MEIOSIS

=Meiosis= Meiosis is a type of cell division that generates cells with half the normal number of chromosomes for sexual reproduction. Meiosis is plays an important role in generating genetic diversity in species that reproduce sexually. *Purpose: to make gametes (sex cells) such as sperm & ovum (egg cells) in humans. *Where: specialised germ cells found in sex organs (testes/ovaries) *Produces: 4 x haploid (n) daughter cells that are genetically different [image:http://i.imgur.com/oWYpsFM.png?4] '''Prophase I''' Chromosomes condense and become visible under a light microscope. Homologous chromosomes can exchange segments in a process referred to as recombination (crossing over). This contributes to genetic diversity as recombination can result in chromosomes that carry unique combinations of alleles [see recombination]. The nuclear membrane disintegrates and spindle fibres begin to form. '''Metaphase I''' Homologous chromosomes line up in pairs across the centre of the cell. The arrangement of each pair (which is on the left vs right) is random and therefore independent of the other homologous pairs. This is known as independent assortment and also contributes to genetic variation as it means each of the resulting sex cells will inherit unique combination of chromosomes and therefore alleles [see independent assortment]. During metaphase the chromosomes attach to the spindle fibres '''Anaphase I''' Homologous chromosomes are pulled apart (this is known as disjunction). The cell elongates (lengthens) in preparation for division down the centre. '''Telophase I''' Homologous chromosomes reach each end of the cell. The spindle fibres disappear and a nuclear membrane forms around the chromosomes at each end of the cell. The chromosomes uncoil and the cell membrane begins to pinch off (in animals cells) and the cell divides. '''Prophase II''' The nuclear envelope disintegrates again. Chromosomes condense (coil up tightly) and spindle fibres form. '''Metaphase II''' Chromosomes attach to spindle fibres and line up across the centre of the cell. '''Anaphase II''' The Chromosomes which consist of two identical copies called sister chromatids divide. The sister chromatids are pulled to opposite ends of the cell. '''Telophase II''' The chromosomes uncoil and the spindle fibres disappear. The Nuclear membrane reforms and the cells divide. Meiosis is now complete and ends up with four new daughter cells. ''note that students studying NCEA are not required to learn the names of each phase''