4.3 Mutations and meiosis

What is a gene mutation?

A change in the base sequence of DNA

Can arise spontaneously during DNA replication

What is a mutagenic agent?

A factor that increases the rate of gene mutations (eg UV light)

Explain how a mutation can lead to the production of a non-functional protein or enzyme

Changes in base triplets in DNA so changes sequence of codons on mRNA

So changes sequence of amino acids in polypeptide

So changes position of hydrogen/ionic/disulphide bonds between amino acids in

So changes proteins tertiary structure

Enzymes - active site changes shape so substrate cant bind and enzyme-substrate complexes cant form

Explain the possible effects of a substitution mutation

Base/nucleotide in DNA is replaced by a different base/nucleotide

This changes one triplet so changes one mRNA codon

So one amino acid in the polypeptide changes - tertiary structure may change if position of hydrogen/ionic/disulphide bonds change

Or amino acid doesn’t change due to genetic code being degenerate or if mutation is in an intron

Explain the possible effects of a deletion mutation

One nucleotide/base removed from DNA sequence

Changes sequence of DNA triplets from point of mutation (frameshift)

Changes sequence of mRNA codons after point of mutation

Changes sequence of amino acids in primary structure of polypeptide

Changes position of hydrogen/ionic/disulphide bonds in tertiary structure of protein

Changes tertiary structure of protein

What are the features of homologous chromosomes?,

Same length and same genes at the same loci but may have different alleles

Describe the difference between diploid and haploid cells

Diploid - have two complete sets of chromosomes (2n)

Haploid - have a single set of unpaired chromosomes (n)

Describe how a cell divides by meiosis

Interphase → DNA replicates to form two copies of each chromosome (sister chromatids), joined by a centromere

Meiosis 1 → Homologous chromosomes separate

• Chromosomes arrange into homologous pairs

• Crossing over between homologous chromosomes

• Independent segregation of homologous chromosomes

Meiosis 2 → Chromatids separate

Four genetically different haploid daughter cells are produced

Why is the number of chromosomes halved during meiosis?

Homologous chromosomes are separated during meiosis 1

How does crossing over of chromosomes during meisois create genetic variation?

Homologous pairs of chromosomes form a bivalent

Chiasmata form (point of contact between chromatids)

Equal lengths of chromatids exchanged between chromosomes (alleles)

Creating new combinations of alleles on chromosomes

How does independent segregation during meiosis create genetic variation?

Homologous pairs randomly align at equator so it’s random which chromosome from each pair goes into each daughter cell

Creating different combinations of maternal and paternal chromosomes/ alleles in daughter cells

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

Mitosis → occurs between stages where chromosome number is maintained (eg diploid (2n) to diploid (2n))

Meiosis → occurs between stages where chromosome number halves (eg diploid (2n) to haploid (n))

Explain chromosome non-disjunction

Homologous chromosomes or sister chromatids fail to separate during meiosis so gametes have an extra copy of a particular chromosome and the others have none

How can the number of possible combinations of chromsomes in daughter cells after meiosis be calculated?

2n (to the power of n) = number of pairs of homologous chromosomes

How can the number of possible combinations of chromosomes after random fertilisation in meiosis be calculated?

(2n)n