lecture_notes_10_15_07 - Mitosis and Meiosis outline...

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Unformatted text preview: Mitosis and Meiosis outline Lecture 3 2007 Mitosis and Meiosis [Reading: SS 18-31; CR 218-228, 243-249] Mitosis in eukaryotes As a means of reproduction, mitosis is: A mechanism of cellular reproduction to form identical cells A process to condense (shorten) chromosomes and deliver identical copies from a parent cell to two daughter cells Karyokinesis divides the nucleus into two equivalent nuclei Cytokinesis follows, to divide the cytoplasm and separate the divided nuclei and cytoplasm into daughter cells Basic chromosome terminology Nuclear membrane (or envelope) surrounds the chromosomes in interphase A centromere or kinetochore (spindle fiber attachment region of a chromosome) which separates the chromosome into 2 arms (sometimes 1) Chromosomes come in pairs of homologues (equivalent chromosomes from each parent), one from each parent Each homologue forms an identical sister chromatid (copy of a chromosome) during replication (duplication of DNA or chromosomes) Chromatids sometimes are referred to as strands in meiosis n: basic haploid chromosome number, number of chromosomes in a gamete e.g. n = 23 for humans 2n: basic diploid chromosome number, number of chromosomes in a zygote e.g. 2n = 46 for humans n varies as a function of life cycle Chromosome shortening = coiling = condensation; elongation = uncoiling = decondensation Moving chromosomes the spindle Microtubules (cytoplasmic protein filaments responsible for intracellular locomotion) form a spindle apparatus (collection of microtubules connecting to centromeres and emanating from the poles of a cell) which attaches to the centromere The Stages of Mitosis Interphase: chromosomes decondensed; each chromosome (in Gap1 or G1 Phase) replicates (Synthesis or S) to form 2 sister chromatids (in Gap2 or G2) Features of Interphase Chromosomes are decondensed (stretched out) Most gene function (transcription) occurs during interphase DNA content doubles with replication from 2C to 4C during S Phase Mitosis and Meiosis outline Lecture 3 2007 Prophase: spindle begins to form and attach to centromeres; chromosomes begin to condense; nuclear membrane disappears; sister chromatids held together from end to end; DNA content is 4C Prometaphase: -microtubules attach to centromeres and move chromosomes to the metaphase plate Metaphase: chromosomes are condensed; centomeres are attached to spindle and align on an imaginary equatorial plate, the metaphase plate, where homologues arrange independently from each other Anaphase: centromeres separate and move to the opposing poles of the cell; "chromatids" become "chromosomes" Telophase: chromosome movement is completed; spindle disappears; nuclear membrane reappears; cytokinesis proceeds Similarities and differences between mitosis and meiosis Mitosis rarely generates new heritable variation Meiosis generates large amounts of new variation by recombination Both processes maintain balanced sets of chromosomes and regulate DNA content Both processes are very complex but very widespread among nearly all eukaryotes Overview of Meiosis One of two key events in the sexual life cycle: meiosis forms gametes and fertilization or syngamy forms zygotes - Meiosis halves the Diploid chromosome number and generates genetic diversity for polymorphic pairs of alleles Haploid Phase of the life cycle Three main events Pairing of homologous chromosomes (= synapsis) Exchange of genetic material between homologues by crossing over, which is visualized cytologically as a chiasma (plural chiasmata, xmta) Segregation of homologues and reduction of chromosome number The ultimate consequence of meiosis: Generation of new combinations of alleles = Recombination The Stages of Meiosis Prophase I A) No chromosome pairing; maximum chromosome decondensation; nuclear membrane present B) Synapsis formation between homologues begins; chromosomes begin to shorten C) Synapsis complete; more shortening; only 2 strands visible D) More chromosome shortening; chiasmata visible For a given pair of synapsed homologues, i.e. bivalent of 2 chromosomes , the doubled nature (all 4 strands or chromatids) can be visible Mitosis and Meiosis outline Lecture 3 2007 E) Homologues begin to repel except at chiasmata; chiasmata terminalization begins, i.e. slippage to the ends of the bivalents; nuclear membrane disappears; human oocytes stop here from birth until puberty (called diplotene) F) Chiasmata terminalized; maximum chromosome condensation and shortening; initiation of spindle attachment Metaphase I: bivalents are held together only by their chiasmata; their centromeres are attached to the spindle at the metaphase plate Homologues are associated with each other in meiosis (unlike mitosis) INDEPENDENT ASSORTMENT Anaphase I: chromatids held together only at their centromeres and move to poles Separation of homologous centromeres SEGREGATION Telophase I: chromosomes move to poles Interphase: no DNA synthesis (replication); incomplete chromosome uncoiling; sometimes new nuclear membrane formed Cytokinesis occurs during female gamete formation in both plants and animals, and in male animal gamete formation, but not in all plant male gamete formation Prophase II: chromosomes condense and begin moving to metaphase plate; nuclear membrane disappears Metaphase II: spindle forms and centomeres align on metaphase plate; unlike mitosis, chromatids are only held together at the centromere Anaphase II: sister centromeres separate to poles; "chromatids" become "chromosomes" Telophase II: nuclear membrane reappears; haploid cells form with cytokinesis ...
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