Cell Division and Reproduction Help (page 2)

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Updated on Aug 21, 2011

Sexual Reproduction

Sexual reproduction involves the production of gametes (gametogenesis) and the union of a male and a female gamete (fertilization) to produce a zygote. In animals, male gametes are sperms and female gametes are eggs, or ova (ovum, singular). Gametogenesis occurs only in the specialized cells (germ line) of the reproductive organs (gonads). In animals, the testes are male gonads and the ovaries are female gonads. Gamete cells are produced through the process of meiosis. Meiosis (Fig. 1-5) consists of two specialized, consecutive cell divisions in which the chromosome number of the resulting cells is reduced by half from a diploid (2n) to a haploid (n) number. This reduction maintains the chromosome number characteristic of the species after fertilization.



Specifically, meiosis involves one DNA replication and two divisions of the chromosomes and cytoplasm. The first meiotic division (meiosis I) is a reductional division that produces two haploid cells from a single diploid cell. The second meiotic division (meiosis II) is an equational division wherein sister chromatids of the haploid cells are separated. Each of the twomeiotic divisions (meiosis I and II) consists of fourmajor phases (detailed below).Note that the DNA replicates during the interphase precedingmeiosis I; it does not replicate between telophase I and prophase II.

  1. Meiosis I. In the beginning of meiosis I, replicated chromosomes thicken and condense. Prophase I of meiosis differs from the prophase of mitosis in that homologous chromosomes come to lie side by side in a pairing process called synapsis. Each pair of synapsed chromosomes is called a bivalent (two chromosomes) or a tetrad (four chromatids). Each chromosome consists of two identical (replicated) sister chromatids at this stage; the cell contains one set of maternally derived and one set of paternally derived chromosomes. During synapsis, chromatids may cross over and exchange genetic material in a process called crossing over and recombination. The events of prophase I are complex and can be subdivided into five stages.
    • Leptotene (thin-thread stage): The long, thin chromosomes start to condense and begin to appear in the formerly amorphous nuclear chromatin material.
    • Zygotene (joined-thread stage): During this stage, homologous chromosome partners are joined together by a ribbonlike protein structure called the synaptonemal complex. This is the beginning of synapsis. Synapsis occurs intermittently along the paired chromosomes at sites where the homologues share similar genetic information. When synaptonemal complexes are not properly formed, synapsis is not as complete and crossing over is markedly reduced or eliminated.
    • Pachytene (thick-thread stage): Synapsis is complete. recombination nodules begin to appear along the chromosomes. At these sites, nonsister chromatids (one from each of the paired chromosomes) of a tetrad cross over, trade DNA strands and reunite, resulting in an exchange of genetic material (see Fig. 6-1). The point of exchange appears under a microscope as a cross-shaped figure called a chiasma (chiasmata, plural). At a given chiasma, only two of the four chromatids cross over randomly. Generally, the number of crossovers per bivalent increases with the length of the chromosome. By the breakage and reunion of nonsister chromatids within a chiasma, linked genes become recombined into crossover-type chromatids; the two chromatids within that same chiasma that did not exchange segments maintain the original linkage arrangement of genes as noncrossover or parental-type chromatids. Crossing over is usually a genetic phenomenon that can be inferred only from the results of breeding experiments.
    • Diplotene (double-thread stage): This stage begins when the synaptonemal complex begins to disappear so that individual chromatids and chiasmata can be more readily seen. Chiasmata are also still visible.
    • Diakinesis (double movement stage): Chromosomes reach their maximal condensation. Nucleoli and nuclear membrane disappear and the spindle apparatus begins to form.
  2. During metaphase I, the synapsed chromosomes orient at randomon the equatorial plane. At anaphase I, the centromeres do not separate, but continue to hold sister chromatids together. The chiasmata begin to dissolve, allowing the homologous pairs of chromosomes to separate and move to opposite poles; i.e., whole chromosomes (each consisting of two sister chromatids) move apart. This movement reduces the chromosome number from diploid (2n) to haploid (n). Telophase I occurs when the nuclear membrane reforms and the chromosomes have reached their polar destinations. Cytokinesis follows and results in a division of the diploid mother cell into two haploid daughter cells. Each haploid cell receives a random assortment of maternal and paternal chromosomes; i.e., the chromosomes in one daughter cell will not be uniformly of eithermaternal or paternal origin. Also, because of crossovers, sister chromatids (still attached at the centromere) may no longer be genetically identical. This ends the first meiotic division.

  3. Interkinesis. The period between the first and second meiotic divisions is called interkinesis. Depending on the species, interkinesis may be brief or continue for an extended period of time. It is important to note one important difference between mitotic interphase and meiotic interkinesis; i.e., no DNA synthesis occurs during interkinesis!
  4. Meiosis II. In prophase II, the spindle apparatus reforms and the chromosomes recondense. By metaphase II, the individual chromosomes have lined up on the equatorial plane. During anaphase II, the centromeres of each chromosome separate, allowing the sister chromatids to be pulled apart in an equational division (mitotis-like) by the attached spindle fibers. During telophase II, the chromosomes gather at opposite poles and the nuclear membrane reappears. Each cell then divides by cytokinesis into two progeny cells. Thus, a diploid mother cell becomes four haploid progeny cells as a consequence of a meiotic cycle (meiosis I and meiosis II). The characteristics that distinguish mitosis from meiosis are summarized in Table 1.2.
  5. Genetic aberrations can occur if mistakes are made during the separation of chromosomes during meiosis. If homologues or sisters fail to come apart (or disjoin) and both migrate to the same pole (called nondisjunction), the resulting gametes will contain two of those chromosomes, instead of just one. When such a gamete fuses with another during fertilization, the resulting zygote will have three of that particular chromosome. This condition is called a trisomy (see Cytogenetics). Most trisomies are lethal; however, trisomy 21 (also called Down syndrome), results in an individual who has three copies of chromosome number 21. This trisomy is not lethal, but produces individuals who are mentally and physically disabled. Trisomies of the sex chromosomes also occur without lethality, but also result in genetic abnormalities.

Practice problems for these concepts can be found at:

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