B9 Genetic Diversity

Question 1

What does meiosis produce and what does it involve

Answer 1

produces daughter cells that genetically Diff from each other.

involves two nuclear divisions and creates four haploid daughter cells from a single diploid parent cell

Question 2

What are 2 mechanisms in meiosis that introduce variation

Answer 2

Independent segregation of homologous chromosomes

Crossing over between homologous chromosomes

Both occur in meiosis I (first round of division)

Question 3

Independent segregation

Answer 3

In meiosis I, homologous pairs of chromosomes line up opposite each other at the equator of the cell.

It is random which side of the equator the paternal and maternal chromosomes from each homologous pair lie.

These pairs are separated, so one of each homologous pair ends up in the daughter cell.

Question 4

Independent segregation - calculation

Answer 4

This creates a large number of possible combinations of chromosomes in the daughter cells produced.

This can be calculated using 2^n

n = number of homologous pairs

Question 5

What is crossing over and how does it work

Answer 5

homologous pairs line up opp each other at equator in meiosis I, parts of chromatids twist around each other.

puts tension on chromatids, pairs of chromatid to break.

broken parts of chromatid recombine with another chromatid.

results in new combo of alleles.

Question 6

Meiosis compared to mitosis

Answer 6

Meiosis
- 2 nuclear divisions
- haploid cells (one set of chromosomes)
- introduces genetic variation

Mitosis
- 1 nuclear division
- diploid cells (2 sets of chromosomes)
- create genetically identical cells

Question 7

Identifying meiosis in a life cycle

Answer 7

Tip: Meiosis involves diploid (2n) parent cell dividing to become haploid (n) cell

Question 8

Final increase in genetic variation

Answer 8

Random fertilisation further increases genetic variation.

There 2n possible chromosome combo in gametes ( 2^23 in humans)

It is random which egg and sperm will fuse is fertilisation. Therefore, variation is increased.

There is altuaily (2n)2 possible combinations of chromosomes when you consider random fertilisation.

For a human this = (2^23)^2 = 7.04 x 10^13

This is before crossing over is considered.

Question 9

What’s non-disjunction

Answer 9

chromosomes or chromatids dont split equally during anaphase.

Question 10

Chromosome mutations

Answer 10

Mutations in number of chromosomes arise spontaneously by chromosome non-disjunction during meiosis.

Question 11

Chromosome non-disjunction

Answer 11

Changes in the number or structure of whole chromosomes can arise spontaneously due to chromosome non-disjunction during meiosis.

This can occur in two forms:
1. Changes in whole sets of chromosomes (polyploidy)
2. Changes in the number of individual chromosomes (aneuploidy)

Question 12

What is polyploidy

Answer 12

Changes in whole sets of chromosomes occur when organisms have three or more sets of chromosomes rather than the usual two. This condition is called polyploidy and mainly occurs in plants.

Changes in whole sets of chromosomes occur when organisms have three or more sets of chromosomes rather than the usual two

Question 13

What is aneuploidy

Answer 13

Changes in the number of individual chromosomes.

Sometimes individual homologous pairs of chromosomes fail to separate during meiosis.

This is non-disjunction and usually results in a gamete having one more or one fewer chromosome.

On fertilisation with a gamete that has the normal number of chromosomes, the resultant zygote will have more or fewer chromosomes that normal in all their body cells. E.g. Down’s syndrome is 3 copies of chromosome 21.

Question 14

Aneuploidy

Answer 14

Changes in the number of individual chromosomes.
On fertilisation with a gamete that has the normal number of chromosomes, the resultant zygote will have more or fewer chromosomes that normal in all their body cells.
E.g. Down’s syndrome is 3 copies of chromosome 21.

Question 15

What’s a mutation

Answer 15

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

Question 16

What’s a gene mutation

Answer 16

Any change to 1 or more nucleotide bases, or a change in the sequence of the bases in DNA

Question 17

What’s produced by a mutation during hte formation of gametes

Answer 17

Mutations occurring during the formation if gametes may be inherited, often producing sudden + distinct differences between individuals.

Question 18

When can a gene mutation arise spontaneously and what do they include

Answer 18

During DNA replication and include base substitution and base deletion

Question 19

What is the gene mutation ‘substitution of bases’

Answer 19

Nucleotide base in DNA molecule is replaced by another nucleotide that has a different base

Question 20

What’s the effect of the gene mutation base substitution of cytosine in GTC with guanine

Answer 20

DNA triplet of bases, GTC codes for amino acid glutamine.

It becomes GTG.

GTG codes for amino acid histidine + replaces original amino acid glutamine.

Polypeptide produced will differ in a single amino acid

Question 21

Why might the base substitution gene mutation not have any effect

Answer 21

The new triplet of bases might still code for the same amino acid as before. So there’s no change in polypeptide produced + so mutation has no effect.

Due to degenerate nature of genetic code

Question 22

What’s the gene mutation deletion of bases

Answer 22

Arises when nucleotide is lost from the normal DNA sequence

1 deleted nucleotide causes all triplets in a sequence to be read differently because each has been shifted to the left by 1 base.

