General Biology/Cells/Sexual Reproduction

Sexual Exclusively eukaryotes Fusion of two haploid genomes Fertilization (= syngamy) Forms new individuals in multicellular organisms as result of fusion of egg and sperm Plants Animals Meiosis yields haploid genomes at some point in life cycle Sexual life cycle Typical animal life cycle Meiosis occurs in germ line cells in gonads producing haploid gametes All other cells are somatic cells Alternation of generations Sexual intercourse Meiosis Gives rise to genetic variation Reduction division: 2n to n Preceded by one round of DNA (chromosome) replication Two rounds of nuclear (& usually cell) division Meiosis I Synapsis of homologs Segregation of homologs Reduction division, 2n to n Meiosis II No chromosome replication Segregation of sister chromatids Formation of 4 haploid (n) cells Two nuclear divisions, usually 2 cell divisions, only one round of replication Meiosis I Prophase: synapsis and crossing over Metaphase Anaphase: chromosome segregation Telophase Meiosis II (mitosis-like) Prophase Metaphase Anaphase: sister chromosome segregation Telophase Prophase I: synapsis Complete alignment of replicated homologs Synapsis occurs throughout the entire length of a pair of homologs Key to chromosome segregation Synapsis, crossing over Subdivided into 5 continuous stages Crossing over Reciprocal, physical exchange between nonsister chromatids Type of recombination; mixes maternal and paternal genes Visual evidence: chiasmata Microtubules and anaphase I During prophase microtubules attach to kinetochores on one side of centromere The metaphase checkpoint insures proper attachment A phosphorylation event initiates motor activity and anaphase Meiosis II Cytologically similar to mitosis No preceding DNA replication Chromatids segregate and move to opposite poles as chromosomes 4 haploid cells produced In animals, these cells differentiate into gametes In plants and many other organisms, these cells divide by mitosis, followed some time later by gamete formation Evolution of sex Asexual reproduction: all offspring genetically identical to parent Sex: recombination destroys advantageous combinations So why sex? Many hypotheses Effect repair of genetic damage? Much pachytene repair as well as gene conversion Some protists form diploid cells in response to stress Recombination breaks up combinations of genes favoring parasites, thus reducing parasitism? Consequences of sex Recombination: generates genetic diversity Crossing over Independent assortment Random fertilization Qualities of gamete usually do NOT reflect qualities of genes enclosed in gamete References This text is based on notes very generously donated by Paul Doerder, Ph.D., of the Cleveland State University.