3.4.3 - GENETIC DIVERSITY (MEIOSIS & MUTATION)

Question 1

define the term mutagenic agent (1)

Answer 1

a factor that increases the rate of mutations

Question 2

give one reason why egg cells produced by meiosis are genetically different (1)

Answer 2

independent segregation OR crossing over

Question 3

genetic variation within a species is increased during meiosis by crossing over and the independent segregation of homologous chromosomes.

apart from mutation, explain one other way genetic variation within a species is increased (2)

Answer 3

1. random fertilisation/random fusion of gametes
2. produces new allele combinations/produces new maternal and paternal chromosome combinations

Question 4

define ‘gene mutation’ and explain how a gene mutation can have:
- no effect on an individual
- a positive effect on an individual (4)

Answer 4

1. definition of gene mutation: a change in the sequence of base pairs in a DNA molecule.
2. change in the base/nucleotide sequence of chromosomes/DNA
3. results in the formation of a new allele
4. it has no effect because the genetic code is degenerate /the mutation is in an intron
5. so amino acid sequence may not change
6. does change amino acid but no effect on tertiary structure
7. new allele is recessive so does not influence phenotype
8. has a positive effect because it results in a change in the polypeptide that positively changes the properties of the protein
9. may result in increased reproductive success/increased survival chances

Question 5

give two differences between mitosis and meiosis (2)

Answer 5

1. mitosis has one division, two divisions in meiosis
2. in mitosis daughter cells are genetically identical, daughter cells in meiosis are not genetically identical
3. in mitosis 2 cells are produced, in meiosis 4 cells are produced
4. separation of homologous chromosomes only in meiosis
5. crossing over only occurs in meiosis
6. independent segregation only in meiosis

Question 6

a change from Glu to Lys at amino acid 1 had no effect on the rate of reaction catalysed by the enzyme. however, the same change at a different amino acid significantly reduced the rate of the reaction catalysed by the enzyme.

use your knowledge of protein structure to suggest reasons for the differences between the effects of these two changes. (2)

Answer 6

1. change at amino acid 1 does not change the shape of the active site/does not change tertiary structure of enzyme
2. the different amino acid may have been in the active site and be required for binding to the substrate

Question 7

explain how the chromosome number is halved during meiosis (2)

Answer 7

1. homologous chromosomes pair (form a bivalent)
2. one chromosome from each pair goes to opposite poles/each daughter cell

Question 8

describe the process of crossing over and explain how it increases genetic diversity (4)

Answer 8

1. homologous pairs of chromosomes associate/form a bivalent
2. chiasma(ta) formation - chromosomes entangle
3. equal lengths of (non-sister) chromatids/alleles are exchanged
4. producing new combinations of alleles

Question 9

describe how the change in chromosome number in downs syndrome is produced (2)

Answer 9

1. in meiosis
2. homologous chromosomes/sister chromatids do not separate (non disjunction)

Question 10

explain why all the cells of the body will have the mutation (2)

Answer 10

ONE PAIR:

1. mutation has occurred in the gametes that formed the zygote
2. all cells in the body are derived from the zygote (a single cell) by mitosis

OR

3. all cells in the body are derived from the zygote (a single cell) by mitosis
4. mitosis produced genetically identical cells (same DNA)

Question 11

describe what happens in division 1 of meiosis (2)

Answer 11

1. chromosomes in homologous pair
2. one chromosome of each pair into daughter cells

Question 12

identify one event that occurs during division 2 but not division 1 of meiosis (1)

Answer 12

1. separation of sister chromatids/division of centromere

Question 13

name two ways in which meiosis produced genetic variation (2)

Answer 13

1. independent segregation of homologous chromosomes (random assortment)
2. crossing over/forming of chiasmata

Question 14

during meiosis, one chromosome from each homologous pair goes to each of the cells produced

explain why this is important (2)

Answer 14

1. to get a haploid number of chromosomes in each cell
2. so that each cell gets one copy of each chromosome
3. so that they have a full set of genes
4. so that when the haploids are formed, a fertilised a diploid zygote is

Question 15

explain the role of independent segregation in meiosis (2)

Answer 15

1. to provide genetic variation
2. allows different combinations of maternal and paternal chromosomes/alleles

Question 16

in prophase I, one of the ways that sexual reproduction enhances the diversity of the offspring is through a process called crossing over.

what entities does this occur between? (1)

Answer 16

non-sister chromatids of homologous chromosomes