Meiosis Notes PDF

Meiosis © Pearson Education, Inc  Cell types: Somatic vs. Gametes SOMATIC CELLS- any body cells other than a sperm or an egg diploid ( 2 sets of chromosomes; notation: 2n) ex. Human somatic/diploid cells contain 46 chromosomes contain hom...

Meiosis © Pearson Education, Inc  Cell types: Somatic vs. Gametes SOMATIC CELLS- any body cells other than a sperm or an egg diploid ( 2 sets of chromosomes; notation: 2n) ex. Human somatic/diploid cells contain 46 chromosomes contain homologous chromosomes (2 copies of a particular chromosome; One from the mother, and one from the father) Karyotype © Pearson Education, Inc GAMETES- -reproductive cells ( sperm and egg) - haploid ( notation: n); contain a single set of chromosomes ex. Human gamete contains 23 chromosomes; Develop in males’ testes and females’ ovaries Credit: tutorvista.com  Meiosis- - cell division resulting in daughter cells with only half the genetic material of the original cell. - genetic material is randomly separated and reorganized to yield genetically different daughter cells. All sexual life cycles involve meiosis. In haplontic species (fungi- bread mold), haploid gametes are produced mitotically by haploid adults. Meiosis results in spores. In plants, meiosis results in haploid spores. Spores then germinate and divide mitotically to form haploid adults, which produce gametes by mitosis. In animals, gametes develop immediately after meiosis. FUNGI PLANTS ANIMALS  Life Cycle of Humans All body cells contain 46 chromosomes The fused cell (zygote) divides by mitosis; organism grows 46 chromosomes Egg and sperm Meiosis in gonads fuse into 1 cell gives rise to haploid during fertilization eggs and sperm, containing 23 chromosomes each 23 chromosomes 23 chromosomes 5 Recall Cell Cycle? Meiosis occurs after interphase! INTERPHASE – G1, S, G2 - Chromosomes replicate during the S phase but remain uncondensed -Centrosomes with centriole pairs replicate S phase Phases of Meiosis- Prophase I Image Credit: OpenStax/Rice University Chromatin condenses Homologous chromosomes come together (process called Synapsis); Protein complex forms between homologs to hold them together The result of synapsis: tetrad (pairs of homologous chromosomes) Non-sister chromatids exchange genetic material ( crossing over) Each tetrad has one or more chiasmata ( X shaped region of crossing over)- that hold homologs together Movement of centrosomes, formation of spindle fibers, breakdown of nuclear envelope, dispersal of nucleoli  Metaphase I Pairs of homologs (tetrads) are arranged across the center of the cell Anaphase I Homologous chromosomes are pulled toward opposite poles of the cell ; Cohesins are cleaved! Telophase I and Cytokinesis Each half of the cell now has a haploid set of chromosomes; but each chromosome is still composed of 2 sister chromatids Cleavage Furrow Nuclear envelope reforms © Pearson Education, Inc Prophase II Spindle fibers form, attach to chromosomes Nuclear envelope breaks down Metaphase II Chromosomes line up on the metaphase plate © Pearson Education, Inc Anaphase II Sister chromatids separate (Cohesins are cleaved) Telophase II and Cytokinesis Chromosomes relax, uncoil. Nuclear envelop and nuclei reform Each cell has a haploid set of chromosomes (unreplicated) Each cell is genetically different © Pearson Education, Inc Spermatogenesis - Development of sperm in testes; begins at puberty Genetically different © Garland Science Oogenesis - Development of oocytes (eggs) in ovaries Source- Molecular Biology of the cell, 5th edition; Figure 21–2 Copyright © Garland Science 2008  Mitosis and Meiosis Compared Mitosis Meiosis Location All tissues Only in testis and ovary Products Somatic cells ( diploid) Gametes ( haploid; sperm/egg) DNA replication and # of 1 time per 1 division 1 time per 2 divisions cell divisions Length of time Short ( varies ~ 24 hours ) 65 days ( sperm) Years ( egg) Pairing of homologs None Yes ( in Meiosis I) Recombination None Yes Characteristics of daughter Genetically identical Genetically different cells Meiosis I Meiosis: Generating Haploid Cells In meiosis, two sets of chromosomes are divided among four daughter cells, each of which has half as many chromosomes as the original cell. The four haploid cells are the result of two successive nuclear divisions. The micrographs show meiosis in the male reproductive organ of a lily; the diagrams show the corresponding phases in an animal cell. (For instructional purposes, the chromosomes from one parent of the original organism are colored blue and those from the other parent are red.) © C.A. Hasenkampf/Biological Photo Service Meiosis II © Pearson