Meiosis Section 8-3
Meiosis process of nuclear division that reduces the number of chromosomes in new cells to half the number in the original cell For example, in humans, meiosis produces haploid reproductive cells called gametes. Human gametes are sperm and egg cells, each which contain 23(1n) chromosomes. The fusion of the sperm and egg results in a zygote that contains 46 (2n) chromosomes
Stages of Meiosis Cells preparing to divide by meiosis undergo the G 1, S, and G 2 phases of interphase Cells begin meiosis with a duplicate set of chromosomes (due to interphase where cells grow to mature size and copies DNA) Because cells undergoing meiosis divide twice, diploid (2n) cells that divide meiotically result in four haploid (1n) cells rather than two diploid (2n) cells.
Meiosis I
Meiosis I (first stage of cell division) At the start of meiosis I, each chromosomes consists of 2 strands of sister chromatids connected at the centromere Homologous pairs of chromosomes come together before meiosis begins. This event is called synapsis Each pair of homologous chromosomes is called a tetrad
Meiosis I (first stage of cell division) ****PROPHASE I*** Portions of chromatids may break off and attach to adjacent chromatids on the homologous chromosomes a process called crossing-over Permits the exchange of genetic material between maternal and paternal chromosomes Crossing-over results in genetic recombination by producing a new mixture of genetic material
Prophase I con t. Each pair consists of four chromatids, because each chromosome in the pair had replicated before meiosis began The nucleoli and the nuclear envelope disappear and the spindle fibers form
Metaphase I Tetrads line up randomly along the middle of the dividing cell Orientation of the homologous pair of chromosomes is random with respect to the poles of the dividing cell Spindle fibers from one pole attach to the centromere of one homologous chromosomes Spindle fibers from opposite pole attach to the other homologous chromosome of the pair
Anaphase I Homologous pairs of chromosomes separate from each other. Spindle fibers pull one member from each pair to opposite ends of the cell Random separation of the homologous chromosomes is called independent assortment
Telophase I Final stage of meiosis I Cytokinesis takes place; each new cell is haploid, containing one chromosome from each pair.
Meiosis II Occurs in each cell formed during meiosis I and is not preceded by the copying of DNA Each diploid cell from meiosis I will go through a second division, forming the four gametes haploid cell
Prophase II Spindle fibers form and begin to move to the chromosomes toward the midline of the dividing cell
Metaphase II The chromosomes move to the midline of the dividing cell, facing opposite poles of the dividing cell
Anaphase II Chromatids separate and move toward opposite poles of the cell
Telophase II Nuclear membrane forms around the chromosomes in each of the four new cells
Cytokinesis II Occurs during telophase II, resulting in four new cells, each of which contains half of the original cell s number of chromosomes
Formation of Gametes In animals, meiosis produces haploid reproductive cells called gametes Only occurs within their reproductive organs In humans, meiosis occurs in the testes and in the ovaries Testes production of male gametes known as sperm cells. Diploid reproductive cell divides meiotically to form four haploid cells called spermatids. Production of sperm cells is called spermatogenesis
Oogenesis is production of mature egg cells Diploid reproductive cell divides meiotically to produce one mature egg cell During cytokinesis I and II of oogenesis, cytoplasm of the original cell is divided unequally between new cells One egg cell is produced by meiosis. Other three products of meiosis called polar bodies, degenerate (break down)
Asexual and Sexual Reproduction Asexual reproduction production of offspring from one parent Does not usually involve meiosis or union of gametes Offspring are identical genetically to the parent Sexual reproduction offspring through meiosis and the union of a sperm and an egg Offspring are genetically different from the parents because genes are combined in new ways in meiosis Sexually produced offspring contain unique combinations of their parents genes (except identical twins)