA hybridisationstudies and amino acid sequencing in proteins, have allowed comparison oforganisms on a molecular basis rather than simply comparing structures.

In the past (around the late 19th century), studies based on structural anatomy(such as studies of the hind limb, ‘knuckle walking’ and enamel on teeth) resultedin orangutans, gorillas and chimpanzees being classified in one family (Pongidae )

and humans being classified in a separate family (Hominidae ).More recently (within the past 40 years), the advanced technology ofamino acid sequencing has been adopted and has shown that proteins suchas cytochrome-c and haemoglobin reveal identical amino acid sequences inchimpanzees and humans, but one amino acid difference between these speciesand gorillas.

With further progress in the understanding of molecular biology, even newertechnologies such as DNA sequencing and DNA hybridisation have confirmedthe results of the amino acid sequencing: chimpanzees show a 97.6% similarity tohuman DNA whereas gorillas show a 96.5% similarity to human DNA.

Therefore advances in technology could be said to have changed the scientificthinking that gorillas and chimpanzees were more closely related to each other,

Darwinian evolution Punctuated equilibrium

Similarities Living things change over long periods of time by a process where selective pressure in

the environment leads to differential survival and reproduction.

Differences Living things evolved gradually over

a long period of time by natural

selection.

Living things evolved by short bursts of rapid

change, punctuated by long periods of

equilibrium.

Proposal Put forward in 1858 by Charles

Darwin and Alfred Wallace, based on

observations of living evidence and

limited fossil evidence.

Put forward in the 1970s by Stephen Jay

Gould and Niles Eldridge, based on fossil

evidence.

Fossil evidence If evolutionary change is gradual, it

could be predicted that there would

be fossilised remains showing theseongoing changes. Darwinists use

transitional forms to support their

perspective of ‘gradualism’.

Many fossilised remains show millions

of years going by without any noticeable

evolutionary change to most species. (Theyargue that if evolution occurs gradually,

as proposed by Darwinists, there should

be a much greater diversity among living

organisms than actually exists.)


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to the currently accepted thinking that chimpanzees and humans are more closelyrelated, based on the assumption that more closely related species share morecommon sequences than do unrelated species.

9. Critically evaluateCritically evaluate the impact of Lamarck’s and Wallace’s work on that of CharlesDarwin.

Answer: Jean Baptiste Lamarck, born in France, paved the way for evolutionarythinking to take its place in society when he proposed a mechanism forevolution—his theory of evolution by ‘the inheritance of acquired characteristics’.

At the time, society was experiencing an industrial and political revolution whichencouraged freedom of thought. The comfortable thinking at the time was thatspecies were created independently and did not change over time and his ideaschallenged this thinking. The mechanism that he proposed opened discussion andquestioning, leading naturalists at the time to question and investigate whether itcould be accepted or discredited. Although his theory was eventually discreditedon the basis that acquired characteristics cannot be inherited, Lamarck’s proposalencouraged people to consider and debate other points of view, opening the wayfor the proposal of further new ideas such as the theory of evolution by naturalselection , proposed by Charles Darwin and Alfred Wallace in the late 19th century.

Although Darwin came up with his idea of evolution by natural selection in1844, he mulled over his ideas in secret and discussed it with only a few trustedcolleagues, despite having found much evidence in support of his ideas. Darwin

was aware that his ideas were progressive and dangerous—a public announcementof a mechanism in support of the scientific thinking that living organisms evolve(as opposed to the idea that each is created individually) would cause an outcry,especially amongst the political and religious sectors of society. His findings ledhim to believe the even more contentious idea that humans were probably also

the result of evolution and this caused Darwin to further delay sharing his ideas with the scientific community in a public forum.

A letter from Alfred Wallace proved to be a catalyst in Darwin making hisfindings public. In 1858, while suffering from malaria on one of his trips toIndonesia, Alfred Wallace wrote a letter to Charles Darwin in which he outlinedhis own theory of evolution by natural selection. Wallace noted that the north-

western Indonesian islands had bird species more similar to those of the closer Asian mainland, whereas in the south-east, the birds were more similar to thosein nearby Australia, which led him to the conclusion that island forms may haveevolved from mainland forms which became isolated.

This theory was remarkably similar to that which Darwin had formulated

independently, 12 years previously and never published. Wallace’s letter was thetrigger that prompted Darwin to put forward his ideas of evolution by naturalselection to the scientific community. With the encouragement of colleagues,Darwin and Wallace’s papers were presented jointly at a seminar in London.Darwin followed this up by completing his book On the Origin of Species ,published in 1859, which detailed the evidence of his findings and the formulationof his theory of evolution by natural selection.

