MEIOSIS

Why Meiosis? Meiosis is a form of cell division that produces haploid cells. A haploid cell contains one full set of chromosomes. In human, the haploid cells are the sex cells – sperm & egg – which contain 23 chromosomes. Sex cells = gametes = germ cells. A regular body cell is a diploid cell that contains two full sets of chromosomes – one from mom and one from dad. The number of chromosomes in a human diploid cell is 46 (2 x 23). Meiosis uses diploid body cells to create the haploid sex cells and, for this reason, is known as reduction division.

Meiosis – Same But Different Meiosis has some similarities to mitosis in terms of the actions and movement of the chromosomes within the cell. The main difference between meiosis and mitosis is that meiosis has two rounds of division and creates four haploid sex cells while mitosis has only one round of division and gives way to two identical, diploid, daughter cells.

The Stages of Meiosis The names of the stages of meiosis are the same as those of mitosis except that there are two rounds of division so you must indicate which division (I or II) you are dealing with. This means that you have Prophase I in the first round of division and Prophase II in the second. Overall… Inter  PI  MI  AI  TI  PII  MII  AII  TII  Inter

Interphase  Prophase I DNA as chromatin. Nucleus in place. Centrioles present but not at poles. Nucleolus present. DNA = chromosomes. Chromosomes in groups of four called tetrads. Nucleus dissolving. Centrioles migrating to the poles of the cell.

Metaphase I  Anaphase I Centrioles at poles of cell. Spindle formed – microtubules attached to tetrads. Nucleus gone. Tetrads are pulled apart. (4  2 pairs)

Telophase I  Prophase II Chromosomes pairs are at opposite ends of cells. Cytokinesis takes place. Cytokinesis is complete. Centrioles migrate to opposite poles.

Metaphase II  Anaphase II Chromosome pairs line up in middle of cell. Centrioles are at poles of cell and form the spindle. Chromosome pairs are pulled apart and moved to opposite ends of cell.

Telophase II  Sex Cells Nucleus begins to reform. Cytokinesis takes place. Four haploid sex cells. Remember… Sex cells = gametes = germ cells

Meiosis Overall

Males vs. Females MEN Spermatogenesis Starts at puberty Testes Four sperm cells Can produce sperm until death WOMEN Oogenesis Starts at puberty Ovaries One egg & three polar bodies Stops at menopause

Males vs. Females

Meiosis vs. Mitosis

MEIOSIS Sex cells Two rounds of division Haploid cells Results in four cells Genetically different Tetrads present MITOSIS Body cells One round of division Diploid cells Results in two cells Genetically identical No tetrads here

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