Chapter 11 Male gametes Female gametes MEIOSIS Meiosis A process of nuclear division that reduces the number of chromosomes in new cells to half the number in the original cell. For example, in humans, meiosis produces haploid reproductive cells, each called gametes. Human gametes are sperm cells and egg cells, each of which contains 23 (1n) chromosomes. The fusion of a sperm and an egg results in a zygote (diploid cell) that contains 46 (2n) chromosomes. Meiosis I Synapsis the pairing of homologous chromosomes during meiosis. Tetrad a pair of homologous chromosomes Crossing over portions of chromatids break off and attach to adjacent chromatids on the homologous chromosome. This permits exchange genetic material between maternal and paternal chromosomes. Genetic Recombination mixing of genetic material; such as crossing over during meiosis. Prophase I DNA coils tightly into chromosomes. Spindle fibers appear. Nucleus and nucleolus dissemble. Chromosomes line up next to their homologue. Crossing over occurs between homologous chromosomes. Draw a diagram of crossing over using two different colored writing implements. Be sure to label a synapsis, tetrad, chromatid, chromosome, crossing over, and recombinant DNA. Metaphase I Tetrads line up randomly along cell midline. Spindle fibers from one pole attach to centromere of one homologous chromosome. Spindle fibers from opposite pole attach to the other homologous chromosome of the pair Anaphase I Each homologous chromosome moves to an opposite pole. Independent Assortment - random separation of the maternal and paternal chromosomes, which result in more genetic variation. Telophase I Chromosomes reach opposite ends of cells. Cytokinesis I occurs. New cells contain a haploid (1n) number. Draw the first four phases of Meiosis I MEIOSIS II Prophase II Spindle fibers form and begin to move the chromosomes toward the midline of the dividing cell. Metaphase II Chromosomes move to the midline of the dividing cell. Anaphase II Chromatids separate and move toward opposite poles of the cell. Meiosis in a lily, anaphase II. (Stained light micrograph.) [ Clare Hasenkampf/BPS.] Telophase II Nuclear membrane forms around chromosomes in each of the 4 new cells. Cytokinesis II occurs forming 4 new cells. Each cell contains half (haploid, 1n) of the original cell's number of chromosomes. Draw Meiosis II using two different colors of writing implements. Formation of Gametes Spermatogenesis the production of sperm cells. Spermatids - immature sperm cells Oogenesis - production of mature egg cells. During meiosis, one egg cell is produced along with 3 polar bodies. Egg cell receives most of the original cells cytoplasm. Polar Bodies 3 degenerate products of ova meiosis. TOOLS OF THE TRADE Pedigrees and Karyotypes What is a Karyotype? A picture of the homologous (paired) chromosomes of an organism arranged from largest to smallest. It is a tool used to diagnose chromosomal disorders. It is performed on a sample of blood, bone marrow, amniotic fluid or the tissue from the placenta. tests the amniotic fluid. Fluid is drawn from the developing placental sac. The given sample is placed on a tray and allowed to grow in the confines of a laboratory. The cells from the growing sample are then stained. Then scientists take a picture of the chromosomes, cut them out and match them up using size, banding pattern and centromere position as guides. Amniocentesis Are these karotypes normal? If so what gender are they? Types of Human Chromosomal Number Disorders Down Syndrome - Trisomy 21 Edward Syndrome Trisomy 18 Patau Syndrome Trisomy 13 Klinefelter syndrome 47XXY Metafemale 47XXX Turner syndrome Monosomy X (45X0) Jacob syndrome - 47XYY Pedigree It is a diagram of family relationships that uses symbols to represent people, and lines to represent genetic relationships. Modern geneticists study the family history using a pedigree because it is easy to follow and visualize a specific trait within families. Pedigree cont. Pedigrees are often used to determine the mode of inheritance (dominant, recessive, or sex-linked) of genetic diseases. A sample pedigree is shown to the right. SKETCH IT! Squares = males, and circles = females. Horizontal lines connecting a male and female represent mating. Vertical lines extending downward from a couple represent their children. Shapes empty or not shaded =individual not affected (homozygous) Shapes partially shaded = heterozygous or carriers Shapes completely shaded = show individual affected by the trait (homozygous) Roman numbers = generations Arabic numbers = individuals Pedigree cont. A Royal Family History The most famous pedigree comes from a European Royal family. Queen Victoria of England had a large number of children. The blood disease, hemophilia, was passed down through the royal family. Hemophiliacs cannot form blood clots and can bleed to death from something as simple as getting pricked by a rose bush! This genetic disease is caused by a recessive allele on the X chromosome.