Biology Unit 2: Meiosis and Genetic Variation

Gametes are the sex cells - sperm and eggs.

They join at fertilisation to form a zygote, which divides and develops into a new organism.

Normal body cells have the diploid number of chromosomes - each cell contains two of each chromosomes - one from each parent.

Gametes have a haploid number of chromosomes - only one copy of each chromosome.

At fertilisation, a haploid sperm fuses with a haploid egg, making a cell with the normal diploid number of chromosomes.

Gametes are formed by meiosis.

1. The DNA unravels and replicates so there are two copies of each chromosome, called chromatids.

2. The DNA condenses to form double-armed chromosomes, made from two sister chromatids.

3. Meiosis I - the chromosomes arrange themselves into homologous pairs.

4. These pairs are then separated, halving the chromosome number.

5. Meiosis II - the pairs of sister chromatids that make up each chromosome are separated.

6. Four haploid cells (gametes) that are genetically different from eachother are produced.

During Meiosis I, homologous pairs of chromosomes come together and pair up.

The chromatids twist around each other and bits of chromatids swap over.

The chromatids still contain the same genes, but now have a different combination of alleles - resulting in greater variation.

1. The four daughter cells formed from meiosis have completely different combinations of chromosomes.

2. When gametes are produced, different combinations of maternal (from the mother) and paternal (from the father) chromosomes go into each cell.

3. This is called independant segregation (separation) of the chromosomes.

Good luck on your exams!