Therefore it seems that, without Lamarck’s ideas that challenged the religiousand social order of the times and the arrival of Wallace’s letter, it is possible thatDarwin would not have considered putting forward his theory of evolution bynatural selection due to his awareness of the social and political upheaval it wouldcause. These events led to Darwin finally laying out his argument in a scientific


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journal and completing his book, to allow the world to judge the validity of histheory. The end result has been the widespread (but still not unanimous) currentacceptance of the theory of evolution by natural selection, proposed by CharlesDarwin and Alfred Wallace in the late 19th century.

10. Analyse Analyse the ways in which theories in biology are tested and validated, using thetheory of evolution as an example.

Answer: A theory is a scientist’s explanation of a principle. Since scientificexplanations are provisional and scientific views at any time depend on theevidence available to support these views, theories may change—we say thereforethat science is ‘tentative’. Biology is a natural science and so its explanations ofnatural phenomena are based on evidence and confirmable data . Scientists lookfor evidence that shows ‘cause and effect’ and they base their inferences on factualinformation that can be observed and/or measured when formulating theories.

A number of predictions are made and then tested, either by experiment or bylooking for irrefutable evidence to support or oppose them. If a significant amountof evidence is gathered that supports the hypotheses being tested (and no evidencearises to the contrary), a theory is put forward that is acceptable to a number ofscientists at the time. As technology advances and understanding increases, newevidence that becomes available may further support a view or it may invalidatethat view, leading to the development of a new theory.

The Darwin–Wallace theory of evolution by natural selection is supported bya large amount of evidence, gathered over more than a century. Since macro-evolution takes place over millions of years, it is impossible to directly test itby experimentation within a lifetime or even over many generations. Thereforeevidence must be gathered to support the theory of evolution—the theorycannot be proved . To validate this theory, scientists have made predictions and

then tested them—so far, their predictions have held true and so the theory isconsidered ‘valid’. There are many strands of evidence which can be appliedto evolution. These include palaeontology (fossil studies), biogeography ( thedistribution of living organisms across the continents), comparative anatomy (similarities and differences in structure), comparative embryology (similarities inearly development) and biochemistry (determining sequences of chemicals such asamino acids in proteins or base pairing in DNA then comparing them in organismsthat may share an evolutionary relationship).

When it was first proposed, difficulties arose in trying to fully explain thetheory of evolution by natural selection because there was no knowledge at thatpoint in time of how these characteristics could be inherited or even what caused

these differences or variations in living organisms. The theory of evolution bynatural selection was proposed before there was any knowledge of genes or anexplanation for how inheritance could come about.

In order to look at the historical development of a theory, we need to examinethe past ideas about the principle or concept compared with the currently acceptedideas . Biological explanations are provisional and biological views at any timedepend on the evidence available to support these views. As technology advancesand understanding increases, new evidence that is gathered may further supporta view or it may refute that view, leading to the development of new biologicalthinking and, at times, new theories.

Mendel’s laws which were rediscovered at the turn of the 20th century haveprovided a greater understanding of the mechanisms of inheritance. These laws


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and the discovery of the chromosome theory of inheritance have led to thedevelopment of the field of genetic studies which has made rapid progress overthe past century. Knowledge and understanding of genes can be used to explainhow random variation arises and therefore provides further evidence for evolution.

CHAPTER

21. Identify and describe Mendel’s hypotheses that were tested (dominance andsegregation) and/or any supporting evidence that was collected.

Answer:

2. State three possible reasons why Mendel’s experimental results were ignored.Answer:

1. He presented his papers to a very small group of scientists that were not veryeminent, in a small town in Moravia called Brünn.

2. His work differed from previous research, appeared to be based on verylittle prior knowledge and the scientists to whom he presented may not haveunderstood is work. Mendel’s use of mathematics and statistics to analyseresults and make predictions in biology was also different.

3. He had no established reputation or recognition in the broader scientific worldbecause he had no prior significant research and no interaction with other

well-known scientists. As a result, his standing as a ‘scientist’ may have beendoubted.

3. Science has been described as empirical (based on experimentation) yet also creative. Assess Assess whether this holds true for the establishment of Mendel’s laws.

Answer: Mendel’s laws were based on experimentation and he employed creativityin his problem-solving techniques, resulting in valid scientific experiments on

which he based his laws.

’

Genetics

2. State t ree possible reasons w y Mendel’s experimental results were ignored.

Hypothesis Supporting evidence collected

Pure-breeding organisms carry the same two

factors for a particular trait.

The offspring of pure breeding lines all resemble

their parents.

Hybrids (offspring of two parents that are pure

breeding for contrasting characteristics) all

resemble only one parent, but carry factors for a

trait from both parents. The characteristic of the

parent that they resemble is dominant and the

other characteristic is masked or recessive.

