Topic 4: Genetic Diversity and Adaptation

Question 1

What is a mutation?

Answer 1

Any change to the quantity or base sequence of the DNA of an organism

Question 2

What is a gene mutation?

Answer 2

Any change to 1 or more nucleotide bases, or a change in base sequences of DNA

Question 3

How do some gene mutations cause non-functional proteins?

Answer 3

Any change in DNA triplets changes the amino acid sequence of the polypeptide (primary structure), so the protein folds into its tertiary structure incorrectly, changing its shape, so it doesn’t function normally anymore

Question 4

What are the different types of DNA mutation?

Answer 4

• Substitution
• Deletion
• Addition
• Chromosome mutations

Question 5

What effect do substitution mutations have?

Answer 5

A nucleotide in a DNA molecule is replaced with nucleotide with a different base.

Polypeptide will differ by 1 amino acid (if any - degenerate code). The effect depends on the role of the original amino acid (e.g if in active site of an enzyme / important in forming bonds of the tertiary structure).
Doesn’t cause frameshift

Question 6

What effect do deletion mutations have?

Answer 6

A nucleotide is lost from the normal DNA sequence.

Causes frameshift - all triplets further on in the sequence are read differently, so the amino acid sequence is entirely different, leading to a non-functional protein

Question 7

What effect do addition mutations have?

Answer 7

A new base is inserted into the DNA sequence where it shouldn’t be.

Causes frameshift - all triplets further on in the sequence are read differently, so the amino acid sequence is entirely different, leading to a non-functional protein

Question 8

What is a chromosome mutation and what types are there?

Answer 8

Changes in the structure or number of whole chromosomes.

Polyploidy, non-disjunction

Question 9

What is polyploidy (in reference to chromosome mutations)?

Answer 9

Changes in whole sets of chromosomes, organisms have 3 or more sets of chromosomes rather than 2

Question 10

What is non-disjunction (in reference to chromosome mutations)?

Answer 10

Changes in the number of individual chromosomes, sometimes individual homologous pairs fail to separate in meiosis, and the gamete will have 1 more or fewer chromosomes

Question 11

What is a mutagen? Give some examples

Answer 11

An agent increasing the rate of gene mutations

E.g some chemicals, carcinogens, ionising radiation (e.g UV)

Question 12

What are the differences between mitosis and meiosis?

Answer 12

Mitosis produces 2 genetically identical daughter cells with the same number of chromosomes as the parent

Meiosis produces 4 genetically different daughter cells with half the number of chromosomes as the parent

Question 13

Why is meiosis important?

Answer 13

Creates gametes with a haploid number (1 complete set of chromosomes).
In sexual reproduction gametes combine, producing a cell with double their chromosome number - if meiosis didn’t produce haploid gametes, the number of chromosomes would double each generation.

Also brings about genetic variation, leading to adaptations increasing survival chances

Question 14

Describe the process of meiosis

Answer 14

Interphase - homologous pairs of chromosomes unravel and replicate

Meiosis division 1:
Prophase 1 - chromosomes condense, homologous chromosomes pair up, crossing over occurs, spindle fibres form, nuclear envelope breaks down

Metaphase 1 - Homologous chromosomes line up in pairs on random sides of the equator (independent segregation)

Anaphase 1/Telophase 1 - 1 chromosome from each homologous pair ends up in each cell after division

Meiosis division 2:
Each daughter cell ends up with a different set of sister chromatids

Question 15

Describe the process of crossing over in meiosis

Answer 15

Chromatids of each chromosome in a homologous pair twist around each other (cross over)
Tensions are created and portions of chromatids break off
Recombination - the broken portions re-join the chromatids of the partner chromosome - equivalent portions exchanged
Creates different portions of alleles

Question 16

Describe the process of independent segregation in meiosis

Answer 16

After DNA has replicated, it is random which side of the equator the maternal/paternal chromosomes from each homologous pair lie.
It is random which combination of these sets each daughter cell gets

Question 17

How would you calculate the number of combinations of chromosomes in daughter cells after meiosis?

Why is it not accurate?

Answer 17

2^n

(where n is the number of pairs of homologous chromosomes)

Only takes into account independent segregation, crossing over increases this number further

Question 18

How would you calculate the number of combinations of chromosomes in a zygote after random fertilisation?

Answer 18

(2^n)^2

(where n is the number of homologous pairs)

Question 19

What is non-disjunction in meiosis?
What are the different types?