Question 23

What are chromosome mutations

Answer 23

Changes in the structure or number of whole chromosomes

Question 24

When can chromosome mutations arise

Answer 24

Spontaneously

Question 25

What are the 2 forms that chromosome mutations can take

Answer 25

- changes in whole sets of chromosomes - changes in the number of individual chromosomes

Question 26

When do changes in whole sets of chromosomes occur

Answer 26

When organisms have 3 or more sets of chromosomes rather than the usual 2 This condition called ‘polyploidy’ + occurs mostly in plants

Question 27

What is non-disjunction

Answer 27

Sometimes individual homologous pairs of chromosomes fail to separate during meiosis. It usually results in gamete’s having either more 1 more or 1 fewer chromosome. On fertilisation with gamete that has hte normal complement of chromosomes, resultant offspring have more or fewer chromosomes than normal in all their body cells.

Question 28

What’s an example of non-disjunction

Answer 28

Down’s syndrome Where individuals have an additional chromosome 21

Question 29

What’s a mutation

Answer 29

An alteration to the DNA base sequence. Often arise spontaneously during DNA replication

Question 30

Why might a mutation not lead to change in the amino acid sequence

Answer 30

- genetic code is degenerate so mutation may end up coding for same amino acid as original triplet - mutation may occur in intron

Question 31

What’s a substitution mutation

Answer 31

When a nucleotide in DNA sequence is replaced by another. This is more likely to be a quiet mutation, meaning no change procures in amino acid sequence

Question 32

What’s a deletion mutation

Answer 32

When a nucleotide in the dna sequence is lost. This is more likely to be harmful and significant, as it leads to a frame shift which means the entire amino acid sequence will be different.

Question 33

What’s a mutagenic agent. Give examples

Answer 33

Factors that increase the rate of gene mutation. X-rays, UV light, gamma rays, certain chemicals e.g. in alcohol and tobacco

Question 34

What’s a polyploidy chromosome mutation

Answer 34

Where an individual has 3 or more sets of chromosomes instead of 2

Question 35

What’s chromosome non-disjunction

Answer 35

When chromosomes fail to separate correctly in meiosis, resulting in gametes with 1 more or less chromosome than normal

Question 36

What’s meiosis

Answer 36

A form of cell division that produces 4 genetically different haploid cells (cells with half the number of chromosomes found in the parent cell) known as gametes

Question 37

How does meiosis differ from mitosis

Answer 37

- Meiosis produces 4 genetically different cells with half the number of chromosomes as the parent cells - mitosis produces 2 genetically identical cells with the same number of chromosomes as the parent cells.

Question 38

What’s happens during meiosis I

Answer 38

1. Homologous chromosomes pair to form bivalents 2. Crossing over (exchange of sections of genetic material ) occurs at chiasmata 3. Cell divides in 2. Homologous chromosomes separate randomly. Each cell contains either maternal/paternal copy

Question 39

What happens during meiosis 2

Answer 39

1. Independent segregation of sister chromatids 2. Each cell divides again, producing 4 haploid cells

Question 40

In which 2 ways does meiosis produce genetic variation

Answer 40

1. Crossing over during meiosis 1 2. Independent assortment (random segregation) of homologous chromosomes & sister chromatids Result in new combination of alleles

Question 41

Define population

Answer 41

All organisms of a particular species that live in the same place

Question 42

What’s an allele

Answer 42

Diff forms of a particular gene, found at same locus (position) on a chromosome. A single gene could have many alleles

Question 43

Define genetic diversity

Answer 43

Total number of diff alleles in a population

Question 44

What advantages does a high genetic diversity provide

Answer 44

Ability to adapt to a change in environment; allows natural selection to occur

Question 45

Explain how natural selection results in development of new characteristics

Answer 45

- random mutations result in new alleles - some alleles provide an advantage, am,ing an individual more likely to survive + reproduce - their offspring receive the new allele and freq continues to increase over many gens

Question 46

What’s directional selection

Answer 46

Occurs when enviro conditions change. Individuals with phenotypes suited to new conditions will survive and pass on their genes. Over time the mean of the population will move towards these characteristics.

Question 47

Give example of directional selection

Answer 47

Antibiotic resistance Bacteria with mutation allowing then to survive in presence of antibiotics will reproduce. Therefore freq of this allele will increase + population will shift to have greater antibiotic resistance

Question 48

What’s stabilising selection

Answer 48

Occurs when enviro conditions stay the same. Individuals closest to the mean are favoured, + any new characteristics are selected against. Results in low diversity

Question 49

Give example of stabilising selection

Answer 49

Birth weight Babies that weigh around 3kg are more likely to survive us than those at lower/higher weights

Question 50

Define a niche

Answer 50

The role of a species within its enviro Species sharing the same niche will compete with each other

Question 51

What are the 3 types of adaptation. Give examples of each

Answer 51

- anatomical (changes to body structure) e.g. oily fur - physiological (changes to bodily processes) e.g. venom production - behavioural (changes to actions) e.g. hibernation