a type of cell division that happens in the reproductive organs to produce gametes.
Meiosis involves a reduction division.
Cells that divide by meiosis are diploid to start with, but the cells that are formed from meiosis are haploid - the chromosome number halves.
Cells formed by meiosis are all genetically different because each new cell ends up with a different combination of chromosomes.
Meiosis involves two divisions - meiosis 1 and meiosis 2. After interphase, the cells enter meiosis 1
prophase 1
The chromosomes condense, getting shorter and fatter. Homologous chromosomes pair up Crossing-over occurs. Just like in mitosis, centrioles start moving to opposite ends of the cell, forming the spindle fibres. The nuclear envelope (the membrane around the nucleus) breaks down.
Metaphase 1
The homologous pairs line up across the centre of the cell and attach to the spindle fibres by their centromeres.
Anaphase 1
The spindles contract, pulling the pairs apart (one chromosome goes to each end of the cell).
Telophase 1
A nuclear envelope forms around each group of chromosomes. Cytokinesis (division of the cytoplasm) occurs and two haploid daughter cells are produced.
meiosis 1
A) Prophase 1
B) metaphase 1
C) anaphase 1
D) telophase 1
E) halved
The two daughter cells undergo prophase 2, metaphase 2, anaphase 2, telophase 2 (and cytokinesis) - these are pretty much the same as the stages in meiosis 1, except with half the number of chromosomes. In anaphase 2, the sister chromatids are separated- each new daughter cell inherits one chromatid from each chromosome. Four haploid daughter cells are produced.
meiosis 2
A) 4 haploid daughter cells
B) haploid daughter cells
The reason meiosis is important is that it creates genetic variation - it makes gametes that are all genetically different. Then during fertilisation, any egg can fuse with any sperm, which also creates variation. This means new individuals have a new mixture of alleles, making them genetically unique.