Answer 19

Where homologous pairs / chromatids don’t separate properly

Polyploidy, aneuploidy

Question 20

What is polyploidy in meiosis and what are the different outcomes after cell division?

Answer 20

Changes in whole sets of chromosomes

Non-disjunction in meiosis 1:
- Homologous pairs don’t separate, 1 daughter cell receives all chromosomes, other receives nothing
Normal meiosis 2:
- 2 daughter cells receive 2 sets of chromatids (2n), 2 daughter cells receive 0

OR

Normal meiosis 1:
- Each daughter cell receives 1 set of chromosomes
Non-disjunction in meiosis 2:
- I cell splits normally, forming 2 normal daughter cells (n). The other doesn’t split sister chromatids, so 1 daughter cell receives nothing, 1 receives 2 sets of chromatids (2n)

Question 21

What is aneuploidy in meiosis and what are the different outcomes after cell division?

Answer 21

Changes in the number of individual chromosomes

Non-disjunction in meiosis 1:
- 1 homologous pair doesn’t separate, 1 daughter cell gets (2n+2), the other gets (2n-2)
Normal meiosis 2:
- Chromatids split normally, 2 daughter cells receive (n+1), the other 2 receive (n-1)

OR

Normal meiosis 1:
- Homologous pairs split normally, each daughter cell gets (2n)
Non-disjunction in meiosis 2:
- 1 cell splits normally, 2 daughter cells get (n)
- 1 chromosome in the other cell fails to split, 1 daughter cell gets (n-1), the other gets (n+1)

Question 22

What is genetic diversity?

Answer 22

The total number (variety of) alleles in a population

Question 23

What is a population?

Answer 23

A group of individuals of the same species that live in the same place and can interbreed

Question 24

What is a species?

Answer 24

A group of organisms that can interbreed to produce fertile offspring

Question 25

Why is genetic diversity a good thing?

Answer 25

Greater genetic diversity means the population is more likely to survive an environmental change because:

• Wider range of alleles + characteristics means there is a greater probability of an individual having a characteristic suited to the new environment = a factor enabling natural selection

Question 26

How does reproductive success affect allele frequency? (natural selection)

Answer 26

• Organisms produce more offspring than can be supported by the environment (food, water etc) but most populations remain constant
• Because of intraspecific competition there is a gene pool within a population
• Random mutations mean some individuals possess combinations of alleles making them more likely to survive in competition
• These individuals more likely to obtain resources + live longer = better chance of breeding, passing on advantageous alleles
• Over many generations, number of individuals with advantageous alleles increases at expense to other individuals
• Frequency of advantageous alleles in the population increases and vice versa

Question 27

What is selection?

Answer 27

The process by which organisms better adapted to their environment tend to survive and reproduce, while those less well adapted tend not to

Question 28

What is the distribution of alleles in a population and why?

Answer 28

A normal distribution curve

Because:
- Most characteristics are influenced by multiple genes (polygenes)
- These are more influenced by the environment than single-gene characteristics
- The environmental effect on polygenes produces individuals in a population that vary about the mean

Question 29

What is directional selection?

Give an example

Answer 29

Selection favours individuals varying in one direction from the mean of the population. Changes the characteristics of the population

e.g antibiotic resistance

Question 30

Describe how antibiotic resistance comes about

Answer 30

• Resistance naturally occurs in a population due to chance mutation (new alleles)
• Allele codes for an enzyme complementary to the antibiotic = mutant bacteria synthesises enzyme that breaks down antibiotic before it can be killed
• When antibiotic is applied susceptible bacteria are killed
• Bacteria with mutant allele survive + reproduce by binary fission, passing on the allele + their population increases in proportion to non-resistant bacteria (they are ‘selected for’)
• Frequency of the resistance allele increases + the normal distribution curve shifts in the direction of greater resistance

Question 31

Describe a graph displaying directional selection

Answer 31

Number of individuals on the y axis, the characteristic on the x axis.

The curve will stay exactly the same shape, but be moved in one direction from the mean

Question 32

What is stabilising selection? Give an example

Answer 32

Selection favours average individuals - eliminates phenotypes at the extremes and so reduces variation around the optimum modal value.

E.g human birth weights (very heavy + very light babies are much more likely to die)

Question 33

Describe a graph displaying stabilising selection

Answer 33

Number of individuals on the y axis, the characteristic on the x axis.

The original graph has the normal distribution curve. The graph after selection has a much narrower, taller curve.

Question 34

What are the different types of adaptation?

Answer 34

• Anatomical (physical features)
• Physiological (biochemical processes)
• Behavioural