3.4.3 genetic diversity -> mutations + meiosis

Define genetic diversity?

Number of different alleles in a population

Genetic diversity within a species can be caused by what?

• Gene mutations

• Meiosis → independent segregation of homologous chromosomes + crossing over)

• Random fusion of gametes during fertilisation

• Chromosome mutations

This genetic diversity is acted upon by natural selection, resulting in what?

In species becoming better adapted to their environment

What are gene mutations?

Changes in the sequence of nucleotide bases in DNA → new alleles of genes are produced by mutations

Gene mutations occur ___ and might arise during….

spontaneously

DNA replication

The mutation rate is increased by what?

Mutagenic agent e.g X-rays, benzene, UV light

What can mutations result in + why?

Different amino acid sequence in the encoded polypeptide → due to altered base sequence coding for a different sequence of amino acids

Why don’t all mutations result in a change to the encoded amino acids?

Some only change one triplet code → may still code for the same amino acid due to degenerate nature of the genetic code

How do some mutations change all triplet codes?

They result in a frame shift → all bases move down/upstream from the mutation

What is substitution?

Replacement of one or more bases by one or more different bases

The substitution of a single base may result in what?

• A new triplet coding for a different amino acid in the polypeptide chain → may result in a non functional protein

• One different amino acid in polypeptide changes but functional protein is still produced

• Same amino acid may e coded due to degeneracy of DNA code → polypeptide is unchanged

• A stop codon → protein synthesis prematurely terminated

What is deletion?

Removal of one or more bases

What is the effect of deletion?

• Results in a frame shift → alteration in all base triplets from point of mutation

• Sequence of amino acids is altered from point of mutation → protein formed is nearly always non-functional

What is addition?

Adding of one or more bases

What does addition result in?

Frame shift → sequence of amino acids is altered from point of addition + protein formed may be non-functional

What is duplication + what does it result in?

Where one or more bases is repeated → frame shift - alteration in base triplets from point of addition

What is inversion?

Where a sequence of bases is reversed → multiple amino acids could be affected + may result in non-functional protein

What is translocation?

Where a sequence of bases is moved from one location in the DNA molecule to another part of the genome

A gene can exist in different forms called…

alleles → code for different types of the same characteristic

If the base sequence of a gene that codes for an enzyme is altered, what effect does this have on the enzyme?

• Change in amino acid sequence may affect position of H/ionic/disulfide bonds which results in a different tertiary structure

• Change in tertiary structure may alter shape of active site → no longer complementary to substrate + ES complexes don’t form - reaction isn’t catalysed

What is meiosis?

A type of nuclear division that produces cells that are genetically different

• Single cell divides twice but DNA replication only occurs once

• 4 cells are produced that are varied + possess half the number of chromosomes (haploid) of original cell

Where is meiosis important?

In the production of haploid gametes → result in the diploid number being restored when gametes fuse at fertilisation to produce a zygote

Compare mitosis + meiosis

How to spot where meiosis takes place in a cell life cycle?

When the diploid number (2n) halves to the haploid number (n)

What is the process of meiosis?

• During late interphase → DNA replication, build up of ATP, protein synthesis + replication of cell organelles

• Meiosis I + meiosis II occur

• Results in 4 haploid genetically different daughter cells

What is separated in each meiosis division?

1. Members of each homologous pair → results in 2 haploid daughter cells with 2 copies of each chromatid

2. Chromatids of each chromosome → results in 4 haploid daughter cells with one copy of each chromatid

What 2 processes in meiosis produce genetically different cells?

• Independent segregation of homologous chromosomes

• Genetic recombination by crossing over within homologous chromosomes

Explain independent segregation

• During first meiotic division (metaphase), homologous chromosomes pair together, lining up opposite each other on the spindle + then separate (anaphase) so one member from each pair enters gamete

• The pairing + segregation of 2 members of a pair is completely independent from the separation of another pair → chromosomes randomly associate within a gamete + so contains varied combination of maternal + paternal chromosomes / alleles

How to work out the number of possible combination of paternal + maternal chromosomes from a known number of homologous pairs?

2 to the power of no. homologous pairs

When does crossing over occur?

During prophase of the first meiotic division

What happens during crossing over?

• 2 members of each homologous pair lie side by side forming a bivalent

• Chromatids of homologous chromosomes intertwine → break + equivalent portions are exchanged → results in exchange of alleles of the same genes + produces new combinations of alleles - recombinants

• Called genetic recombination

• Chromosomes then separate

What is a chiasma?

The place where crossing over occurs

Why is the number of recombinants formed is relatively low?

Crossing over is relatively rare