Meiosis

Cancer -cells lose the ability to control growth. Internal regulator – proteins that respond to events inside the cell. Allow the cell cycle to proceed only when certain processes have happened inside the cell. External regulator – proteins that respond to events outside the cell. Direct cells to speed up or slow down the cell cycle.

Meiosis Sex cell division -the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell. -involves 2 divisions Meiosis I and Meiosis II 1 cell turns into 4 cells

Meiosis I -prior to this step the chromosomes are duplicated.

Meiosis I Prophase I: each chromosome pairs with its corresponding homologous chromosome to form a structure called a tetrad. (4 chromatids in a tetrad) Synapsis – pairing of homologous chromosomes. chromatids twist around each other and switch chromosomes with each other called. crossing over.

Meiosis I Metaphase I: Spindle fibers attach to the chromosomes. Each composed of 2 chromosomes (four chromatids) in center of cell.

Meiosis I Anaphase I: The fibers pull the homologous chromosomes toward opposite ends of the cell.

Meiosis I Telophase I and Cytokinesis: nuclear membranes form. The cell separates into two cells.

Meiosis II *No chromosome replication in this division. (No S Phase) Prophase II: 2 cells with half the number of chromosomes as the original cell.

Meiosis II Metaphase II: The chromosomes line up in a similar way to the metaphase stage of mitosis. Anaphase II: sister chromatids separate and move toward opposite ends of the cell.

Meiosis II Telophase II: and Cytokinesis: Meiosis II results in four haploid (N) daughter cells.

The Difference Between Mitosis and Meiosis Mitosis results in the production of two genetically identical diploid cells, whereas meiosis produces four genetically different haploid cells. Cells produced in Meiosis are sperm and egg cells. When the egg is fertilized the new organism has the whole set of chromosomes.

Gametes – reproductive cells, haploid. -occurs only in reproductive organs. Testis: -After Meiosis occurs 4 haploid cells are formed called spermatids. -Spermatids develops into a mature sperm cell called. Spermatogenesis. Ovaries: -only one egg cell is made during meiosis. -Oogenesis – production of mature egg cells. -only one cell forms because the cells split unequally, one cell takes most of the cytoplasm. -the other three cells are called polar bodies and they break up and dissolve.

Reproduction Sexual – offspring from 2 parents. Asexual – offspring from one parent. Homework: pg. 156 Questions 1-6

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