4.3 Genetic diversity can arise as a result of mutation or during meiosis

Question 1

What is a gene mutation?

Answer 1

• A change in the base sequence of DNA (on chromosomes)
• Can arise spontaneously during DNA replication (interphase)
• Involves base deletion / substitution

Question 2

How can a mutation lead to the production of a non-functional protein/enzyme?

Answer 2

• Change in base / triplet sequence of DNA / gene
• Changes sequence of codons on mRNA
• Changes sequence of amino acids in the primary structure of the polypeptide
• Changes position of hydrogen / ionic / disulphide bonds in tertiary structure of protein
• Changes tertiary structure / shape of the protein (and active site if enzyme)
• (if enzyme) substrate can’t bind to active site and form an enzyme-substrate complex

Question 3

What happens during base deletion?

Answer 3

• One nucleotide/base removed from DNA sequence
• Changes triplet/codon sequence from the point of mutation (frameshift)
• Changes sequence of codons on mRNA after point of mutation

Question 4

What are the two different types of base substitution?

Answer 4

1. Changes one mRNA codon and one amino acid -> sequence of amino acids in primary structure of polypeptide changes
2. Due to the degenerate nature of the genetic code, the new triplet may still code for the same amino acid so the sequence of amino acids in the primary structure of the polypeptide remains unchanged

Question 5

What are the two different types of base substitution?

Answer 5

1. Changes one mRNA codon and one amino acid -> sequence of amino acids in primary structure of polypeptide changes
2. Due to the degenerate nature of the genetic code, the new triplet may still code for the same amino acid so the sequence of amino acids in the primary structure of the polypeptide remains unchanged

Question 6

What are mutagenic agents?

Answer 6

• Increase the rate of gene mutation (above the rate of naturally occurring mutations)
  Eg/ ultraviolet light or alpha particles

Question 7

What are mutagenic agents?

Answer 7

• Increase the rate of gene mutation (above the rate of naturally occurring mutations)
  Eg/ ultraviolet light or alpha particles

Question 8

What happens before meiosis starts?

Answer 8

DNA replicates so there are two copies of each chromosome, called sister chromatids, joined by a centromere
- 2 x 2n

Question 8

What happens before meiosis starts?

Answer 8

DNA replicates so there are two copies of each chromosome, called sister chromatids, joined by a centromere
- 2 x 2n

Question 9

What happens in meiosis I?

Answer 9

Separated homologous pairs

Question 10

What happens in prophase 1?

Answer 10

• Chromosomes condense and become visible
• Chromosomes come together in homologous pairs (bivalent) (synapsis)

Question 11

What happens in metaphase 1?

Answer 11

• Chromosomes line up across the equator in their homologous pairs
• The orientation of the pair is random (not influenced by any other pair), each member of the pair faces on opposite pole of the cell
• The chromosomes attach to the spindle by their centromeres

Question 12

What happens in anaphase 1?

Answer 12

• Spindle pulls homologous pairs apart (opposite ends of the cell) (centromere don’t divide)

Question 13

What happens in telophase 1?

Answer 13

• Cytokinesis
• Nuclear envelope may reform

Question 14

What happens in meiosis II?

Answer 14

Separate chromatids
- Creates 4 haploid cells that are genetically varied

Question 15

What happens in prophase 2?

Answer 15

• Chromosomes condense
• Nuclear envelope breaks down
• Centrioles replicate and new spindle starts to form

Question 16

What happens in metaphase 2?

Answer 16

• Chromosomes line up across the centre of the cell and attach to the spindle

Question 17

What happens in anaphase 2?

Answer 17

Spindle pulls centromeres apart

Question 18

What happens in telophase 2?

Answer 18

• Nuclear envelope reform
• Cytokinesis

Question 19

How does meiosis create genetic variation?

Answer 19

Crossing over between homologous chromosomes (prophase 1)
- Alleles exchanged between chromosomes
- Creates new combinations of maternal and paternal alleles on chromosomes

Independent segregation of homologous chromosomes (metaphase 1)
- Random alignment of homologous pairs at equator→random which chromosome from each pair goes to each daughter cell
- Creates different combinations of maternal and paternal chromosomes and alleles in daughter cells

Random fertilisation when two gametes fuse to form a zygote

Question 20

What is the importance of meiosis?

Answer 20

• Two divisions – creates haploid gametes (half number of chromosomes)
• Diploid number restored at fertilisation
• Maintains chromosome number from one generation to the next
• Independent segregation and crossing over creates genetic variation

Question 20

What is the importance of meiosis?

Answer 20

• Two divisions – creates haploid gametes (half number of chromosomes)
• Diploid number restored at fertilisation
• Maintains chromosome number from one generation to the next
• Independent segregation and crossing over creates genetic variation

Question 21

What is non-disjunction mutation?

Answer 21

• Homologous chromosomes fail to separate during meiosis I OR sister chromatids fail to separate during meiosis II
• One gamete has an extra copy of this chromosome and the other has none
• Upon fertilisation, zygote has one fewer (dies) or one extra chromosome (survives)
• Arises spontaneously
• Causes genetic diseases e.g. down’s syndrome in humans – extra copy of chromosome 21

Question 22

What is non-disjunction mutation?

Answer 22

• Homologous chromosomes fail to separate during meiosis I OR sister chromatids fail to separate during meiosis II
• One gamete has an extra copy of this chromosome and the other has none
• Upon fertilisation, zygote has one fewer (dies) or one extra chromosome (survives)
• Arises spontaneously
• Causes genetic diseases e.g. down’s syndrome in humans – extra copy of chromosome 21