Meiosis Meiosis is the type of cell division for that produces the reproductive cells (egg or sperm) which are also known as gametes. Two important characteristics of meiosis is that it reduces the number of chromosomes to half of the parent cell and it creates genetic diversity.

Somatic Cells vs. Gametes Somatic cells are body cells (nerve cells, muscle cells, epithelial cells…). These cells all have different appearances and functions but have the same genetic information. Gametes are the reproductive cells - the egg or the sperm. Gametes only have half as many chromosomes as somatic cells.

If Meiosis Did NOT Occur If two parental cells combined that had the parents full set of 46 chromosomes: The first generation of offspring would have 92 chromosomes. The next generation of offspring would have 184 chromosomes…

Homologous Chromosomes Most cells in a human (and in most eukaryotic organisms) contain two copies of each chromosome. These chromosomes are not identical (one is from the mother and one from the father), but they contain genes for the same traits. These pairs are called homologous chromosomes. Homologous chromosomes may contain different forms of a gene for the same trait. Brown eyes Green eyes

Humans have 23 pairs of chromosomes, for a total of 46 chromosomes in each somatic (body) cell. Autosomal chromosomes: Chromosomes 1-22, which form homologous pairs in both males and females. Sex chromosomes: Chromosome 23. Determines gender (XX= female, XY= male). For males, this pair is not homologous (different length and type of genes).

Diploid and Haploid Cells Diploid cells have homologous pairs of chromosomes. Human somatic (body) cells are diploid. (di = 2) Haploid cells just have one copy of each chromosome. Gametes (egg and sperm) cells are haploid. When fertilization occurs (fusion of egg and sperm) the diploid number is restored.

Diploid cells are represented by 2n and haploid by n, where n is the number of different chromosomes. Examples: Organism Gamete (n) Body cell (2n) Human 23 46 Pea Plant 7 14 Fruit Fly 4 8 Dog 39 78 Indian Fern 630 1260

Chromosome number in different organisms http://morgan.rutgers.edu/morganwebframes/level1/page2/ChromNum.html

Pair-Share Compare and contrast mitosis with meiosis. What are homologous chromosomes? What kind of cells have homologous chromosomes? What do the terms diploid and haploid mean? What kind of cells are diploid? What kind of cells are haploid?

Interpreting Genetics Diagrams Homologous chromosomes are indicated by length. Sister chromatids (identical copies produced during S phase of interphase) are shown connected by centromere. Genes on the chromosome are represented with letters. Example: R/r represents gene for pea seed shape Different forms of the same gene are represented with capital versus lower case letters. Example: R represents round shape form of the gene, r represents wrinkled shape form of the gene.

Overview of Meiosis In meiosis, one replication of chromosomes is followed by 2 cell divisions, to produce 4 haploid cells.

Phases of Meiosis As with mitosis, meiosis is preceded by an interphase period, during which the chromosomes are duplicated. The two sister chromatids are identical copies. The homologous pairs have the same genes, but could have different alleles (forms of the gene). At the start of meiosis, there are 4 copies of each gene.

During Prophase I, the chromosomes condense to become visible During Prophase I, the chromosomes condense to become visible. HOMOLOGOUS PAIRS JOIN TOGETHER TO FORM TETRADS (4 STRANDS). (Homologous pairs never form tetrads during mitosis). Prophase I

Metaphase I In metaphase I, the HOMOLOGOUS PAIRS line up along the middle of the spindle.

Anaphase I: HOMOLOGOUS PAIRS separate. Telophase I Nuclei reform, producing 2 haploid daughter cells. But each cell still has 2 sister chromatids for each chromosome.

Telophase/ cytokinesis II Meiosis II separates the sister chromatids. Prophase II Metaphase II Telophase/ cytokinesis II Anaphase II

Meiosis Video Click once image to start http://www.youtube.com/watch?v=D1_-mQS_FZ0

Meiosis Animation http://www.cellsalive.com/meiosis.htm

Genetic Variation from Meiosis Meiosis is important for providing genetic variation because it allows for genetic recombination- new combinations of genes. Parental combinations Recombinants

Sources of Genetic Variation from Meiosis Independent Assortment: Each homologous pair lines up independently on the spindle in Meiosis I. So all of the possible arrangements of chromosomes are equally likely. In humans there are 223 possible chromosome combinations from each parent.

If an organism has gene forms Y and y on one homologous pair, and R and r on a different homologous pair, then due to independent assortment the gametes produced will be 25 % YR, 25% yr, 25% Yr and 25% yR

Independent Assortment Video Click once image to start http://www.youtube.com/watch?v=kZyK2eWhpfQ

http://www. sumanasinc http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html

Sources of Genetic Variation from Meiosis cont. Crossing Over: During prophase I, strands of non-sister chromatids can be exchanged while the homologous pair are joined in tetrads. Crossing over allows recombination of genes on the same chromosome.

Crossing Over Video Click once on image to start http://www.youtube.com/watch?v=op7Z1Px8oO4

Crossing over animation

Comparing Mitosis and Meiosis Purpose Location # of DNA replications # of cell divisions #/type of cells produced Growth and Repair Gamete Production Throughout body Only in germ cells 1 1 1 2 4 haploid cells 2 diploid cells

Comparing Mitosis and Meiosis Video Click once on image to start http://www.youtube.com/watch?v=Ba9LXKH2ztU

Comparing Mitosis and Meiosis Cont. NOVA site comparing mitosis and meiosis.

M E I O S Mnemonic for Meiosis akes ggs n varies perm n permatocytes (precursor cells for male gametes)

Spermatogenesis: produces four equal sperm cells through meiosis Spermatogenesis: produces four equal sperm cells through meiosis. A greater quantity of sperm is produced, but each sperm has less stored energy. Oogenesis (egg production): meiotic division produces 1 larger egg which receives most of the organelles and cytoplasm of the parent cell. The other three cells produced in the meiotic division are smaller polar bodies. Fewer eggs need to be produced so more energy is invested per egg (mitochondria etc.).