Molecular Evolution

Question 1

What is Natural selection?

Answer 1

The effects of a wide range of factors on the frequency of heritable changes in a species

Question 2

What is fitness?

Answer 2

How well a species is able to reproduce in its particular environment.

Question 3

State whether heritable changes within a population will be selected for or against depending on how these changes affect fitness

Answer 3

• Changes that increase fitness will be selected for within a population
• Changes that decrease fitness will be selected against within a population
• There are also neutral changes that don’t affect an organisms fitness and so aren’t selected for or against within a population

Question 4

What is genetic variation?

Answer 4

The difference in DNA sequences between individuals within a population. It is the main source of heritable changes within a species.

Question 5

What are the 4 processes that affect how frequently genetic variation occurs?

Answer 5

• Selection
• Mutation
• Migration
• Genetic Drift

Question 6

How does selection affect the frequency of genetic variation?

Answer 6

Genetic variants that confer a positive advantage will be selected for within a population. This is because that variant is more fit for its environment. This means that populations in different environments will have different genetic variants that are advantageous to them, and will therefore be selceted for, because different environments exert differerent selection pressures on the populations that inhabit them. This therefore creates genetic variation between the populations.

Question 7

Give some examples of genetic variants that may be selected for within a population?

Answer 7

• Resistance to disease
• Ability to metabolise a new food source
• Antibiotic resistance
• Change in appearance that enhances mate choice

Question 8

What is a mutation and how do they affect genetic variation?

Answer 8

A mutation is defined as a change within the structure of a gene. Muations affect genetic variation because they result in the formation of different genetic variants within a population because different people will accumulate different mutations within their genome.

Question 9

What factors determine the frequency of a particular variant within a population?

Answer 9

• How they are selected for within a population
• When they first arose

Question 10

Suggest reasons why a genetic variant may be rare within a population?

Answer 10

• A variant may have arisen very recently and so wouldn’t have had a chance to spread throughout the population.
• ​The variant might make the individuals that have it less fit for its environment so will be selected against within the population.

Question 11

Define the term “migration”

Answer 11

The physical movement of people from one place to another in order to settle in the location they have migrated to.

Question 12

How does migration affect the frequency of genetic variation?

Answer 12

Migration results in new pools of variants being introduced into an existing population - this is called admixture. This admixture results in increased variation between individuals within a population

Question 13

Define the term “genetic drift”

Answer 13

How the frequency of a variant changes in a population due to chance

Question 14

How does genetic drift affect the frequency of genetic variation?

Answer 14

Not all organisms in a population will pass on their genetic variants to the next generation. Even when we do pass on our genetic variants mechanisms such as genetic recombination will also result in not all variants being passed on.

Question 15

Does every single gene/DNA sequence within an organism show genetic variation?

Answer 15

No, DNA sequences that are vital to the survival of an organism don’t normally show much evidence of genetic variation.

Question 16

What normally happens to variants that arise within DNA sequences that are vital for the survival of an organism and why?

Answer 16

Most variants in these regions will be selected against as they are likely to have a strongly deleterious effect which will affect chances of having children and therefore reduce fitness.

Question 17

Are there any regions within DNA sequences important for survival where genetic variation can occur without it having a negative impact?

Answer 17

Genetic variation can occur within the third base of a triplet codon of these DNA sequences as some amino acids are coded for by multiple codons meaning a change in the third codon might not change the amino acid sequence.

Question 18

What are the regions of a gene that will be highly conserved?

Answer 18

Coding regions (not exons as these contain non-coding regions)

Question 19

What are the regions of a gene that will show intermediate conservation?

Answer 19

Promoter, 5’ untranslated region (UTR), 3’ UTR, terminator, splice sites

Question 20

What are the regions of a gene that will show low conservation?

Answer 20

Introns, 3rd base of codons, terminator

Question 21

The following diagram shows intron 6, exon 7 and intron 7 of the NAMPT (Nicotinamide phosphoribosyltransferase) gene. Explain how this diagram shows conservation, or lack of conservation, of the gene between different organisms.

