CHAPTER 11 MEIOSIS

Meiosis and sexual reproduction Gametes are sex cells of complex organisms. They are formed by meiosis Sex cells are haploid(monoploid) in chromosome number. That is they would be represented by the letter “n”. In humans “n” would equal 23. Remember all other cells in the body are diploid and are represented by “2n” where 2n = 46 in humans.

Meiosis and sexual reproduction Sperm and egg cells are haploid cells and their nuclei fuse to form a zygote at fertilization. A zygote is a diploid cell which is the result of the fusion or joining of the nuclei of 2 gametes or the fusion of the egg and sperm. When the nuclei of the two gametes fuse, the process is called fertilization. The single cell formed as a result of fertilization is called the zygote.

Meiosis and sexual reproduction Remember somatic cells are body cells – 2n or diploid chromosome number. They are never the cells involved in sexual reproduction. We have 23 pairs of chromosomes in our somatic cells. The chromosomes that make up each pair are called homologous chromosomes

46 human chromosomes

Meiosis and sexual reproduction Homologous chromosomes: –Pairs of chromosomes in body or somatic cells –Similar in size and shape –Carry information for the same traits –Not found in gametes –Same staining patterns

Meiosis and sexual reproduction Diploid refers to cells that have all the homologous chromosomes that are characteristic of the species. “2n” Haploid (monoploid) “n” cells with only one chromosome from each pair. Meiosis produces gametes in humans and spores in plants. Meiosis guarantees half the number of chromosomes that are present in the parent cells.

Meiosis and sexual reproduction Meiosis is a reduction division. –Beginning with a cell that has a 2n chromosome number – each cell divides twice There are 2 stages to meiotic cell division –Meiosis I –Meiosis ll

Meiosis and sexual reproduction First division: –Prophase I –Metaphase I –Anaphase I –Telophase I Interkinesis Second division –Prophase II –Metaphase II –Anaphase II –Telophase II »cytokinesis

Meiosis and sexual reproduction First division: –Prophase I: Chromosomes already replicated Each pair of chromatids line up with its homologous pair. They fasten at their centromeres Synapsis takes place - Process when homologous pairs of chromosomes are paired and fastened at their centromeres forming tetrads. Four chromatids = one tetrad Crossing over takes place – when the strands of a tetrad twist about each other and exchange segments; Nuclear membrane disappears Spindle fibers form End of prophase – tetrads move toward equator of cell

Meiosis and sexual reproduction Metaphase I –Tetrads fastened to spindle microtubules at their centromeres –Centromeres of tetrads line up on the equator

Meiosis and sexual reproduction Anaphase I – homologous chromosomes separate from each other and move to opposite ends of the cell. –Process of separation of homologous chromosomes is called disjunction. »Failure of the chromosomes to separate correctly is called non disjunction

Meiosis and sexual reproduction Telophase I– –Marks the end of the first meiotic division –Cytoplasm divides (cytokinesis) – daughter cells have half the number of chromosomes as the parent cells –Nuclear membrane forms – cells either go through a brief interphase ( interkinesis) or go right to the second division.

Meiosis and sexual reproduction Second meiotic division: Prophase II – –Each daughter cells forms a spindle –Double stranded chormosomes move toward the middle of the spindle Metaphase II – –Chromosomes fasten to spindle at centromeres –Centromeres line up on equator –Chromosomes are still 2 strands

Meiosis and sexual reproduction –Anaphase II Centromeres divide and chromatids separate Single stranded chromosomes move to opposite ends of spindle Telophase II –Daughter cells divide resulting in 4 haploid cells –All chromosomes return to interphase state –Nuclear membrane forms