Dominance: when parents that are pure

breeding for contrasting characteristics (such

as tallness and shortness) are crossed, the

offspring all resemble one parent (in this

case the tall parent). There is not blending of

characteristics.

These factors pass as unmodifi ed ‘units’ to

successive generations according to set ratios

(that is they segregate during gamete formation

and recombine during fertilisation).

Segregation: when two hybrid plants are

crossed, one characteristic (dominant) appears

three times as frequently in the offspring as the

other (recessive) characteristic. That is, the

average ratio observed in offspring is 3:1. This

can be mathematically explained only if there

are two factors responsible for each trait and they separate or segregate and each gamete

receives only one factor for the trait.


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Science is empirical and this criterion is evident in Mendel’s experiments,for example he counted plants to obtain quantitative results and then appliedmathematical formulae to establish his laws objectively. Mendel’s experimentsdisplay the criterion of creativity, particularly in his problem-solving approach.For example, he removed stamens from plants to prevent self-pollination andensure that only cross-pollination occurred and this creative thinking improved his experimental technique.

Mendel’s careful analysis of results, combined with his creative thinking,enabled him to link his observations to the process of sexual reproduction, and byapplying his understanding, he consequently established the Mendelian laws ofinheritance which still hold true today.

4. ExplainExplain how advances in scientific understanding brought about by Mendel’sdiscoveries provide evidence for the Darwin–Wallace theory of evolution bynatural selection (PFA H1-based question.)

Answer: At the time that Darwin and Wallace proposed their theory of evolution bynatural selection, Darwin recognised that there was no known way of explaininghow variation could be passed on from parent to offspring. Mendel’s experimentsshowed that:■ each inherited trait in an individual is determined by a pair of ‘factors’ which

pass as unmodified units’ (individual Mendelian factors today are called ‘genes’)to successive generations according to set ratios

■ these factors are passed on in gametes—characteristics are carried from parentsto offspring by gametes (sex cells) and when gametes combine, they may bringtogether a different combination of alleles for the same gene, for example thegene for stem length in pea plants may differ in each parent. The segregationand later fusion in fertilisation further increase the variation that can be

inherited in individuals, an essential ingredient in the process of evolution.■ some factors may be masked (recessive), whereas others are expressed

(dominant), accounting for the variations which appear within individuals in apopulation. Darwin described the ‘random variation’ on which natural selectionacted, but he could not explain how this variation arose within a population orhow it was passed from one generation to the next.

By advancing the understanding of the hereditary nature of variation, Mendel’sfindings (when later rediscovered) provided an explanation of how naturalselection could lead to changes within a population, providing further evidence forthe Darwin–Wallace theory of evolution by natural selection.

5. Look at the following pairs of alleles and identify which would have a similar

appearance if the alleles presented by the capital letters are dominant over thealleles presented by the small letters:

AA Aa aa Bb BB bbAnswer: AA and Aa; BB and Bb


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6. If you were carrying out breeding experiments with a group of organisms thatare heterozygous for a particular gene that has one dominant and one recessiveallele:

(a) How many different phenotypes of offspring would there be? IdentifyIdentify themand give their expected ratios.

(b) How many different genotypes of offspring would there be? IdentifyIdentify themand give their expected ratios.

Answer: (a) There would be two phenotypes: individuals displaying the dominant

phenotype and individuals displaying the recessive phenotype (no ‘blending’of phenotypes).

Phenotypic ratio: three dominant to one recessive.(b) There would be three genotypes: one homozygous dominant: two

heterozygous dominant: one homozygous recessive. Phenotypic ratio: three dominant to one recessive.

7. A homozygous pea plant with purple flowers is crossed with a pea plant with white flowers.(a) Which colour is dominant? Justify Justify your answer.

(b) Use a Punnett square to show the possible genotypes that would result fromthis cross.

(c) Calculate the ratios of phenotypes and genotypes of the offspring.Answer: (a) Purple. The heterozygous individual displays the dominant phenotype—in this

instance, purple flowers.(b) Let P = purple and p = white flowers. Parents: Pp × pp

Gametes: Pp × pp

P p

p Pp pp

p Pp pp

(c) Phenotype ratio: one purple to one white flower Genotype ratio: one homozygous purple to one heterozygous purple to two

white flowers

8. ExplainExplain why each of the following statements is false: (a) Offspring resulting from self-fertilisation are genetically identical. (b) In a monohybrid cross Bb × Bb, there is a 25% chance of a child being bb. If

the first child is bb, there is less of a chance that the second child will be bb.Answer: (a) During sexual reproduction, cells undergo meiosis to produce gametes.