Answer 21

• The purple plot shows that most of the conservation within this region of the human NAMPT gene is observed within the exon within the 2 introns showing very low levels of conservation.
• The plots of the conservation of exon 7 of the NAMPT gene in other species show that as the species get less closely related to humans the conservation in exon 7 decreases.

Question 22

Define the term “Phylogenetics”

Answer 22

The study of the evolutionary history and relationships (how closely related they are) among individuals or groups of organisms.

Question 23

In phylogenetics how are evolutionary relationships/evoultionary history illustrated?

Answer 23

There are usually demonstarted using a phylogenetic tree

Question 24

On a phylogenetic tree what does the distance between two species/strains/sequences represent?

Answer 24

• Represents how similar those 2 entities are
• Can also represent evolutionary pressure and time - You may see large difference in distance between 2 organisms if one is under a lot more pressure to change even if time-wise they may not be far apart

Question 25

What is gene duplication?

Answer 25

Duplication of a DNA sequence containing a gene

Question 26

How can gene duplication contribute affcet genetic variation?

Answer 26

After the duplication one copy of the gene can continue the original function while the other copy can evolve new function(s) by changes in the coding sequence and/or control sequences.

Question 27

Explain the mechanism that leads to gene duplication

Answer 27

The mechanism that leads to gene duplication is called unequal crossing over.

It occurs during meiosis between homologous chromosomes that are not paired properly

The result of unequal crossing over is that on one chromosome, at the site of exchange, you have a duplication of a sequence whilst on the other chromosome, at the site of exchange, you have the deletion of that same sequence.

Question 28

Name the 2 clusters of globin genes, what chromosome they are on and the number of genes/pseudogenes within each cluster

Answer 28

• Alpha-like: on chromosome 16 – 4 genes and 3 pseudogenes
• Beta-like: on chromosome 11 – 5 genes and 1 pseudogene

Question 29

How are the globin genes within each cluster arranged on their respective chromosomes?

Answer 29

They are arranged in order of expression during development

Question 30

What mechanisms contributed to the evolution of the globin genes?

Answer 30

The globin genes have evolved through duplication and accumulation of mutations (divergence).

Question 31

What does the fact that divergence the globin gene promoters as well as divergence of the genes themselves occured during their evolution mean?

Answer 31

It means that the globin gene promoters bind different transcription factors at different times and allow expression of genes at different stages of development e.g. from embryo to foetus to postnatal

Question 32

What is a psuedogene and how do they form?

Answer 32

A psuedogene is a gene that has lost functionality.

They form after a gene duplication event in which the gene that doesn’t keep its original function accumulates mutations to the point of non-functionailty.

Question 33

Why is it that the symptoms of sickle cell disease usually occur at 5-6 months of age?

Answer 33

Because the mutation that causes sickle cell occurs in the 𝛃 - globin gene which is very highly expressed in HbA but hardly expressed at all in HbF. At 5-6 months is when the levels of HbA within the blood increase massively whilst the levels of HbF decreases meaning the effect of having mutated Hb A (HbS) would begin to manifest at this point.

Question 34

Describe the mutation that results in development of sickle cell disease.

Answer 34

• The mutation is a single base change in the 𝛃-globin gene of Haemoglobin A.
• The single base change causes the GAG codon to change to a GTG
• This codon change causes a Glutamine to become a Valine at position 7 of the protein

Question 35

What is Haemoglobin A known as once it becomes mutated during sickle cell disease?

Answer 35

Becomes known as Haemoglobin S (HbS)

Question 36

Why is the HbS variant selected for in populations within places like Africa, the Middle East and the Mediterranean?

Answer 36

Because one copy of the HbS variant confers resistance to severe malaria and malaria is very prevalent in these areas. In these places people with one copy of the HbS variant have what is known as a “Heterozygote advantage”