Octopus Sex and Other Stories

1. Organisms that reproduce asexually can rely on chromosome duplications and mitotic cell divisions.



2. Sexually reproducing organisms must prepare sex cells with reduced chromosome quantities so that fusion at fertilization will maintain the diploid chromosome number.

10.1   Comparing Sexual with Asexual Reproduction

1. In asexual reproduction, one parent passes a duplicate of its genetic information to its offspring, which can only be genetically identical clones of the parent.



2. In sexual reproduction, each parent contributes one gene for each trait.
   
   
   1. Genes for each trait come in slightly different forms called alleles, originally produced by mutations.
   
   
   
   2. Meiosis shuffles the alleles during gamete formation, and fertilization produces offspring with unique combinations of alleles.
   
   
   
   3. The variation generated by sexual reproduction is the testing ground for natural selection and is the basis for evolutionary change.

10.2   How Meiosis Halves the Chromosome Number

1. Think "Homologues"
   
   
   1. Meiosis begins with diploid (2n = 46 in humans) germ cells and produces haploid gametes (n = 23).
      
      
      1. In 2n cells there are two chromosomes of each type, called homologous chromosomes.
      
      
      
      2. Homologous chromosomes line up (even unequally matched sex chromosomes!) during meiosis.
   
   
   2. Meiosis produces gametes that have one of each pair of homologous chromosomes.


2. Two Divisions, Not One
   
   
   1. In some ways meiosis resembles mitosis:
      
      
      1. The chromosomes are duplicated during interphase to form sister chromatids held together at the centromere.
      
      
      
      2. Chromosomes are moved by the microtubules of the spindle apparatus.
   
   
   2. Unlike mitosis, meiosis has two series of divisions--meiosis I and II.
      
      
      1. During meiosis I, homologous chromosomes pair and cytokinesis follows.
         
         
         1. Each of the two daughter cells receives a haploid number of chromosomes.
         
         
         
         2. Each chromosome is still duplicated.
      
      
      2. In meiosis II, the sister chromatids of each chromosome separate; cytokinesis follows resulting in four haploid cells.

10.3   Visual Tour of the Stages of Meiosis

[This section consists entirely of a two-page figure of the stages of meiosis with explanatory captions.]

10.4   A Closer Look at Key Events of Meiosis I

1. Crossing Over in Prophase I
   
   
   1. Homologous chromosomes pair up (synapsis).
      
      
      1. Nonsister chromatids exchange segments in a process called crossing over.
      
      
      
      2. Because alleles for the same trait can vary, new combinations of genes in each chromosome can result; this is one source of genetic variation.
   
   
   2. After crossing over, the nonsister chromatids begin to partially separate but remain attached by chiasmata.


2. Metaphase I Alignments
   
   
   1. During metaphase I, homologous chromosomes randomly line up at the spindle equator.
   
   
   
   2. During anaphase I, homologous chromosomes (still duplicated) separate into two haploid cells each of which has a random mix of maternal and paternal chromosomes.

10.5   From Gametes to Offspring

1. Gamete Formation in Plants
   
   
   1. Germ cells within plant tissues produce haploid spores by meiosis.
      
      
      1. Each spore undergoes mitosis to produce a haploid gametophyte.
      
      
      
      2. Gametophytes produce haploid cells--eggs or sperm.
   
   
   2. Fertilization results in a diploid sporophyte (example: pine tree).


2. Gamete Formation in Animals
   
   
   1. The life cycle of multicelled animals proceeds from meiosis to gamete formation >>> fertilization >>> growth by mitosis.
   
   
   
   2. In males, meiosis and gamete formation are called spermatogenesis.
      
      
      1. Germ cell (2n) >>> primary spermatocyte (2n) >>> MEIOSIS I >>> two secondary spermatocytes (n) >>> MEIOSIS II >>> four spermatids (n).
      
      
      
      2. Spermatids change in form; each develops a tail to become mature sperm.
   
   
   3. In females, meiosis and gamete formation are called oogenesis.
      
      
      1. Germ cell (2n) >>> primary oocyte (2n) >>> MEIOSIS I >>> secondary oocyte (n, and large in size) plus polar body (n, and small in size) >>> MEIOSIS II >>> one large ovum (n) plus three polar bodies (n, small).
      
      
      
      2. The single ovum is the only cell capable of being fertilized by a sperm; the polar bodies wither and die.


3. More Gene Shufflings at Fertilization
   
   
   1. The diploid chromosome number is restored at fertilization when two very different gamete nuclei fuse to form the zygote.
   
   
   
   2. The variation present at fertilization is from three sources:
      
      
      1. Crossing over occurs during prophase I.
      
      
      
      2. Random alignments at metaphase I lead to millions of combinations of maternal and paternal chromosomes in each gamete.
      
      
      
      3. Of all the genetically diverse gametes produced, chance will determine which two will meet.

10.6   Meiosis and Mitosis Compared

1. Mitotic cell division produces clones; this type of division is common in asexually reproducing organisms and in the growth process.



2. Meiosis occurs only in the germ cells used in sexual reproduction; it gives rise to novel combinations of alleles in offspring.

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