Meiosis / Why Sex? Biol 1011 - General Biology October 31, 2011 Both mitosis and meiosis result in divided nuclei and cells, but they differ in their fundamental results. Mitosis: duplication of chromosomes followed by cell division; - each daughter cell has exact copy of chromosomes from the parent cell. Meiosis: duplication of chromosomes followed by two cell divisions; - four daughter cells each contain half the chromosome number of the parent cell - crossing over and independent assortment result in variation in chromosomes of each daughter cell - daughter cells become gametes (sperm and eggs) MITOSIS MEIOSIS Diploid parent cell Chromosome replication Prophase Metaphase Anaphase and Telophase Two diploid daughter cells of mitosis Prophase I - tetrads form - crossing over Metaphase I Anaphase I and Telophase I Meiosis II Four haploid daughter cells of meiosis Each species has a specific number of different chromosomes; a karyotype is a display of the number and type of chromosomes. Nucleus Karyotype (human) Chromosomes (condensed) Ploidy refers to the number of chromosome sets a cell contains. e.g. chromosome 5 one from one from Gene for eye formation (allele for rounder eyes) Gene for eye formation (allele for narrower eyes) Diploid (2n) – two sets Haploid (n) – one set Homologous chromosomes (homologs) etc: Triploid (3) Tetraploid (4) Polyploid MEIOSIS Diploid (2n) Haploid (n) Amount of hereditary material reduced by half Haploid gametes (n) Diploid adult (2n) Normal amount of hereditary material restored MITOSIS Zygote (2n) Sperm Egg FERTILIZATION Meiosis consists of two cell divisions, meiosis I and meiosis II. First each chromosome in the diploid (2n) parent cell is duplicated. Meiosis overview 1 Maternal chromosome Paternal chromosome Nuclear envelope MEIOSIS I Replication Replicated chromosome Centromere Sister chromatids Homologous chromosomes Parent cell contains homologous pair of chromosomes (2n) Homologs separate Sister chromatids separate Daughter cells contain just one homolog (n) Four daughter cells containing one chromosome each (n). In animals, these cells become gametes. MEIOSIS II MEIOSIS I Meiosis overview 2 The results of meiosis are haploid cells, which will form the gametes which recombine at fertilization to form a diploid zygote. Meiosis overview 3 Female gamete (egg) (n) Fertilization Male gamete (sperm) (n) Diploid offspring contains homologous pair of chromosomes Zygote (2n) Details of Meiosis • Meiosis I – Homologous chromosomes separate • Meiosis II – Sister chromatids separate Meiosis details. Tetrad 1. Interphase 2. Early Prophase I - synapsis 3. Late 4. Metaphase I 5. Anaphase I Prophase I - tetrads form - metaphase - migration - crossing over plate Meiosis details. 6. Telophase I & 7. Prophase II 8. Metaphase II 9. Anaphase II 10. Telophase II & Cytokinesis - sister Cytokinesis - metaphase - spindle chromatids plate formation - migration - migration separate Crossing over is very common; it usually occurs at least once in each non-sister chromatid. Process: Chromosome replication, synapsis, and crossing over Sister chromatids Centromere 1. Replication (interphase) Chromosomes Homolog one Synaptonemal complex Tetrad 2. Synapsis (prophase I) Homolog two Non-sister chromatids Protein complex Crossing over in two chromatids 3. Crossing over (prophase I) Q. Why should meiosis (and sexual reproduction) be such a widespread strategy for many types of organisms? Binary fission - bacteria Budding - Hydra Vegetative reproduction - plants Budding - yeast UC Berkeley Q. Why should meiosis (and sexual reproduction) be such a widespread strategy for many types of organisms? A. Sexual reproduction introduces genetic variation into organisms. Mechanisms for creating genetic variation: 1. Separation and random distribution of homologous chromosomes during meiosis I - independent assortment (random distribution of chromosomes from maternal and paternal sets) 2. Crossing over (meiosis I) 3. Fertilization Independent assortment: random distribution of homologous chromosomes from paternal and maternal sets during meiosis I. e.g. Maternal chromosome Maternal chromosome Paternal chromosome Allele that contributes to black hair Allele that contributes to brown eyes Allele that contributes to green eyes An eye-color gene Paternal chromosome Allele that contributes to red hair A hair-color gene Independent assortment: random distribution of homologous chromosomes from paternal and maternal sets during meiosis I. During meiosis I, tetrads can line up two different ways before the homologs separate. OR Brown eyes Black hair Green eyes Red hair Brown eyes Red hair Green eyes Black hair Crossing over, independent assortment and fertilization produce tremendous potential for genetic variation in gametes. Independent assortment: How many combinations of maternal and paternal chromosome homologs could result from meiosis? 2n, where n = haploid number e.g. organism X; n = 2 Humans; n = 23 2n = 4 2n = 223 ≈ 8.4 million combinations Fertilization: (8.4 X 106) X (8.4 X 106) = 7.06 X 1013 = 70.6 trillion combinations Crossing over: ~ 1/chromosome/ meiosis What is the evolutionary advantage of having all of this potential variation in the genes in gametes and offspring? Meiosis and sexual reproduction may be evolutionary adaptations to an ever-changing environment. What changes? e.g. Weather/climate Moisture Food supplies Predators Diseases - bacteria - viruses - parasites Pathogens (disease-causing organisms) tend to evolve rapidly - this means that our mechanisms for fighting disease (e.g. immune system) also has to evolve quickly Meiosis and sexual reproduction may be evolutionary adaptations to an ever-changing environment. What changes? e.g. Weather/climate Moisture Food supplies Predators Diseases - bacteria - viruses - parasites Trypanosoma - causes sleeping sickness - evolves extremely rapidly to evade the immune system Occasionally homologous chromosomes fail to separate during meiosis I. This is termed nondisjuction. n+1 n+1 n-1 2n = 4 n=2 n-1 1. Meiosis I 2. Nondisjunction. 3. Meiosis II 4. Aneuploidy starts normally. occurs normally. results. Nondisjuction syndromes occur fairly commonly in human populations. e.g. Down syndrome - Trisomy 21 Turner syndrome - X0 Klinefelter syndrome - XXY Jacob syndrome - XYY Incidence of Down syndrome per number of births Maternal age has a striking effect on the prevalence of Down syndrome. 1 12 1 16 1 20 1 36 1 1000 1 300 1 180 1 100 1 60 Age of mother (years)