Meiosis and Sexual Reproduction Mrs. Cook Biology

What makes you-you?? How can two children look different and act different but come from the genetically same mom and dad???

Formation of Haploid Cells Some organisms reproduce by joining gametes to form the first cell of a new individual. The gametes are haploid- they contain one set of chromosomes.

Formation of Haploid Cells Meiosis- is a form of cell division that halves the number of chromosomes when forming specialized reproductive cells, such as gametes in animal or spores in plants. Involves two divisions of the nucleus Meiosis I Meiosis II

Formation of Haploid Cells Before meiosis begins, the DNA in the original cell is replicated- the cell undergoes G 1, S, & G 2 of Interphase I. Meiosis starts with homologous chromosomes.

Formation of Haploid Cells Meiosis I Prophase I DNA coils into chromosomes Spindle Fibers appear Nucleolus and Nuclear membrane dissolve Every Chromosome lines up next to its Homologue- This is called Synapsis.

Formation of Haploid Cells Each pair of Homologous Chromosomes is called a Tetrad. In the Tetrad formation, Chromatids are aligned lengthwise so that the genes on one chromosome are adjacent to the corresponding gene on the other chromosome.

Formation of Haploid Cells During Synapsis, the Chromatids in a homologous pair twist around one another and portions break off and reattach to the other chromatids in a process called Crossing-Over. Crossing-Over allows the exchange of genetic materials between maternal and paternal chromosomes. Genetic Recombination is when a NEW mixture of genetic material is created.

Crossing over and Genetic Recombination

Formation of Haploid Cells Metaphase I Tetrads line up randomly along the equator Spindle fibers from one pole attach to the centromere of one of the homologous chromosome, and fibers from the other pole attach to the other homologue.

Formation of Haploid Cells Anaphase I Each homologous chromosome (each with two chromatids attached by a centromere) moves to opposite poles. The chromatids DO NOT separate at their centromeres- Yet. The genetic material, however, has recombined.

Formation of Haploid Cells Telophase I During Telophase I, the chromosomes reach opposite poles and cytokinesis begins. The new cells contain a haploid number of chromosomes

Formation of Haploid Cells At the end of Meiosis I, the original cell produces two new cells each containing one chromosome from each homologous pair. The new cell contains half the number of chromosomes of the original cell But each new cell contains two copies (as chromatids)

Formation of Haploid Cells Meiosis II Chromosomes do not replicate between meiosis I and meiosis II Prophase II Spindle fibers form and begin to move the chromosomes toward the equator.

Formation of Haploid Cells Metaphase II The chromosomes line up at the equator with each chromatid facing opposite poles.

Formation of Haploid Cells Anaphase II The chromatids separate at the centromere and move toward opposite poles.

Formation of Haploid Cells Telophase II A nuclear membrane forms around the chromosomes in each of the four new cells. Cytokinesis II results in four new genetically different, Haploid Cells.

Formation of Haploid Cells

Meiosis and Gamete Variation Meiosis is an important process that allows for the rapid generation of new genetic combination. Three mechanisms make key contributions to genetic variation: 1.Independent assortment 2.Crossing over 3.Random fertilization

Meiosis and Gamete Variation Most organisms have more than one chromosome. Human gametes receive 1 chromosome from each of the 23 pairs of homologous chromosomes.

Meiosis and Gamete Variation Independent Assortment- Which of the 2 chromosomes from the 23 pairs a gamete receives is by chance. The random distribution of homologous chromosomes during meiosis is called Independent Assortment.

Meiosis and Gamete Variation To find the possible combinations of Chromosomes in each cell you take 2 n (2 because we have 2 copies of each chromosome. n= the number of different chromosomes.) Examples: Find the possible number of chromosomes combinations in a cell that starts with 4 pairs of chromosomes. 2 4 = 2 * 2 * 2 * 2= 16 possible combinations Cell with 8 chromosomes? 2 8 = 2 * 2* 2* 2* 2* 2* 2* 2= 256 possible combinations

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Meiosis and Gamete Variation Crossing-over and Random Fertilization The DNA exchange during crossing-over adds even more recombination to the independent assortment of chromosomes. This makes the number of genetic combinations that can occur among gametes practically unlimited. The zygote that forms a new individual is created by the random joining of two gametes. It is no wonder we are all so different!

Meiosis and Gamete Formation Meiosis in Males- Sperm cells are formed by meiosis in the testes. This process is called Spermatogenesis. A diploid cell first increases in size & becomes a large immature cell (germ cell.) The germ cell undergoes meiosis I. Two cells are produced and undergo meiosis II. A Total of 4 haploid cells are produced The 4 cells change in form and produce a tail to become male gametes and called Sperm.

Meiosis and Gamete Formation Meiosis in Females- Egg cells are formed by meiosis in the ovaries and is called Oogenesis. A diploid reproductive cell divides by meiosis to produce one mature egg cell (ovum) and three polar bodies. Diploid Reproductive cells go through Meiosis I even before the female is born. They remain in a resting state until she reaches maturity, then 1 cell goes through Meiosis II once a month during ovulation.

Meiosis and Gamete Formation During Cytokinesis, the cytoplasm divides unevenly, so only one actual egg cell is produced. The polar bodies eventually degenerate. The larger cell develops into a gamete called an ovum (plural is ova), commonly called an egg. The ovum has rich storehouse of nutrients to nourish the young organism that develops if fertilization occurs.

Sexual Reproduction Sexual Reproduction is the production of offspring through meiosis and the union of sperm and egg cells. Offspring will be genetically different from parents because of crossing-over and independent assortment.