3.4.3 Meiosis & Mutations

Question 1

MUTATIONS:

What is a gene mutation

Answer 1

• change in DNA base sequence of a chromosome
• happen spontaneously during DNA replication
• rate of mutation increases by mutagenic agents (e.g. ionising radiation and chemicals)

Question 2

Different types of base mutations

Answer 2

Substituted -
may have no effect (degenerate code), change an a.a and so the tertiary structure, or introduce a stop codon

Deleted or added -
Causes a frameshift, so all the codons after the mutation are changed

Question 3

What can cause a change in the number of chromosomes

Answer 3

• non-disjunction: chromosomes do not separate during anaphase in meiosis
• e.g. Down’s syndrome

Question 4

MEIOSIS

Describe the general process of meiosis

Answer 4

• produces daughter cells - genetically different from each other
• 2 nuclear divisions
• 4 haploid daughter cells from 1 diploid parent cell
• preceded by DNA replication - chromosomes consist of 2 identical (sister) chromatids, joined at the centromere

Question 5

MEIOSIS

Behaviour of chromosomes in prophase 1

Answer 5

• chromosome condense + become visible
• homologous chromosomes attach to each other at chriasmata (non-sister chromosomes join together), forming bivalents
• spindle fibres form

Question 6

Behaviour of chromosomes in Metaphase 1

Answer 6

• homologous pairs of chromosomes attach to spindle fibres at the equator of the cell

Question 7

Anaphase 1

Answer 7

• homologous pairs of chromosomes separate, & individual chromosomes are pulled to the poles of the cell
• spindle fibres contract, pulling the chromosomes by the centromeres

Question 8

Telophase 1

Answer 8

• chromosomes uncoil
• spindle fibres break down
  Haploid (2) nuclei have formed

Question 9

Prophase II

Answer 9

• chromosomes condense & become visible
• spindle fibres form

Question 10

Metaphase II

Answer 10

• chromosomes line up at the equator of cell & attach to spindle fibres by the centromeres
• spindle fibbers contract, pulling the chromatids by the centromeres

Question 11

Telophase II

Answer 11

• chromosomes uncoil
• spindle fibres break down

Question 12

GENETIC VARIATION

How does ‘independent segregation of homologous chromosomes (metaphase I) cause G.V

Answer 12

• maternal + paternal chromosomes can be positioned either side of the equator of the cell
• this is random + independent for each chromosome pairs
• results in new combinations of alleles on the chromatids

Question 13

How does ‘crossing over’ (prophase & metaphase I) cause genetic variation

Answer 13

• sections of non-sister chromatids can be exchanged when the chiasmata (X shaped structure, joins chromosomes) break
• results in new combinations of alleles on the chromatids

Question 14

How does ‘random fertilisation’ (after meiosis) cause genetic variation

Answer 14

-each individual organism produces genetically varied gametes
- in each fertilisation event there is an equal probability of any female gamete fusing with any male gamete
- increases genetic variation within a species

Question 15

Anaphase II

Answer 15

• chromosomes separate, and chromatids are pulled to the poles
• spindle fibres contract, pulling the chromatids by the centromeres