Meiosis

Meiosis: An Introduction An organism must inherit a single copy of every gene from its parents. Meiosis –number of chromosomes are cut in half through the separation of homologous chromosomes in a diploid cell.

Chromosome Number Homo= same Different organisms have different numbers of chromosomes. Humans have 46 chromosomes, 22 pairs of autosomes and one pair of sex chromosomes. Homologous chromosomes- A matching pair (or set) of chromosmes (one from each parent) Homo= same

Homologous chromosomes vs Sister chromatids Homologous chromosomes are separate chromosomes Sister chromatids are attached by the centromere.

The “Diploid” Condition A cell that contains both sets of homologous chromosomes is said to be diploid.= 2N For humans, the diploid number is 46, which can be written as 2N=46. Diploid

The “Haploid” Condition Meiosis results in the production of haploid gametes (sex cells) which have only 23 chromosomes. The diploid cell that entered meiosis has become 4 haploid cells (N).

Meiosis I Interphase I Interphase I Cells undergo a round of DNA replication, forming duplicate chromosomes.

Meiosis I: Prophase I During Prophase I each chromosome pairs with its homologous chromosome forming a tetrad. There are 4 chromatids in a tetrad. Prophase I Tetrad

Meiosis I: Synapsis Tetrads formed during Prophase I exchange portions of their chromatids in a process called crossing over. Increase variation in multicellular organisms. Tetrad Chromatid Crossing-over occurs during meiosis. (1) Homologous chromosomes form a tetrad. (2) Chromatids cross over one another. (3) The crossed sections of the chromatids are exchanged. Allele 10

Meiosis I: Metaphase I How is this different from Mitosis? During Metaphase I, the 23 homologous pairs (tetrads) line up along the equator of the cell. Metaphase I

Meiosis I: Anaphase I Homologous pairs separate and move toward the poles of the cell. The chromatids are still in tact at the centromere. Anaphase I

Telophase I and Cytokinesis Meiosis: Telophase I Nuclear membranes form and the spindle breaks down. Cleavage results in two cells. The two haploid (N) cells produced by meiosis I have chromosomes from each homologous pair. The cells are different from each other. Telophase I and Cytokinesis

Meiosis II The 2 haploid cells produced by meiosis I now enter a second meiotic division which results in the production of 4 genetically different haploid cells.

Meiosis II Telophase II and Cytokinesis II Telophase I and Cytokinesis I Prophase II Metaphase II Anaphase II

Meiosis II During Prophase II, spindle fibers reform and move chromosomes toward cell equator. 16

Meiosis II During Metaphase II Chromosomes move to Midline. 17

Meiosis II During Anaphase II, chromatids separate and move toward opposite poles of the cell. 18

Meiosis II During Telophase II, the nuclear membrane reforms around chromosomes in each of the four new cells, and a second round of cytokinesis produces four new haploid cells

Sperm or Ovum 20

Gamete Development… Spermatogenesis: four new cells from meiosis called spermatids in males develop into mature sperm cells

Gamete Development (cont.) Oogenesis: During cytokinesis following meiosis in females, cytoplasm is divided unequally among the four new cells. One cell receives most of the original cell’s cytoplasm and develops into a mature egg called an ovum. The other three cells called polar bodies die off.

Gamete Development (cont.)

Sexual Reproduction When the nuclei of the two gametes combine during fertilization, the 2n number is restored (2n = 46). The fusion of two gametes produces the first cell of the new animal called a zygote.

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