Meiosis & Mutations

Question 1

What is the genome?

Answer 1

The complete set of genes in a cell (including those in the mitochondria or chloroplasts)

Question 2

What is the proteome?

Answer 2

The full range of proteins that a cell can produce (coded for by the cells DNA/genome)

Question 3

What is a gene mutation?

Answer 3

A change in the base sequences of DNA (on Chromosomes), which can arise spontaneously during DNA replication

Question 4

What is a mutagenic agent?

Answer 4

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

Question 5

Explain how a mutation can lead to a non functional protein or enzyme

Answer 5

1. Changes sequence of base triplets in DNA (in a gene) so changes sequence of codons on mRNA
2. So changes sequence of amino acids in the polypeptide
3. So changes position of hydrogen / ionic / disulphide bonds (between amino acids)
4. So 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 6

Explain the possible effects of a substitution mutation?

Answer 6

• Base in DNA is replaced by another base
• Changes one triplet so changes one mRNA codon
• One amino acid in polypeptide changes
• Tertiary structure may change if position of disulphide bonds is changed
• However, there may be no change due to the degenerate nature of the genetic code or if mutation occurs in an intron

Question 7

Describe how a cell divides by meiosis

Answer 7

Meiosis I separates homologous chromosomes
- Chromosomes arrange into homologous pairs
- In Prophase I, Crossing over occurs
- In metaphase 1, independent assortment of homologous chromosomes
- In Meiosis 2, chromatids are separated
- Produces 4 genitically unique daughter cells, which are haploid if parent cell is diploid

Question 8

Explain why the number of chromosomes is halved during meiosis

Answer 8

Homologous pairs of chromosomes are separated in Meiosis I

Question 8

Explain how crossing over creates genetic variation

Answer 8

• Homologous pairs of chromosomes form a bivalent
• Chiasmata form
• Alleles exchanged between chromosomes
• Creating new combinations of paternal & maternal alleles on chromosomes

Question 9

Explain how independent assortment creates genetic variation?

Answer 9

• Homologous pairs randomly align at the equator
• So random which chromosome from each pair goes into each daughter cell
• Creating different combinations of paternal & maternal alleles in each daughter cell

Question 10

Describe how mutations in the number of chromosomes arise

Answer 10

• Spontaneously by chromosome non disjunction during meiosis
• Homologous chromosomes (meiosis I) or sister chromatids (meiosis II) fail to separate
• So some gamete’s may have an extra copy (n+1) of a particular chromosome or no copy (n-1)