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Meiosis

1. Importance of meiosis
   1. must maintain a constant number of chromosomes per cell for each generation
   2. provides a mechanism for comparing the two copies of each chromosome for the purpose of error correction (repair)
   3. produces new combinations of chromosomes and new combinations of alleles at different genetic loci
   4. birds do it, bees do it, even educated fleas do it (and, of course, humans do it)
2. Before meiosis begins
   1. the spermatogonium or oogonium cell must go through the S period of the cell cycle such that all of the chromosomes have been doubled but the chromatids are held together at the centromere
   2. sexual differences
      • females - the beginning of meiosis occurs while the individual is in the fetal stage
      • males - meiosis does not begin until puberty and then is a continuing process
3. Stages of meiosis
   1. meiosis consists of two consecutive nuclear and cell divisions without an intervening period of DNA synthesis (S)
   2. the first division is termed Meiosis I or MI and the second division is termed Meiosis II or MII
   3. Animation of meiosis (5.3 MB .mov file from UC Santa Barbara)
   4. Another animation of meiosis (an amateur production by R. Huskey)
4. Meiosis I
   1. Prophase I
      • doubled chromosomes begin to condense
      • homologous chromosomes (those containing genes for the same end products) begin the"pairing process" at their ends ("telomeres") via attachment to the nuclear membrane
      • homologous chromosomes pairing ("synapsis") is mediated by a protein complex called the synaptonemal complex
      • non-sister chromatids exchange parts of chromosomes (crossing over) in humans there are about 49 chiasmata per meiosis or more than 2 per homologous pair
      • in females, division stops here until receipt of hormonal signals to continue - this cessation will last between 12 and 50 years (!)
      • near the end of Prophase I, the synapsis apparatus disappears and the homologues tend to repulse but are held together by points of crossover or chiasmata
      • near the end of Prophase I, the nuclear membrane disappears
      • the spindle has formed and kinetochore microtubules attach to kinetochores
   2. Metaphase I
      • paired homologues consisting of two, doubled chromosomes align on the metaphase plate
      • at this stage, the paired homologues are held together by the remains of the crossovers (chiasmata)
      • as the paired homologues are pulled and pushed by the spindle fibers, chiasmata appear to move toward the ends of the chromosomes (terminalize)
      • the orientation of one homologous pair is independent of any other homologous pair
        • the alignment of the homologous pair Mother #1 and Father #1 is not related to the alignment of the homologous pair Mother #2 and Father #2
        • this corresponds to Mendel's Second Law of Independent Assortment
        • the number of possibilities for segregation of chromosomes is given by the equation 2^N where N = haploid # of chromosomes; for humans this is 2²³ or 8,388,608
   3. Anaphase I
      • homologous chromosomes separate from each to go to opposite poles of the cell
      • centromeres do not divide so each chromosome remains in the doubled state; apparently the DNA associated with the centromere was not replicated in the previous S phase
      • at this point the number of chromosomes has been halved
      • since the chromosomes are doubled, they appear to have"four" arms as they move to the opposite poles of the cell
   4. Telophase I
      • doubled chromosomes arrive at the poles of the cell
      • doubled chromosomes may de-condense somewhat
      • spindle disappears and the nuclear membrane may reappear
      • cytokinesis occurs
      • the two cells do not go through a G1, S, and G2 cycle but proceed into Meiosis II
      • in animal females, cell division is grossly asymmetric producing a small polar body and a large egg cell
      • human eggs arrest at this point and do not complete Meiosis until ovulation and ferilization occur
5. Meiosis II
   1. Prophase II
      • the chromosomes, which had doubled prior to Meiosis I, re-condense but do not pair since there is no homologue
      • the spindle appears and the nuclear membrane, if present, disappears
      • in human females, this is triggered by fertilization
   2. Metaphase II
      • doubled chromosomes align on the metaphase plate through attachment of spindle fibers to the kinetochores
   3. Anaphase II
      • finally, the centromeres divide resulting in the division of the doubled chromosomes by separation of chromatids; at some point the centromeric DNA has been replicated
      • chromosomes move to opposite poles of the cell
      • this resembles a mitotic anaphase except that the number of chromosomes has been halved
   4. Telophase II
      • chromosomes de-condense
      • nuclear membrane reappears
      • cell division occurs - cytokinesis
      • in females this division is asymmetric again such that only one egg cell is produced
      • in males, sperm maturation of all four cells follows
6. Summary
   1. the spermatogonium or oogonium containing doubled chromosomes (2N = 46 in humans) undergoes two consecutive divisions without an intervening period of DNA synthesis
   2. the end products of meiosis are four cells with a haploid number (N = 23 in humans) of chromosomes
   3. fertilization of a sperm (N = 23) with an egg (N = 23) creates a zygote (2N = 46)
   4. DNA/cell goes from 2C to 4C to 2C to C as ploidy level goes from 2N to N to N+N
7. Errors in Meiosis
   1. the process of homologue separation in Meiosis I is more prone to errors than mitotic anaphase
   2. the phenomenon is termed "non-disjunction" since the homologous chromosomes failed to separate or disjoin
   3. the result is a meiotic product with more or less than the expected number of chromosomes
      • aneuploidy
        • if the effect is limited to one or a few chromosomes it is termed aneuploidy or"not true ploidy"
        • having one extra chromosome is termed trisomy; one less chromosome is monosomy
        • in animals, having one extra or one less chromosome is not usually compatible with normal development and usually results in an early termination of pregnancy
        • in humans, there are a few trisomies for autosomes (non-sex chromosomes) that may develop to birth although they each have a distinctive set of developmental differences but no monosomies for autosomes
          1. Trisomy 21 or Down's Syndrome
          2. Trisomy 13 or Patau's Syndrome
          3. Trisomy 18 or Edward's Syndrome
          4. the rate of non-disjunction appears to be related to the age of the mother
        • aneuploidies for the sex chromosomes are more frequent and have less drastic phenotypes
          1. XO or Turner's Syndrome
          2. XXY or Klinefelter's Syndrome
          3. XXX or XXXX Syndrome
          4. XYY Syndrome?
      • polyploidy
        • if all of the chromosomes are involved in non-disjunction, the result would be a change in the ploidy level or a triploid
        • higher ploidy levels can arise from a failure of cytokinesis following mitosis
        • some organs regularly have cells with higher ploidy levels - liver
        • higher ploidy levels result in a higher number of gene copies which is an effective way to increase gene product production
        • for most animals, triploidy and higher level ploidy states are not compatible with normal development
        • plants seem to be able to tolerate ploidy level changes and tend to rely on ploidy differences

