AS Biology: Meiosis and Genetic Variation

Meiosis is a type of cell division.

The products of meiosis: Four daughter cells each with half the number of chromosomes as the parent cell. Meiosis is necessary as it maintains the same number of chromosomes for all adults of a species.

If this division didn't occur, when gametes fuse at fertilisation, the cell would have 92 chromosomes. This doubling would continue onto the next generation.

Every diploid cell of an organism has one set of chromosomes from each parent.

To ensure that there is the diploid number of chromosomes when two gametes fuse at fertilisation, each gamete must have the haploid number of chromosomes.

This is achieved as a result of meiosis:

Chromosome pairs seperate so that only one chromosome from each pair enters each gamete (haploid number). When these fuse, the diploid number is restored.

Meiosis is broken down into two nuclear divisions:

1

-Homologous chromosomes pair up

-Their chromatids wrap around eachother (a process called crossing over)

-Equivalent portions of the chromatids are exchanged

-By the end, the homologous chromosomes will have seperated

-Each chromosome going into one of the two daughter cells

2

-The chromatids seperate and move apart

-Each will be contained in one of the four daughter cells (each cell containing 23 chromatids). Each are genetically different

Meiosis produces genetic variation in two ways:

Independent segregation of homologous chromosomes:

-During meiosis 1, chromosomes line up alongside it homologous pair

-In humans, this means we have 23 pairs of chromosomes lined up inside the cell

-The way in which they line themselves up is totally random

-One chromosome from each pair is then passed into one of the two daughter cells

-Which one of each pair goes into the cell, and with which one of any other pair, depends on how the pairs are lined up

-Since this is random, the combination of chromosomes that go into each daughter cell is also completely random.

-INDEPENDENT SEGREGATION

-Each pair of homologous chromosomes has exactly the same genes, therefore determining the same characteristic

-However the alleles of these genes may differ

-The random distribution of the chromosomes therefore produces new genetic combinations

Genetic recombination by crossing over

-During meiosis 1, chromosomes line up alongside their homologous partner

-The chromatids of the chromosomes begin to twist around one another

-The tension that is created causes portions of the chromatids to break off

-These portions then rejoin with the chromatids of its homologous partner

(usually equivalent portions are swapped)

-This causes new genetic combinations to be produced