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Meiosis & Sexual Reproduction Heredity Transfer of characteristics from parent to offspring through their genes • Genome- Complete genetic material of an organism. • Chromosome- carry genes. • Gene– “unit of heredity” made of DNA. Chromosomes & Genes • Cellular DNA is organized in chromosomes. • Genes have specific places on chromosomes. Homologues Chromosomes exist in homologous pairs in diploid (2n) cells. Exception: Sex chromosomes (X, Y). Other chromosomes, known as autosomes, have homologues. Karyotype A karyotype is the complete set of all chromosomes of a cell of any living organism. • Upper right is a typical karyotype of a human male. • Lower right is a karyotype of a human sperm. Sexual Reproduction • Fusion of two gametes to produce a single zygote. • Introduces greater genetic variation, allows genetic recombination. • With exception of selffertilizing organisms, zygote has gametes from two different parents. Peter + Lois = Stewie In humans … • At fertilization, 23 chromosomes are donated by each parent. (total = 46 or 23 pairs). • Gametes (sperm/egg): – Contain 22 autosomes and 1 sex chromosome. – Are haploid (haploid number “n” = 23 in humans). • Fertilization results in diploid zygote. – Diploid cell; 2n = 46. (n = 23 in humans) • Most cells in the body are produced by mitosis. • Only gametes are produced by meiosis. Mitosis vs. Meiosis • 2n • 1n • Clone • Daughter cells different from parent cell and from each other. • Same genetic information in parent cell and daughter cell. • Give me another one just like the other one! • Daughter cells have ½ the number of chromosomes as somatic cell. • Shuffling the genes (Mix it up!) Meiosis - Sex Cell (Gamete) Formation In meiosis, there are 2 divisions of the nucleus: meiosis I & meiosis II Meiosis Animations: http://www.sumanasinc.com/webcontent/animations/content/meiosis.html http://www.johnkyrk.com/meiosis.html Meiosis Animations: http://www.sumanasinc.com/webcontent/animations/content/meiosis.html http://www.johnkyrk.com/meiosis.html Mitosis vs. Meiosis Meiosis creates genetic variation Mitosis • During normal cell growth, mitosis produces daughter cells identical to parent cell (2n to 2n) Meiosis • Results in genetic variation by shuffling of maternal and paternal chromosomes. • No daughter cells formed during meiosis are genetically identical to either mother or father. • During sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring. How is this “shuffling” accomplished? Genetic shuffling of Meiosis 1 1. Crossing over in Prophase I 2. Independent assortment in Metaphase/Anaphase I. Crossing Over You will also see the terms “synapsis” & “chaisma” associated with this process. Homologues break at identical locations, then rejoin opposite partners. This creates new combinations of genes on each chromosome. Occurs randomly several times on every chromosome. Results in mixing of the genes you inherited from your parents. Crossing over Two homologous chromosomes pair up during meiosis. Genetic material exchanged between non-sister chromatids. Crossing over produces recombinant chromosomes (genes have recombined). Independent Assortment Meiosis & Sexual Life Cycle • Life Cycle = sequence of stages in organisms reproductive history; conception to reproduction. • Somatic cells = any cell other than gametes, most of the cells in the body. • Gametes produced by meiosis.