Mendelian Inheritance

8. Introduction
   Clearly some traits of organisms are handed down from generation to generation. Baby lions look like lions; baby armadillos look like armadillos; you resemble your parents and your grandparents more than you resemble your classmates (unless you happen to be an identical twin).
   Note: there is a slightly higher probability that you resemble your mother and your maternal grandparents than your father and your paternal grandparents. This is a consequence of the near 25% illegitimacy rate among the US middle class.
   It was not clear how traits are inherited until Gregor Mendel did a systematic investigation of inheritance in the 1850's and 1860's.
9. Why did Mendel succeed when others had failed?
   1. Mendel used a plant in which he could control the matings (no experiments of nature).
   2. He used discrete, discernible traits that were consistent within a strain.
   3. He kept meticulous records and counted large numbers of progeny.
   4. He repeated his experiments and obtained consistent results.
10. What did he do?
    1. Obtained several pure-breeding strains (seven that he used) of pea that differed from each other in a single, distinct trait.
    2. Mated (allowed no unaccounted for matings) strains differing in a single character.
    3. Observed and counted the characters in the progeny.
    4. Allowed the progeny to self-fertilize (a peculiarity of some plants)
    5. Observed and counted the characters in the second generation progeny
11. What did he find?
    1. The results of his experiments are shown in Table 10.1 of the text.
    2. The first generation hybrid showed the trait of one of the two parents.
    3. The results were the same independent of which parents was the pollen donor and which parent the pollen recipient.
    4. The character not present in the first generation hybrid reappeared in the second generation at a frequency of 25% (one-quarter, 1/4).
12. What did he conclude?
    1. Traits do not blend or disappear but can be masked.
    2. So, some traits are dominant and others are recessive but the recessive traits are not obliterated.
13. How did he explain all of this?
    Mendel's First Law: The Law of Segregation
    1. Each plant has two determinants (alleles) for each trait.
    2. Each plant transmits in a random manner only one of its determinants to each of its progeny plants. The determinants segregate from each other and are transmited separately.
    3. For some determinants (alleles), the presence of one copy (heterozygous) is enough to express the trait (dominant). For other determinants, two copies (homozygous) must be present for the trait to be expressed (recessive).
14. What proof did he offer for his hypothesis?
    1. The second generation progeny with the dominant trait should consist of two different determinant compositions (genotypes).
    2. There should be 1/3 that are homozygous and therefore pure-breeding and 2/3 that are heterozygous and therefore should produce progeny results identical to the second generation results.
    3. The data are consistent with the prediction.