Meiosis Unit 4

Mendel’s principles of genetics Each organism must inherit a single copy of every gene from both “parents” When an organism produces its own gametes, those two sets of genes must be separated from each other so that each gamete only contains 1 set of genes.

Chromosome Number Example: Drosophila Has 8 chromosomes 4 from each parent The two sets are homologous, they have a corresponding chromosome from each parent. A diploid cell contains both sets of homologous chromosomes (2N) The gametes of a sexually reproducing organism has only 1 set of chromosomes making them haploid (N)

Phases of Meiosis Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell Meiosis has two stages Meiosis I Meiosis II

Meiosis I Prior to meiosis I each chromosome is replicated Prophase I Each chromosome pairs up with its corresponding homologous chromosome to form a tetrad (4 chromatids in a tetrad) Chromosomes may exchange portions of chromatids in crossing over results in new combinations of alleles

Meiosis I Metaphase I Spindle fibers attach to the chromosomes

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 2 cells. This results in 2 haploid daughter cells, each with ½ the number of chromosomes as the original cell.

Meiosis II The two cells produced by meiosis I now enter a second meiotic division. Unlike meiosis I, neither cell goes through chromosome replication. Each of the cell’s chromosomes has 2 chromatids.

Meiosis II Prophase II Spindle fibers form

Meiosis II Metaphase II The chromosomes line up in the center of cell.

Meiosis II Anaphase II The sister chromatids separate and move toward opposite ends of the cell.

Meiosis II Telophase II and Cytokinesis Nuclear membranes form and each cell separates into 2 new daughter cells. Meiosis II results in 4 haploid daughter cells.

Gamete Formation Female Gametes Male Gametes 4 equal-sized sperm cells Female Gametes 1 large egg cell 3 polar bodies

Comparing Mitosis & Meiosis Cells produced by mitosis have the same number of chromosomes and alleles as the original cell. Mitosis allows an organism to grow and replace cells. Some organisms reproduce asexually by mitosis. Mitosis results in the production of two genetically identical diploid cells. Meiosis Cells produced by meiosis have half the number of chromosomes as the parent cell. These cells are genetically different from the diploid cell and from each other. Meiosis is how sexually reproducing organisms produce gametes. Meiosis produces four genetically different haploid cells.

Review During meiosis, the number of chromosomes per cell is cut in half through the separation of Daughter cells Homologous chromosomes Gametes Chromatids

The formation of a tetrad occurs during Anaphase I Metaphase II Prophase I Prophase II

If a human body cell contains 46 chromosomes, a single sperm cell should have Between 23 and 46 chromosomes

In many female animals, meiosis results in the production of Only 1 egg 1 egg and 3 polar bodies 4 eggs 1 egg and 2 polar bodies

Compared to egg cells formed during meiosis, daughter cells formed during mitosis are Genetically different, while eggs are genetically identical Genetically different, just as egg cells are Genetically identical, just as eggs cells are Genetically identical, while egg cells are genetically different