Meiosis introduces genetic variation—by crossing over and by independentassortment and random segregation . The result is that each gamete carriesa different mixture of paternal and maternal genes. Furthermore, the genotypesof the offspring are determined at the time of fertilisation by which male andfemale gametes fuse. All of these stages in sexual reproduction (those words


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listed in bold print) introduce genetic variety by mixing the genes present inthe parent individual. (Genetically identical offspring only result from asexualreproduction such as propagation.)

(b) The order in which the offspring are born is not relevant—the birth of oneoffspring does not affect the birth of the next, however if enough offspringare born, then the above ratios may be attained. Every time meiosis occursand gametes are formed, there is an equal chance of the genes segregatingin a particular way and recombining during fertilisation. Just as each time

you spin a coin, you have a 50% chance of getting heads—any one spin isnot influenced by the spin before it—and so each gamete formation andfertilisation process in living things is independent of the previous one.

9. Draw the standard symbols used in a pedigree to represent: (a) parents (both unaffected)

(b) twins, one unaffected male and one affected female (c) the Roman numeral indicating the generation of grandchildren.Answer:

(a)

father mother

(b)

unaffected female male(c) III

10. IdentifyIdentify three uses of pedigrees.Answer:

■ Animal pedigrees: —select suitable individuals for stud breeding by identifying any desirable traits —predict how closely related the parent animals are —verify the thoroughbred status of animals by breeding societies.

■ Human pedigrees: —determine if a particular family trait is genetically inherited

—trace the occurrence of a genetic disorder, abnormality or disease within afamily over several generations

—deduce genotypes by determining the probability that prospective parents are

heterozygous for (and therefore carriers of) a particular defective allele —predict the likelihood of a family member inheriting a trait/developing a

disorder.


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CHAPTER

31. How did the work of Sutton and Boveri increase our understanding of the role ofchromosomes in inheritance? (PFA H1-based question)Answer:

■ Boveri increased our understanding by demonstrating that a full set ofchromosomes, found in the nucleus, is needed for normal development ofoffspring.

■ Sutton and Boveri independently showed that each chromosome containsnumerous hereditary factors that are passed from parent to offspring bygametes, as a result of meiosis.

■ Sutton changed previous thinking that all chromosomes were the same size andshape and also noticed the similarity between the separation of chromosomesin meiosis and Mendel’s law of segregation.

■ Both Sutton and Boveri arrived at the previously unknown conclusion thatmany of Mendel’s factors were present on each chromosome.

2. ExplainExplain, in genetic terms: (a) how alleles are responsible for the differences or variation seen in individuals

within a population (b) how such characteristics could be passed on from one generation to the

next.Answer:

(a) Genes or the environment or a combination of both genes and theenvironment are responsible for the variation in individuals within apopulation. Variation is of evolutionary advantage only if it has a genetic basis.

Genes on chromosomes determine characteristics that are inherited. Alleles aredifferent forms (variations) of the same gene and these alleles occur in pairsin individuals. Alleles may be recombined to introduce new mixes of geneticmaterial, giving variation. Mutations are a source of new variations arising (asopposed to the reshuffling of genetic material to introduce variability within apopulation).

Genetic variation in individuals (and variability in a population) arises asa result of sexual reproduction (a recombination of alleles that alreadyexist ) or as a result of mutation (new alleles arise). Sexual reproductioninvolves gamete formation (by meiosis, with crossing over, segregationand independent assortment accounting for variation by recombining genes),

followed by fertilisation (fusion of male and female gametes which furtherrecombines genes). New alleles of a gene may arise by mutation , introducingeven more variation into a population.

(b) Haploid gametes carry one copy of each allele from parents to offspring,resulting in genetic recombination, which leads to variations seen in differentindividuals in a population. Variation within a population leads to differentialreproduction (those individuals that survive reproduce and therefore passon their alleles). Beneficial mutations are maintained in a gene pool becausethey confer some selective advantage. Harmful mutations are eliminated fromthe gene pool in nature because they reduce the chances of the individual’ssurvival while neutral or slightly harmful mutations may be carried in apopulation.

Chromosomes—the key to inheritance


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The gene pool of a population contains genetic variation that has beenintroduced by mutation and maintained by recombination. If mutations can beinherited, they provide the variation on which natural selection acts, in orderfor evolution to occur.

3. ExplainExplain how meiosis introduces variation during gamete formation.Answer: Variation is introduced during sexual reproduction by:■ meiosis: crossing over—homologous chromosomes exchange genes and so the

resulting combinations of alleles on chromatids differ from those originally onthe parent chromosomes

■ independent assortment: genes on different chromosomes sort independently,giving different gene combinations in gametes (different from those of parents).

Meiosis results in a reshuffling of genetic material, introducing variation in apopulation.

