Biology Lecture Notes

Lecture 11: Meiosis Introduction to heredity Heredity: Transmission of traits from one generation to the next (inheritance) Variation: Differences between individuals Genetics: the study of hereditary variation Gametes: Reproductive cells that transmit genes from one generation to the next Reproduction is asexual or sexual Asexual Reproduction: Single parent produces offspring no crossing Unicellular - split Multicellular - budding or fragmentation Results from mitotic division 1 diploid (2n) parent - 2 diploid offspring 1 haploid (n) parent - 2 haploid offspring Produces clones - offspring genetically identical to parent Advantages of Asexual Reproduction FAST Low E required Safe Lots of offspring Well adapted? Don’t change. Sexual Reproduction Fusion of 2 gametes to form a zygote gamete (n) + gamete (n) = fertilization = zygote (2n) Gametes usually from different parents (but not always) Offspring NOT genetically identical to parents Disadvantages of Sexual Reproduction Slow High E Required Dangerous ~ predation, disease Often fewer offspring Well Adapted? Offspring get half of your genes - gene dilution Advantage of Sexual Reproduction Genetic Variation Offspring represent novel combinations of parents’ genes Better able to respond to change or stress A Problem If gametes have the same chromosome # as parents Chromosome # doubles each generation Solution: Meiosis Reduction division Cell divides twice 1 diploid (2n) cell - 4 haploid (n) cell Variation increases chances of survival Chromosomes in Heredity Karyotype: orderly display of chromosomes Mitotic chromosomes, stained Homologous Chromosomes Same length, centromere position, staining pattern Homologous chromosomes contain: THE SAME GENES Human Karyotype Somatic cells: 46 chromosomes 44 autosomes or 22 pairs of autosomes 2 Sex chromosomes X and Y determine sex Female: XX Male: XY Small homologous region in XY chromosomes Life Cycles Life cycle: Sequence of stages from generation to generation Fertilization and meiosis: Occur in all sexual life cycles Alternate Timing Varies Meiosis Reduction division 4 stages and involves 2 cell divisions Interphase Meiosis I Interkinesis Meiosis II Interphase Like before mitosis, chromosome and centrioles duplicate Each chromosome now 2 sister chromatids (still chromatin) Meiosis I (and cytokinesis) First meiotic division - homologous chromosomes separate, ploidy reduced Each meiotic division includes the same stages of mitosis except prometaphase First and second meiotic divisions are indicated in the name of each stage Prophase I, Metaphase I, Anaphase I, Telophase I Prophase II, Metaphase II, Anaphase II, Telophase II Prophase I Includes crossing over/recombination Synapsis: Homologous chromosomes pair up Genes in chromosome align Synaptonemal complex forms - protein structure Results in tetrad: structure made of 2 homologous chromosomes (4 chromatids) held together What is the point of synapsis? Crossing over (homologous recombination) Enzymes break and rejoin DNA Exchanges between non-sister chromatids Results in new combination of genes Important source of genetic diversity Crossing over at Gene Level The point: new combinations Also During Prophase I Chromatin condenses Centromeres and kinetochores of homologous chromosomes separate Sister chromatids still attached Chiasmata form - sites where homologous chromosomes still attached due to crossing over Nuclear Envelope breaks down and spindle forms At end of Prophase I in Humans Humans 2n = 46 So how many chromosomes? 46 How many tetrads? 46 x 2= 92 chromosome/4= 23 How many chromatids? 92 Metaphase I Tetrads align at metaphase plate Homologous chromosomes orient towards opposite poles Both sister kinetochores of one chromosome - spindle for same pole Kinetochore of homologous chromosome - spindle for opposite poles Anaphase I Disjunction: Homologous chromosomes separate Sister chromatids still connected Chromosomes act independently Direction depends on orientation of tetrad Nondisjunction Homologs fail to separate 2 homologs go to the same pole Relatively common error in meiosis Telophase I Chromosomes may decondense Nuclear envelope reforms Cytokinesis occurs Results in 2 haploid cells, each has duplicated chromosomes At end of Telophase I in humans Humans 2n = 46 So how many chromosomes in each nucleus? 23 How many chromatids? 46 How many tetrads? 0 Meiosis I summary Start: 1 cell , 2n, duplicated chromosomes End: 2 cells, n, duplicated chromosomes Crossing over prophase I Homologous pairs line up at metaphase plate, separate in anaphase I Meiosis I is when diploid - haploid happens! Interkinesis Time between 1st and 2nd meiotic divisions Short (usually) interphase-like stage No S phase, no DNA synthesis occurs Meiosis II 2nd Meiotic division Chromatids separate into daughter cells Very similar to mitosis Stages of Meiosis II Prophase II Metaphase II Anaphase II Telophase II Meiosis II summary Start: 2 cell, n, duplicated chromosomes End: 4 cells, n, unduplicated chromosomes Each daughter cell is genetically unique No crossing over in meiosis II Amount of DNA per cell reduced, ploidy does not change