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  1. 1. MEIOSISUnlike single celled prokaryotes, most organisms are diploid and multicellular.At a particular time in their life cycle certain cells will differentiate, viameiosis, into haploid cells. During sexual reproduction two haploid cellscalled gametes will unite, thus restoring the diploid chromosome number in thenext generation. Meiosis is especially important because it produces geneticvariation, the raw material for evolution.I. Halving the chromosome numberA. Sexual reproduction1. Requires gamete formation and then fusion of gametes (syngamy) to form azygote a. Gamete = a sex cell (sperm or egg) b. Zygote = first diploid cell resulting from syngamy (fertilization)2. If gametes contained same number of chromosomes as body cells, doublingwould soon fill cellsB. Life cycles 1. Life cycle refers to all reproductive events between one generationand next 2. Mitosis = nuclear division that maintains a constant chromosomenumber 3. Meiosis = nuclear division reducing chromosome number from diploid(2n) to haploid (n) number 4. Meiosis occurs at different points during life cycle of variousorganismsC. Three basic types of life cycles: 1. gametic meiosis - meiosis give rise to gametes which fuse togetherin syngamy (fertilization). Characteristic of animals and some types of algae.Therefore, in animals, sexual reproduction involves a regular alternationbetween meiosis and syngamy. 2. sporic meiosis - in true plants (and some algae) meiosis results inthe production of haploid spores, cells that can grow directly into a newhaploid individual. Therefore, in plants there is a regular alternation ofgenerations between haploid and diploid individuals. a. In most plants meiosis takes place within flowers. 3. zygotic meiosis - haploid individuals (or cells) fuse to form adiploid zygote which immediately undergoes meiosis. This is the simplest typeof life cycle and occurs in unicellular haploid organisms such as some algaeand all fungi.D. Chromosomes occur in homologous pairs1. In a diploid cell, chromosomes occur as pairsa. Each set of chromosomes is a homologous pair; each member is a homologouschromosome or homologue
  2. 2. Homologous chromosomes - corresponding chromosomes, one from each parent,which contain the same genesb. Homologues look alike; they have same length and centromere position; havesimilar banding pattern human karyotypec. A locus (location) on one homologue contains the same types of gene whichoccur at the same locus on the other homologue2. Chromosomes duplicate just before nuclear divisiona. Duplication produces two identical parts called sister chromatids, heldtogether at centromere3. One member of each homologous pair is inherited from either male or femaleparent; one member of each homologous pair is placed in each sperm or eggII. Overview of meiosisA. Purpose of Meiosis1. Meiosis keeps chromosome number constant across the generations2. Makes sure that each gamete contains only one member of each homologous pairB. Meiosis has two divisions1. Since meiosis involves two nuclear divisions it produces four haploiddaughter cells; each, containing half the total number of chromosomes as thediploid parent nucleus2. Meiosis I - the first nuclear divisiona. Prior to meiosis I, DNA replication occurs and each chromosome has twosister chromatidsb. During meiosis I, homologous chromosomes pair; come together and line up insynapsisc. During synapsis, the two sets of paired chromosomes lay alongside each otheras bivalentsd. While paired up the chromosomes have equal exchanges of genetic material;this is called crossing overe. After crossing-over occurs, sister chromatids of a chromosome are no longeridentical3. Meiosis II - second nuclear division, nearly identical to mitosisa. No replication of DNA needed between meiosis I and II because chromosomeswere already doubledb. During meiosis II, centromeres divide; daughter chromosomes derived assister chromatids separatec. Chromosomes in the four daughter cells have only one chromatidC. Crossing-over produces genetic variation, the raw material for evolution1. Crossing-over results in exchange of genetic material between nonsisterchromatids
  3. 3. 2. Due to crossing-over, daughter chromosomes derived from sister chromatidshave different mix of genesPHASES OF MEIOSISI. Prophase I A. Nucleolus disappears; nuclear envelope fragments; and spindle fibersassemble B. Homologous chromosomes undergo synapsis forming bivalents; crossing-overmay occur at this time in which case sister chromatids are no longer identical C. Chromatin condenses and chromosomes become visibleII. Metaphase I A. Bivalents independently align themselves at the equatorial plane(metaphase plate) B. This independent assortment during metaphase I is another form ofgenetic recombination; produces more genetic variation C. The different possibilities can be calculated using the formula 2nwhere n = haploid chromosome number. E.g. in humans 8,388,608 different wayshomologous pairs can line up (223) D. Note centromeres are on opposite sides of the equatorial planeIII. Anaphase I A. The homologues of each bivalent separate and move toward opposite poles B. Each chromosome still has two chromatids C. Note the centromeres do not divideIV. Telophase IA. Only occurs in some speciesB. When it occurs, the nuclear envelope reforms, nucleoli reappear andchromosomes may decondenseV. InterkinesisA. Period between meiosis I and meiosis IIB. No DNA replication occursTHE SECOND MEIOTIC DIVISION IS NEARLY IDENTICAL TO MITOSISVI. Prophase II A. If need be the nuclear envelope and nucleoli dissolve and chromatincondenses into chromosomesVII. Metaphase II A. The chromosomes align with their centromeres on the equatorial planeVIII. Anaphase II
  4. 4. A. Centromeres divide and daughter chromosomes move toward the polesIX. Telophase II A. Nuclear envelope reforms, nucleoli reappear and chromosomes decondense B. Cytokinesis produces four haploid daughter cellsX. Comparison of meiosis and mitosis: A. Mitosis occurs more often because it allows growth and repair of bodytissues in multicellular organisms; meiosis only occurs at certain times in thelife cycle of sexually reproducing organisms B. DNA is replicated only once before both mitosis and meiosis; in mitosisthere is only one nuclear division; in meiosis there are two nuclear divisions C. There is no crossing-over in mitotic prophase; there is crossing-over inprophase I of meiosis D. Duplicated chromosomes align on metaphase plate in mitosis; bivalentsalign on the metaphase I plate E. Sister chromatids separate to form daughter chromosomes in anaphase ofmitosis; homologous chromosomes separate in anaphase I of meiosis F. Meiosis II is like mitosis except the meiosis nuclei are haploid G. Mitosis produces two daughter cells; meiosis produces four daughter cells H. In mitosis, two daughter cells have same chromosome number as parentcell; in meiosis, four daughter cells are haploid I. In mitosis, the daughter cells are genetically identical to each otherand to the parent cell; in meiosis, the daughter cells are not geneticallyidentical to each other or to the parent cellXI. Significance of MeiosisA. Meiosis produces genetic variation1. Without meiosis, chromosome numbers would continually increase2. Meiosis ensures daughter cells receive one of each kind of gene; preciselyhalves the chromosome number3. Independent assortment provides 2n possible combinations of chromosomes indaughter cells4. In humans with 23 haploid chromosomes, 2n = 223 = 8,388,608 possiblecombinations.5. Variation is added by crossing-over; if only one crossover occurs withineach bivalent, 423 or 70,368,744,000,000 combinations are possible6. Fertilization also contributes to genetic variation; (223)2 =70,368,744,000,000 possible combinations without crossing-over7. With fertilization and crossing-over, (423)2 =4,951,760,200,000,000,000,000,000,000 combinations are possibleB. Advantages of Meiosis
  5. 5. 1. Tremendous storehouse of genetic variation provides for adaptations tochanging environment2. Asexual organisms depend primarily on mutations to generate variation