4. State two genetic consequences of fertilisation.

Answer: During fertilisation the male and female gametes combine—the possibilityof many dif ferent combinations of gametes fusing brings about variation.

The genetic consequences of fertilisation are that:■ the original (diploid) chromosome number is restored■ the many possible combinations of gametes that fuse are a significant source of

variation.

5. IdentifyIdentify one source of variation within living organisms that does not have agenetic basis. Give an example of this type of variation and explain why it cannotplay a role in natural selection.

Answer: Variation between individuals in a population may be due to the influenceof environment, for example a person may increase the size of their biceps by

exercising in a gym, lifting weights to increase the size and strength of theirmuscles. The offspring of a person who works out in the gym do not inherit thatperson’s large muscles. Therefore this type of variation, which does not have agenetic basis, will not help the offspring to adapt to the environment and cannotbe acted upon by natural selection.

(Without genetic variation being passed from one generation to the next, therecan be no inherited differences in the ability to reproduce and, therefore, nonatural selection.)

6. Watson and Crick’s working style was more successful than that of Franklinand W ilkins. DiscussDiscuss what made the working style of Watson and Crick moresuccessful.

Answer: Watson and Crick took a creative and collaborative approach whichinvolved sounding ideas off each other and building a model. They used anyresearch available to them, in the best interests of solving the scientific problem

with which they were faced. They discussed ideas with each other and questionedany other colleagues who were working in similar fields, communicating broadlyand well. They did what we would call ‘brainstorming’ today—they gaveconsideration to every idea put forward, even those that sounded impossible— believing that the correct idea would be amongst those that they came up with.

The advantages of this approach were that:■ they gathered a large amount of information on DNA as a result of their broad

communication—both their own findings and that of other people


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■ they filled in all the missing gaps in their knowledge and understanding byusing chemistry textbooks and talking to people

■ their model building allowed them to visualise their proposals—they looked atthe overall picture when interpreting available research

■ their non-judgemental approach and freedom to bounce ideas off each otherenhanced their creative thinking

■ their correct scientific approach of making predictions and testing their modelled them to reject their first incorrect model and develop a later one that provedto hold true for predictions such as the mechanism for self-replication.

The disadvantages of this approach were that:■ their work was not always orderly and methodical—they had to rely on the

meticulous detail obtained from the expert crystallography skills of RosalindFranklin to fill in the missing part of their puzzle; measurements and details

which Watson wrote down and Crick recognised as giving DNA its doublehelical nature.

The team work of Watson and Crick, which they later acknowledged as havingplayed such an important part in their discovery, contrasted with the strained

working relationship between Franklin and Wilkins. Wilkins’s expertise was inisolating high-quality DNA which could be used for X-ray diffraction; Franklin’sstrengths were her X-ray diffraction skills and her logic and ability to analyse

X-ray crystallography photos, and her intense and meticulous effort in collectingdata. At the time of Watson and Crick’s discovery, Franklin and Wilkins had bothindependently come across evidence that DNA was a helix in the ‘wet’ form, butthey did not discuss their findings effectively and their communication was poor.

By processing information from other researchers, long discussions and manyhours spent manipulating models, Watson and Crick’s idea took shape and seemed

to work—they discovered the detailed structure of the now famous DNA doublehelix, as well as the discovery that bases are always paired in a particular way which provided a mechanism for DNA to self-replicate.

The successful working style of Watson and Crick was therefore a majorcontributing factor that led to them being awarded the Nobel prize in physiologyand medicine in 1962 for their work on the discovery of DNA.

7. ExplainExplain why DNA is sometimes referred to as the ‘blueprint of life’.Answer: A blueprint is a plan of the design of a building, usually created byan architect. The plan or ‘blueprint’ determines the eventual structure (andfunctionality) of the building. DNA is the chemical (a macromolecule) found in thenucleus of each cell and it codes for the production of proteins and determines the

ultimate structure and functioning of living organisms. It is therefore referred to asthe ‘blueprint of life’ as it holds in a coded form the ‘plan’ or ‘design’ of the livingorganism in which it occurs.

8. DistinguishDistinguish between the terms ‘nucleotide’ and ‘nucleic acid’.Answer: A nucleotide is a basic subunit of nucleic acids—it is made up of a sugar,a phosphate and a base. For example, adenine, thymine, cytosine and guanine arethe types of bases that form part of the nucleotides that make up the nucleic acidDNA.

A nucleic acid is a large molecule (polymer), such as DNA or RNA, made up ofrepeating units of nucleotides.


