Meiosis & Mutations

Homologous pairs

This term refers to one of each pair of chromosomes belonging to the father, and the other of the pair being provided by the mother.

This makes the total number of chromosomes 46, diploid

Meiosis 1

Prophase 1 - Homologous chromosomes pair up, and the chromatids cross over via a chiasma and randomly swap over. This crossing over leads to variation.

Metaphase 1 - Homologous pairs line up along the equator and spindle fibres attach to the centromere. The chromosomes are independently assorted, meaning their homologous pairs orientate themselves randomly, so the maternal and paternal chromosomes are randomly combined when the cell divides . This process leads to variation.

Anaphase 1 - The spindle fibres contract and pull the homologous pairs apart towards opposite poles of the cell.

Telophase 1 - Cytokinesis occurs and 2 daughter cells are formed, with one chromosome from each homologous pair going into each cell.

Meiosis 2

Prophase 2 - DNA does not replicate.

Metaphase 2 - The chromatids line up at the equator, and spindle fibres attach to the centromere.

Anaphase 2 - Chromatids are pulled apart and pulled towards opposite poles of the cell.

Telophase 2 - Cytokinesis occurs, splitting each of the daughter cells into 2, resulting in 4 haploid ‘granddaughter’ cells being produced, each with 23 chromosomes. These cells are not genetically identical and have no homologous pairs.

Mutations

A mutation is a change to the base sequence in DNA.

Mutations are almost always harmful as they interrupt the normal function of a protein.

Mutations caused in DNA replication are spontaneous.

Mutagenic agents can increase the chance of a mutation occurring - chemicals ,exposure to ionising radiation (UV)

Substitution mutation

Substitution mutations are where one or more bases are changed in the DNA sequence.

e.g. ATC → ACC

The proteins are typically still functional:

• degenerate code - different triplets can code for the same amino acids

• if the amino acid is different, it may be similar / will make a small difference to the protein structure.

Deletion mutation

Deletion mutations are where one or more bases are removed from the DNA sequence.

e.g. ATC → XTC

This causes a shift in the triplet codons.

Addition mutation

Addition mutations are where one or more bases are added to the DNA sequence.

e.g. ATC → ATCT

This causes a frame shift of the triplet codons.

Chromosome mutation

Changes in the structure or number of chromosomes are called chromosome mutations.

These can occur spontaneously during meiosis:

• Changes in the whole set of chromosomes- occur when organisms have 3 or more sets of chromosomes rather than 2. This is referred to as polyploidy

• Changes in the number of individual chromosomes- sometimes individual homologous pairs fail to separate during meiosis. This failed process is called non-disjunction