3.4.3 Genetic diversity can arise as a result of mutation or during meiosis ( 3.4 Genetic information , variation and relatoonships between organisms)

Question 1

What is a gene mutation?

Answer 1

A change in the base sequence of DNA (on chromosomes).

Examples include base deletion or substitution.

Question 2

How can a mutation occur?

Answer 2

Mutations can arise spontaneously during DNA replication (interphase).

Question 3

What is a mutagenic agent?

Answer 3

A factor that increases the rate of gene mutation, e.g., ultraviolet (UV) light or alpha particles.

Question 4

How can a mutation lead to a non-functional protein or enzyme?

Answer 4

1. Changes sequence of base triplets in DNA (in a gene) so changes sequence of codons on mRNA.
2. Changes sequence of amino acids in the polypeptide.
3. Changes position of hydrogen/ionic/disulphide bonds (between amino acids).
4. Changes protein tertiary structure (shape) of protein.
5. Enzymes - active site changes shape so substrate can’t bind, enzyme-substrate complex can’t form.

Question 5

What happens in a substitution mutation?

Answer 5

1. DNA base/nucleotide (pair) replaced by a different base/nucleotide (pair).
2. This changes one triplet so changes one mRNA codon.
3. One amino acid in polypeptide changes.

Example: TAC GTC AAA CGG CAT changes to TAC GTC AAT CGG CAT.

Question 6

What are the possible effects of a deletion mutation?

Answer 6

1. One nucleotide/base (pair) removed from DNA sequence.
2. Changes sequence of DNA triplets from point of mutation (frameshift).
3. Changes sequence of mRNA codons after point of mutation.
4. Changes sequence of amino acids in primary structure of polypeptide.
5. Changes position of hydrogen/ionic/disulphide bonds in tertiary structure of protein.
6. Changes tertiary structure/shape of protein.

Question 7

What are the features of homologous chromosomes?

Answer 7

Same length, same genes at same loci, but may have different alleles.

Question 8

What is a diploid cell?

Answer 8

A cell that has 2 complete sets of chromosomes, represented as 2n.

Question 9

What is a haploid cell?

Answer 9

A cell that has a single set of unpaired chromosomes.

Question 10

Describe how a cell divides by meiosis.

Answer 10

In interphase, DNA replicates - 2 copies of each chromosome (sister chromatids), joined by a centromere.

Meiosis I separates homologous chromosomes, and Meiosis II separates chromatids.

Question 11

What is the outcome of Meiosis I?

Answer 11

4 genetically varied daughter cells.

Question 12

What is crossing over in meiosis?

Answer 12

The exchange of genetic material between homologous chromosomes.

Question 13

What is independent segregation in meiosis?

Answer 13

The random distribution of homologous chromosomes to daughter cells.

Question 14

What should you be able to do with diagrams of meiosis?

Answer 14

Complete diagrams showing the chromosome contents of cells after the first and second meiotic division, given the chromosome content of the parent cell.

Question 15

How do chromosomes appear after DNA replication in meiosis?

Answer 15

They appear as X shapes due to DNA replication.

Question 16

Explain why the number of chromosomes is halved during meiosis

Answer 16

Homologous chromosomes are separated during meiosis I

Question 17

Explain how crossing over creates genetic variation

Answer 17

• Homologous pairs of chromosomes form a bivalent
•non sister chromatids form
• alleles / chromatids exchanged between chromosomes
• Creating new combinations of alleles on chromosomes

Question 18

Explain how independent segregation creates genetic variation

Answer 18

• Homologous pairs randomly align at equator - so random which chromosome from each pair goes into each daughter cell
• Creating different combinations of maternal and paternal chromosomes / alleles in daughter cells

Question 19

Other than mutation and meiosis , explain how genetic variation within a species is increased

Answer 19

• Random fertilisation
• Creating new allele combinations

Question 20

Explain the different outcomes of mitosis and meiosis

Answer 20

1) mitosis produces 2 daughter cells , whereas meiosis produces 4 daughter cells
• as 1 division in mitosis , whereas 2 divisions in meiosis
2) Mitosis maintains the chromosome number (diploid-diploid or haploid to haploid ) whereas meiosis halves the chromosome number ( diploid to haploid)
• As homologous chromosomes separate in meiosis but not mitosis
3) Mitosis produces genetically identical daughter cells whereas meiosis produces genetically varied daughter cells
• As crossing over and independent segregation happen in meiosis but not mitosis

Question 21

Explain the importance of meiosis

Answer 21

• Two divisions creates haploid gametes
• So diploid number is restored at fertilisation - chromosome number maintained between generations
• Independent segregation and crossing over creates genetic variation

Question 22

How can you recognise where meiosis and mitosis occur in a life cycle

Answer 22

• mitosis occurs between stages where chromosome number is maintained
• meiosis occurs between stages where chromosome number halves

Question 23

Describe how mutations in the number of chromosomes arise

Answer 23

• Spontaneously by chromosome non disjunction during meiosis
• Homologous chromosome or sister chromatids fail to separate during meiosis
• So some gametes have an extra copy of a particular chromosome and others have none

Question 24

Suggest how the numbers of possible combinations of chromosomes in daughter cells following meiosis can be calculated

Answer 24

2^n
where n = number of pairs of homologous chromosomes ( half the diploid number )

Question 25

Suggest how the number of possible combinations of chromosomes following random fertilisation of two gametes can be calculated

Answer 25

(2^n)^2 where n = number of pairs of homologous chromosomes ( half the diploid number )