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CHAPTER

41. Explain how the biochemical process of protein synthesis that occurs in cells is

related to macroscopic changes in the organism.Answer: The biochemical pathway of protein synthesis involves transcriptionof the coded instructions in DNA into messenger RNA, which then moves intothe cytoplasm where it is translated into protein. Genes are therefore expressedin terms of the protein products that they produce. Many of these proteins areenzymes, which control chemical functioning of cells. Other proteins that areproduced may form a structural part of the cell (e.g. the protein in cell membranes,collagen in connective tissue, silk in insect cocoons) and some proteins formessential chemicals such as hormones (e.g. insulin), defence proteins (e.g.antibodies) and transport proteins (e.g. haemoglobin). DNA directs the productionof these proteins which in turn determine the macroscopic structure of theorganism. Any change in the DNA (e.g. by mutation) will lead to a correspondingchange in the messenger RNA and this may lead to a difference in the aminoacid sequence of a polypeptide (or a difference in the length of the polypeptidechange or its three-dimensional configuration). A change in the polypeptide maybring about a variation of the usual protein and this new variation may conferstructural differences; for example it may be an enzyme which no longer functionsas it should (or at all), or it may be a pigment protein that now changes colour.Therefore we see that a microscopic change in DNA structure could lead tomacroscopic changes in the organism as a result of changes to the structure of oneor more proteins that are encoded by the DNA.

An enzyme called DNA polymerase enzyme is usually involved in checkingthe biochemical process of transcription and corrects any bases that have beenincorrectly inserted, but at times this cross-checking does not work effectively.Mistakes in DNA replication may also lead to macroscopic changes within anorganism, but these will be limited to all cells arising from that defective parentcell. However, a mutation in a germ line cell means that all or most of the cellsthat form as a result of division of this cell will duplicate the error and so all ofthem (most or all cells in the developing organism) will show the macroscopicchange.

2. DistinguishDistinguish between DNA and mRNA.Answer: DNA is a double-stranded helix, whereas RNA is single stranded.

DNA contains the nitrogenous bases thymine, adenine, cytosine and guanine, whereas RNA contains the nitrogenous base uracil in place of thymine (the otherthree bases are the same for RNA as for DNA).

The sugar that forms part of the sugar-phosphate backbone in DNA isdeoxyribose sugar, whereas in RNA it is ribose sugar. It is the sugar that gives themtheir names—DNA is deoxyribonucleic acid and RNA is ribonucleic acid.

DNA functioning—changes in DNA structure arerefl ected in phenotype


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3. Match the phrase in the first column with the correct term in the second columnin the table below.

Answer:

Phrase Term

1. DNA makes an identical copy of itself. (a) translation

2. DNA acts as a template for mRNA. (b) replication

3. mRNA triplet of bases that determines which amino acid will be

brought in.

(c) transcription

4. Carries DNA code from cytoplasm to nucleus. (d) mutation

5. Where transcription takes place. (e) mitochondrion

6. Triplet of bases on one end of tRNA. (f) ribosome

7. Material which genes are made of. (g) codon

8. Contains non-nuclear DNA. (h) anticodon

9. Change in nucleotide sequence in a gene. (i) DNA

10. mRNA is decoded and directs the sequencing of amino acids

to make a polypeptide chain.

(j) mRNA

1 2 3 4 5 6 7 8 9 10

b c g j f h i e d a


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4. Draw a flow chart to outlineoutline the process by which DNA controls the productionof polypeptides. Indicate at which points enzymes are involved and name theseenzymes.

Answer:

5. In the form of a table, comparecompare the processes of DNA replication and proteinsynthesis.

15

.

Process DNA replication Protein synthesis

When it occurs Immediately before cell division. When a cell needs to manufacture a protein

product.

Type of cell in which it

occurs

A somatic (body) cell about to undergo mitosis or

a germ cell about to undergo meiosis.

A differentiated somatic cell.

Main steps involved DNA unzips (whole molecule).

Each original strand gives rise to a complementary

strand of DNA.

DNA unzips only in that part of a chromosome

where the required gene occurs.

The gene on one strand of DNA (the non-coding

strand) gives rise to a complementary strand of

mRNA.

The mRNA moves into the cytoplasm and is

translated into a protein (with the help of tRNA).

Result All chromosomes within a cell are replicated,

forming two identical genomes.

Protein products that were encoded by the gene

are produced in the cell, for cell functioning or to

form a structural part of the cell.

transcription

translation

enzyme: helicase

enzyme: RNApolymerase

DNA in nucleus unzips in region of desired gene

mRNA nucleotides pairwith nucleotides on DNA non-coding strand

cytoplasmmRNA nucleus

mRNA on ribosomes

tRNA aligns according to

mRNA base sequence

amino acids joined in sequence

polypeptide


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6. (a) De neDefine a mutagen and give examples.(b) DistinguishDistinguish between spontaneous and induced mutations and give an

example of each.Answer:

(a) A mutagen is any environmental factor that induces mutations in DNA that isexposed to the factor. For example ionising radiation such as X-rays, gammarays and ultraviolet light; certain chemicals such as benzene, tar in tobaccosmoke, nitrites and biological agents such as certain viruses, for example thehepatitis B virus.

