CH 13: Meiosis & Sexual Life Cycles

• Most chromosomes in a diploid somatic cell are homologous.
• The identical nature of the homologues is that within each homologue, the gene locus and the trait the gene "codes" for are identical, but the INFORMATION within the particular gene is NOT necessarily identical!.
• Autosome -- homologous chromosomes in the somatic cell.
• Sex chromosome -- determine the gender of the offspring. Females have homologous sex chromosomes, males do not.
  • In humans, the sex chromosome is designated as an X chromosome, so diploid female cells have XX, whereas the male chromosome is designated Y, thus males are XY. The Y is much smaller than the X.
• Fertilization -- fusion of n sperm and n ovum nuclei. If gametes come from two parents, it is syngamy (two marriage), if from one parent, autogamy (self marriage).
• Zygote -- 2n cell produced by fertilization.

Meiosis

• Meiosis ("the process of making smaller") -- Production of n gametes from 2n cells.
• This means that one from each homologous pair of autosomes, plus one sex chromosome, wind up in each gamete at the end of meiosis.
• For humans, each gamete contains 23 chromosomes, 22 autosomes plus either one X or one Y sex chromosome.
• In sexually reproducing organisms, meiosis followed by fertilization leads to the growth of an individual via mitosis/cytokinesis.
• In many groups of organisms, sexual reproduction of offspring (meiosis/fertilization) may ALTERNATE with asexual reproduction of new offspring (mitosis/cytokinesis). We will discuss this in Biology II.
• Meiosis is also known as double or reduction division.
• Meiosis exhibits two series of phases, Meiosis I (M I) and Meiosis II (M II). Each consists of Prophase, Metaphase, Anaphase and Telophase sequences. Interphase I is Interphase in Meiosis I, Interphase II is Interphase in Meiosis II, Interphase is Interphase in Mitosis.
• Prior to the onset of meiosis I, the cells are in Interphase and go through the typical interphase periods.
• Sister chromatids are replicated during the S stage of Interphase.
• Meiosis I -- This is initiated at different times depending on the species. (Figure 13.7, p 240-241)
  • In some animals, meiosis does not begin until after sperm penetration of the ovum.
  • In humans, woman ovulates a cell that has completed Meiosis I. Meiosis II is not started until sperm penetrates. Meiosis does not begin until sexual maturity (puberty).
• Prophase I -- The basic processes of prophase/prometaphase as described for mitosis apply here (formation of spindle fibers, breakdown of nuclear membrane [yet still visible], etc.). This is the long phase of Meiosis. Several MAJOR differences:
  • Synapsis -- Where the sister chromatids of each homologous pair are coiling, they are bound together by a special protein "zipper" the synaptonemal complex.
  • Crossing Over -- While joined, the INNER chromatids of each homologue physically cross cross and exchange pieces. (Figure 13.10, p 245).
  • Results in Geneetic Recombination, which increases the variability of characteristics that an organism may exhibit.
  • After crossing over, the synaptonemal complex breaks down.
  • However, the two sets of chromatids still remain bound together by two protein connectors, one at each end, called terminal chiasma.
  • With four chromatids bound together in this fashion, the resulting structure is referred to as a tetrad.
• Metaphase I -- Tetrads line up at equator and snap apart simulataneously.
  • However, because only the outer two kinetochores can have spindle fibers attached, two sets of chromatids separate.
  • These are non-sister chromatids, also known as TWO dyads or double stranded chromosomes.
• Anaphase I -- Dyads pull themselves to the poles, cell elongates.
• Telophase I/Cytokinesis -- Dyads uncoil, disappear, nclear membranes reform, cells divide in two.
• Interkinesis -- An interphase-like period (referred to as Interphase in your text) but without any further replication of chromosome material.
• Meiosis II -- Each of the two cells formed in Meiosis I go through P,M,A,T II. The sequence of events during these phase is now very much like mitosis except that at the end of telophase II there are 4 cells, each with 1/2 the original chromosomal number.
  • Ploid = # of sets of chromosomes. ie. diploid = 2 sets of chromosomes; haploid = 1 set of chromosomes. Count the centromeres to keep the numbers straight.
  • Mitosis, always 1 cell becomes 2 cells. Meiosis 1 cell to 4 cells.
  • Mitosis: 2n (diploid in 1 cell) X 2 / 2 (cells) = 2n (chromosomes)
  • Meiosis: 2n (diploid in 1 cell) X 2 / 4 (cells) = n (chromosomes)
  • Because of crossing over, genetic information in each of the four cells may differ, leading to additional variability in offspring.
  • Two other general interactions (which will be discussed more in Chapter 14) are the independent assortment of chromosomes and random fertilization.
  • For example, in humans, which have 23 chromosomes in their gametes, there are about 8 million (2n=23) possible assortments for the production of gametes.
  • Fertilization of one sperm and one ovum increase the possible zygote (2n cell produced by fusion of n sperm and n ovum nuclei) combinations to over 70 trillion fertilization combinations.

Sperm & Ovum Production

• The process of forming n gametes from 2n cells is meiosis.
• Another general term for the process is gametogenesis: in males, it is called spermatogeneis, in females, oogenesis.
• The overall process of meiosis is exactly the same in both males and females.
• However, the number of functional gametes produced differs.
• In spermatogenesis, all 4 of the gametes produced are functional sperm. (figure 46.11, p. 985)
• In oogenesis, only one functional ovum plus 3 non-functional cells called polar bodies are produced. Polar body has no significant organelles and no nucleus.
• A first polar body is formed by extrusion of chromatids from one cell during MI with virtually no cytoplasm.
• In MII, this polar body will divide in two.
• During MII, the larger cell will extrude a third polar body at the end of TII.
• Due to independent assortment of chromosomes, which genetic combination is present in that ovum is TOTALLY RANDOM!
• Due to random fertilization, which genetic combination is present in the sperm that fertilizes that ovum is also TOTALLY RANDOM!

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