Education, Inc  Meiosis and Genetic Diversity Mutations (changes in an organism’s DNA) are the PRIMARY source of genetic diversity The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation 3 mechanisms contribute to genetic variation 1. Independent assortment of chromosomes 2. Crossing over 3. Random fertilization  Independent Assortment of Chromosomes Homologous pairs of chromosomes orient randomly at metaphase I of meiosis In independent assortment, each pair of chromosomes sorts maternal and paternal homologs into daughter cells independently of the other pairs The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes Show allele segregation and independent assortment for the following genotypes: AaBb → AB, Ab, aB, ab AABb → AB, Ab © Pearson Education, Inc  Crossing Over Genetic variation, in addition to that due to independent assortment, can be increased by CROSSING OVER events, during Prophase I Crossing over produces recombinant chromosomes, which combine DNA inherited from each parent Homologous chromosomes pair up, and two non-sister chromatids trade places  The frequency of crossing over between two genes is controlled by the distance between genes. The probability of crossing over increases as the distance between those genes increases. An arbitrary unit of measure, the map unit, is used to describe the distance. 1 map unit = 1% Recombination Frequency (crossover) Genes are “linked” if RF% < 50% * They do not assort independently * They sit close together on the same chromosome Example- Parental Generation: AABB x aabb F1: Aa Bb Perform a “test” cross: AaBb x aabb Results: ab # observed Type AB AaBb 130 Parental Ab Aabb 45 Recombinant aB aaBb 35 Recombinant ab aabb 140 Parental Total offspring: 350  Random Fertilization Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg) The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations Crossing over adds even more variation Each zygote has a unique genetic identity Source- Molecular Biology of the cell, 5th edition; Figure 21–2 Copyright © Garland Science 2008  Meiotic Errors – also contribute to variation Nondisjunction - Failure to separate in Anaphase I or Anaphase II - Leads to aneuploidy- abnormal number of chromosomes ( trisomy or monosomy) © Pearson Education, Inc Turner’s Syndrome Human Conditions- due to nondisjunction ( Monosomy) http://2.bp.blogspot.com/_hhUdKwzDmA4/Sw4GLwPyVTI/AAAAAAAAATI/kW_Rd5Y_jYA/s400/do.jpg Down Syndrome- Trisomy 21 © Pearson Education, Inc Polyploidy Most organisms are either diploid (2n) or haploid (n). Triploid (3n)- 3 sets of chromosomes, Tetraploid (4n)- 4 sets of chromosomes, or higher are possible in certain species. This occurs in a variety of ways: extra round of DNA replication, or lack of spindle formation Polyploidy might be a desired trait in crops, ornamental plants because it leads to more robust plants with larger flowers, fruits, and seeds. As with many polyploid plants, this triploid orange daylily (Hemerocallis fulva) is particularly large and robust, and grows flowers with triple the number of petals of its diploid counterparts. (credit: Steve Karg; https://cnx.org/contents/[email protected]:kfWJNVvv@7/Chromosomal-Basis-of-Inherited-Disorders) Cultivated bananas (3n) – sterile fleshy seedless fruit The cultivated banana is believed to have been derived from a cross between a diploid species Musa acuminata and the tetraploid species M. balbisiana, both of which produce seeded fruit, some 1000 years ago in southeast Asia. This gave rise to a sterile triploid plant with large seedless fruit and enormous food-producing properties. Propagation of the cultivated banana occurs by dividing its root system. (Credit: ScienceDirect.com) Chromosomal mutations also contribute to variation - mostly undesirable changes in phenotype- especially if they result in the loss of function of genes or DNA sequences that are needed for survival. Deletion Duplication Inversion Translocation  JUST FYI…. Meiosis can be modified in different ways. Animal species that have no males – the resulting gamete is diploid and does not need to be fertilized to develop. These animals are called “ parthenogenetic” ( Greek, “virgin birth). In the fly Drosophila mangabeirai- one of the polar bodies acts as a sperm and “fertilizes” the oocyte. In the lizard Cnemidophorus uniparens, the oogonia further doubles their chromosomes, so that the gametes are diploid. Grasshopper Pycnoscelus surinamensis forms a diploid egg by two mitotic divisions. In bees, wasps, and ants- unfertilized haploid eggs develop into males. If fertilized- they develop into females. Developmental Biology, 9th edition © by Sinauer Associates, Inc.