(b) A spontaneous mutation is a mistake that arises randomly during DNAreplication—it occurs approximately once in every billion base pairs replicatedand are also known as ‘random errors’ in cell division.

An induced mutation is a mistake that arises during DNA replication as aresult of the influence of an environmental factor such as ionising radiation orcertain chemicals. The environmental factor causes a greatly increased rate of

mutation compared with the rate of spontaneous mutation.

7. CompareCompare gene mutations with chromosome mutations, giving an example ofeach.

Answer: A gene mutation involves a change in the nucleotide bases within agene—this could be as a result of the rearrangement of the bases or a deletion,insertion or duplication of one or more base pairs. For example, colour blindnessand cystic fibrosis in humans.

A chromosomal mutation involves a change in many genes—that is, arearrangement, deletion, insertion or duplication of many genes (a whole portion)of a chromosome.

Changes in chromosome number could be considered chromosomal mutations,

since all the genes on one chromosome may be deleted or duplicated, for exampleDown’s syndrome, which features an additional copy of chromosome 21.

8. The following scientists all made major scientific breakthroughs that are acceptedin the fields of genetics and evolution today:(a) Darwin and Wallace

(b) Mendel (c) Beadle and Tatum. (i) For each scientist or pair of scientists listed above, identifyidentify the scientific

hypothesis, theory or law that they proposed. (ii) For each scientist or pair of scientists, analyseanalyse and outlineoutline the way in

which they tested and validated their hypothesis, theory or law.

Answer:

Scientific hypothesis,

theory or law that was

proposed

The way in which the hypothesis, theory or law was

tested and validated

Darwin and

Wallace

The theory of evolution by

natural selection

Original theory based on observations of numerous

organisms including evolution on islands of birds.

Predictions held true based on various strands

of evidence such as palaeontology, biogeography,

comparative anatomy, comparative embryology and

biochemical evidence.

continued . . .


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Scientific hypothesis,

theory or law that was

proposed

The way in which the hypothesis, theory or law was

tested and validated

Mendel Genetics—the lawof dominance and

independent assortment

Mendel made and tested numerous predictions usingseven characteristics in garden peas; his experiments

were rediscovered and tested (repeated successfully)

35 years later (in the early 1900s) and subsequent to

that and have held true.

Beadle and

Tatum

‘One gene—one

polypeptide’ hypothesis

Beadle and Tatum’s original hypothesis, the ‘one gene—

one protein’ hypothesis was tested against predictions

and did not hold true for all cases—while the basis to

their theory held true, it changed in detail as geneticists

came to a greater understanding of the structure and

functioning of genes and proteins. It had to be amended

with the discovery that some proteins consist of more

than one polypeptide chain and one gene may code

for one polypeptide chain—only part of a protein. The

hypothesis seems to be holding true, but may change

again in the light of recent discoveries in molecular

genetics.

CHAPTER

51. Define selective breeding, artificial pollination, artificial insemination andhybridisation.

Answer: Selective breeding can be thought of as a form of artificial selectionimposed by humans when they conduct deliberate crosses in living organisms toobtain a combination of desirable characteristics in the offspring. Genetically, itcan be described as a reproductive technique applied to manipulate the phenotypeof offspring. An example of selective breeding is crossing a Fresian bull (a variety

where the females have a high milk yield) with a Jersey cow (a breed known forthe ability to produce creamy milk) in an attempt to breed offspring that producelarge amounts of creamy milk.

Artificial pollination is a selective breeding technique used in plants wherepollen is taken from a plant with one desired characteristic (e.g. a plant variety thatis disease resistant) and brushed onto the female parts (stigma) of another plant

with another desired characteristic (e.g. a long-flowering season).

Artificial insemination is a technique whereby semen containing sperm isintroduced with a syringe into the reproductive tract of a female. It is ‘artificial’because it involves human intervention and it is used as a modern selectivebreeding technique to produce offspring with favourable characteristics (it issimilar to selective breeding, except that only the sperm is transported as opposedto the entire male animal).

Hybridisation involves cross-breeding two genetically different individuals.If the organisms that are cross bred are from the same species (but different

varieties), this is termed intraspecific hybridisation, but if they belong to differentspecies, it is interspecific hybridisation. Hybridisation occurs both in nature andartificially—it occurs as a result of human manipulation of breeding to try to

improve the quality of plants and animals, a phenomenon known as hybrid vigour.

