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Transcript
Mitosis vs. Meiosis Meiosis • The form of cell division by which gametes, with half the number of chromosomes, are produced. • Diploid (2n) → haploid (n) • Meiosis is sexual reproduction. • Two divisions (meiosis I and meiosis II). Fertilization • The fusion of a sperm and egg to form a zygote. • A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote Meiosis • Sex cells divide to produce gametes (sperm or egg). • Gametes have half the # of chromosomes. • Occurs only in gonads (testes or ovaries). Male: spermatogenesis Female: oogenesis • Meiosis is similar to mitosis with some chromosomal differences. Spermatogenesis human sex cell meiosis I sperm meiosis II Meiosis I (four phases) • Cell division that reduces the chromosome number by one-half. • four phases: a. prophase I b. metaphase I c. anaphase I d. telophase I Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad sister chromatids Homologous Chromosomes • Pair of chromosomes (maternal and paternal) that are similar in shape and size. • Homologous pairs (tetrads) carry genes controlling the same inherited traits. • Each locus (position of a gene) is in the same position on homologues. • Humans have 23 pairs of homologous chromosomes. a. b. 22 pairs of autosomes (somatic cells) 01 pair of sex chromosomes • Gene- unit of heredity that is transferred from parent to offspring and is held to determine characteristics of the offspring • Trait- feature or characteristic of a living thing • Locus- specific location of a gene, or DNA sequence, on a chromosome Homologous Chromosomes eye color locus eye color locus hair color locus hair color locus Paternal Maternal Karyotype • A method of organizing the chromosomes of a cell in relation to number, size, and type. Humans have 23 Sets of Homologous Chromosomes Each Homologous set is made up of 2 Homologues. Homologue Homologue Autosomes (The Autosomes code for most of the offspring’s traits) In Humans the “Autosomes” are sets 1 - 22 Sex Chromosomes The Sex Chromosomes code for the sex of the offspring. ** If the offspring has two “X” chromosomes it will be a female. ** If the offspring has one “X” chromosome and one “Y” chromosome it will be a male. In Humans the “Sex Chromosomes” are the 23rd set XX chromosome - female XY chromosome - male Boy or Girl? The Y Chromosome “Decides” Y chromosome X chromosome Crossing Over • Crossing over (variation) may occur between nonsister chromatids at the chiasmata. • Crossing over: segments of nonsister chromatids break and reattach to the other chromatid. • Chiasmata (chiasma) are the sites of crossing over. Crossing Over - variation nonsister chromatids chiasmata: site of crossing over Tetrad variation Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment. Question: • In terms of Independent Assortment how many different combinations of sperm could a human male produce? Answer • Formula: 2n • Human chromosomes: 2n = 46 n = 23 • 223 = ~8 million combinations Anaphase I • Homologous chromosomes separate and move towards the poles. • Sister chromatids remain attached at their centromeres. Non Disjunction • Occurs when chromosomes fail to separate properly – Some cells end up with one more or one less pair of chromosomes 21 trisomy – Downs Syndrome Can you see the extra 21st chromosome? Is this person male or female? Telophase I • Each pole now has haploid set of chromosomes. • Cytokinesis occurs and two haploid daughter cells are formed. Telophase I Meiosis II • No interphase II (or very short - no more DNA replication) • Remember: Meiosis II is similar to mitosis Prophase II • same as prophase in mitosis Metaphase II • same as metaphase in mitosis metaphase plate metaphase plate Anaphase II • same as anaphase in mitosis • sister chromatids separate Telophase II • Same as telophase in mitosis. • Nuclei form. • Cytokinesis occurs. • Remember: four haploid daughter cells produced. gametes = sperm or egg Telophase II Variation • Important to population as the raw material for natural selection. • Question: What are the three sexual sources of genetic variation? Answer: 1. crossing over (prophase I) 2. independent assortment (metaphase I) 3. random fertilization Remember: variation is good! Question: • A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? Answer: • 10 chromosomes (haploid) Question: • A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? Answer: • 10 chromosomes