meiosis

• meiosis is a type of cell division
• takes place in reproductive organs
• cells that divide by meiosis are diploid to start with, but the cells that are formed from meiosis are haploid - the chromosome number halves
• 1. before meiosis starts the DNA unravels and replicates so there are two copies of each chromosome, called chromatids
• 2. the DNA condenses to form double-armed chromosomes, each made from two sister chromatids, joined by a centromere
• 3. meiosis I (first division) - the chromosomes arrange themselves into homologous pairs
• 4. these homologous pairs are then separated, halving the chromosome number
• 5. meiosis II (second division) - the pairs of sister chromatids that make up each chromosome are separated (the centromere is divided)
• 6. four haploid cells (gametes) that are genetically different from each other are produced
• homologous pairs - humans have 46 chromosomes, 23 pairs. one chromosome in each pair came from each parent. the chromosome that make up the pair are the same size and have the same genes, although they could have different versions of those genes
• homologous pairs have genes positioned at the same loci on the same chromosome
• alleles - different versions of the same gene
• chromatids cross over in meiosis I