Applications and implications of genetics


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2. DistinguishDistinguish between gene cloning and whole organism cloning.Answer: Gene cloning is a technique that results in the production of multipleidentical copies of a gene. It is part of the process of genetic engineering and takesplace at a cellular level.

Whole organism cloning is an asexual reproductive technology that uses theDNA (or nucleus) of a mature organism (the parent organism) to produce agenetically identical, fully developed identical organism. The clone is a geneticallyidentical ‘twin’ of the original organism, for example the cloning of seedless

watermelons and bananas.

3. Each time a mammal is cloned, the process of somatic cell nuclear transfer(SCNT) involves three animals. IdentifyIdentify and describedescribe the role played by each ofthese animals during SCNT.

Answer:

1. The mature animal to be cloned (genome donor): this animal provides anucleus which contains the DNA instructions to be passed on to the new(cloned) individual.

2. The female egg donor: she provides an egg cell whose nucleus is removed— termed an enucleated egg cell. The cytoplasm of this egg cell is needed toactivate all genes in a nucleus, even those that may have been shut down in amature cell.

3. The surrogate mother: the cloned embryo is implanted into her uterus todevelop and grow; she will give birth to the embryo, but is not its biologicalmother.

4. What is a transgenic species? Give an example to illustrate your answer.Answer: A transgenic species is a species of organism that contains in its genomeone or more genes from another species, genes that have been artificiallyintroduced by genetic manipulation (genetic engineering). A species is onlyconsidered to be transgenic if it passes the inserted gene on to its offspring inreproduction. An example of a transgenic species is Bt cotton—a bacterial gene(from Bacillus thuringiensis ) has been inserted into the cotton genome to makethe cotton resistant to the caterpillar of the Helicoverpa zea moth. Another exampleis sheep that carry the human insulin gene—they produce insulin in their milk.

5. It is interesting to note that the second-cloned horse, Pieraz-Cryozootech, wascloned from a gelding (a castrated male horse). Consider the implications andadvantages of this application of cloning as a reproductive technology.

Answer: Cloning allows the breeding of castrated male animals and the mainadvantage of this is to override the problem experienced in endurance racing

whereby champions could not be bred as they had been sterilised. Theimplications are that this could decrease genetic diversity within the race horses,but also that they could be used to study the effects of environment (such as typeof training and/or nutrition), since all animals cloned from one parent would begenetically identical. The racing fraternity would need to decide on the legal issuesof racing cloned horses.

6. ExplainExplain how developing a transgenic species can introduce wide-scale resistanceto a disease.

Answer: Biologists and breeders in agriculture can insert a gene for resistanceto a particular disease into an organism that is not normally resistant, but whichgives a high crop yield. If these organisms compete successfully with the wild


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type organisms, they will survive, reproduce and pass on the gene for resistanceto their offspring. Therefore the transgenic species will become widespread andcan introduce wide-scale resistance to a particular disease. An example where atransgenic species has been successfully created to resist an insect pest is that ofBt cotton. In this case a bacterial gene (from Bacillus thuringiensis ) is insertedinto the cotton genome to make the cotton resistant to the pest of the Helicoverpazea moth. Cotton plants with this gene kill the Helicoverpa zea caterpillar becausethe gene allows them to produce a protein that is lethal to the caterpillar wheningested. To prevent resistance arising in the caterpillars, two genes have beeninserted into the new generation cotton plants as it is less likely that caterpillars

will become resistant to both genes. This has reduced the need for pesticides incotton, while introducing wide-scale resistance to the pest.

7. ExplainExplain how the genetic diversity of a species may be increased using:(a) cloning

(b) transgenic species.Answer: In both cloning and the creation of transgenic species, individuals aresubjected to artificial selection for a limited number of traits over many generationsand only a small number of parental organisms contribute to the genetic make upof the next generation.(a) In cloned individuals, the continued use of one parent (an artificial form

of asexual reproduction) or only a few parents limits the gene pool anddecreases genetic diversity, since all offspring resulting from one parent haveidentical combinations of genes. However, the cloning of animals that areextinct, endangered or with a limited lifespan can be used to increase theexisting gene pool.

(b) The technology of creating transgenic species allows scientists to artificially

select the specific traits that they want expressed and to recombine these within an individual. The potential impact on genetic diversity depends onhow well the transgenic organism with a newly combined genome competesin the wild. In the short term, creating transgenic organisms increases the genepool because it introduces a genotype that did not exist in the wild. However,if these organisms out compete wild-type organisms, the genomes of the latter

will be lost from the environment, leading to a decrease in biodiversity. Also, iftransgenic organisms are selectively inbred to maintain parent lines, this couldlead to a decrease in biodiversity.

Blueprint of Life ANSWERS

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