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Transcript
Meiosis: Cell division for gamete (sperm or egg) formation. Gametes are genetically unique and contain half the number of chromosomes (haploid - n). In humans, meiosis only occurs in specialized cells in reproductive organs that make the egg or sperm. Fertilization (fusion of an egg and sperm to form a zygote) returns to a cell with a full set of chromosomes (diploid , n + n = 2n) a A Homologous Chromosomes: b B C c - Example: A/a Skin Pigment Normal pigment (A) (a) Albino B/b Eyebrow thickness (B) Bushy/ (b) thin C/c Chin dimples (C) undimpled (c) dimpled - Two copies of each chromosome (one from the mother and one from the father). Chromosomes ARE NOT identical, but they contain genes for the same traits. Homologous chromosomes may contain different forms (ex: dominant or recessive) of a gene for the same trait called alleles. Humans have 23 homologous pairs, for a total of 46 chromosomes in each somatic (body) cell. 23 Homologous Pairs in Humans Organized by size. Organized picture is called a karyotype. Used to detect chromosomal mutation. Diploid and Haploid Cells Diploid cells have homologous pairs of chromosomes. ex. Human somatic (body) cells are diploid. (di = 2) Haploid cells just have one copy of each chromosome pair. ex. Gamete (egg and sperm) cells are haploid. Diploid cells = 2n Haploid cells = n Interpreting Genetics diagrams 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 Alleles of the same gene are located at the position on each homologous chromosome. Example: A pea plant that is heterozygous for seed shape (Rr) is in the picture above. In meiosis, after the cell has prepared to divide (double size, organelles, and DNA) it goes through 2 cell divisions, to produce 4 haploid cells. Phases of Meiosis: Meiosis also has an interphase period, during which chromosomes are duplicated (sister chromatids). 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 genes (the original 2 alleles plus the 2 copies) • This organism has 3 genes represented in the picture (X, Y, and Z). • It is heterozygous for each of these traits (Xx, Yy, Zz). • As pictured, the chromosomes have copied so each chromosome (blue and red) has two copies of each allele (XXxx, YYyy, ZZzz). Meiosis I – Separating homologous chromosomes Prophase I During Prophase I, the chromosomes condense to become visible. HOMOLOGOUS PAIRS JOIN TOGETHER SIDE BY SIDE TO FORM TETRADS (4 chromatids). Sources of Genetic Variation from Meiosis Crossing Over: During prophase I, sections of non-sister chromatids can break off and be exchanged while homologous pairs are in tetrads. Recombination of genes on the same chromosome can occur due to crossing over. In humans, this happens about 10 times in different locations per chromosome pair. Metaphase I In metaphase I, the HOMOLOGOUS PAIRS line up along the middle (side by side). Anaphase I: Homologous chromosome pairs separate. Telophase I Nuclei partly reforms, producing 2 haploid daughter cells. But each cell still has 2 sister chromatids for each chromosome. Why are the resulting cells haploid? Meiosis II- Separating sister chromatids Prophase II Anaphase II Metaphase II Telophase/ cytokinesis II Comparing Mitosis and Meiosis Purpose MITOSIS MEIOSIS Growth and Repair Gamete Production # of DNA 1 replications # of cell 1 divisions #/type of cells 2 diploid cells produced 1 2 4 haploid cells