• Review of the cell cycle and mitosis

• Meiosis and Sexual Reproduction

• Meiosis Tutorial from the University of Arizona (still incomplete)

• National Institute of Health Gene Testing Glossary

• The Dictionary of Cell Biology

1. Discuss the relationship between sex and reproduction, and the consequences of sexual and asexual reproduction.

2. Draw a diagram that illustrates the relationships among the terms: chromosome, DNA, genes, chromatids, centromeres, homologous chromosomes (homologs), and alleles.

3. Recognize the essential elements of the process of meiosis, and the role of the process in an organism's life cycle.

4. Describe your own life history in terms of a general sexual life cycle.

5. Indicate where and when in your body meiosis occurs.

6. Use common objects such as paper clips or scraps of paper to model the changes in and movements of chromosomes during meiosis.

7. Compare the timing, location, numbers of cells, chromosome numbers, and genetic outcomes of mitosis and meiosis.

Crossing over: The exchange of genetic material between homologous chromosomes during the first stage of meiosis. It results in genetic variation in populations greater than that which might result from independent assortment alone.

Daughter cell: A cell which results from division of another cell (a mother cell), either in meiosis of mitosis.

Diploid: A cell with two copies of each of its chromosomes.

Embryo: The stage of an organism's development in which tissues and organs develop beginning with a fertilized egg.

Gamete: In animals, a haploid cell which results from the second stage of meiosis. In plants, the haploid cells proceed through an intermediate, multicellular stage before producing gametes. Male gametes are sperm; female gametes are eggs.

Haploid: A cell with only a single copy of each chromosome.

Homologous chromosomes: Chromosome pairs within cells which have the same sequence of genes. One chromosome of each pair comes from each of the parents through its gamete.

Independent assortment: The random arrangement and partitioning of homologous chromosomes during the first cell division stage of meiosis.

Zygote: The fused egg and sperm; the result of fertilization. In humans, this is also called the pre-embryo and the term is applied to the dividing cells during the first two weeks of development.

• What is it?

• Why is it important?

• Are sex and reproduction always linked?

1. Reproduction and sex are not obligatorily linked. Most of the organisms in the world can probably reproduce without sex (asexual reproduction):
   • Microbes (bacteria, fungi)
   • Many insects
   • Many plants
   • But not mammals (and very rarely in other vertebrates)
   Sex without reproduction is also possible, even common, in bacteria.
   But most commonly, sex and reproduction are linked somehow.

2. Reproduction without sex allows a population to be made entirely of genetically identical individuals.
   • If this is a successful individual, and the environment is steady for a long time, this could be a "good idea".
   • But if the environment changes, a population of identical individuals might not be the "best idea".

3. Sexual reproduction is the only way to assure genetic variability within a population.
   • Variability is evolutionarily beneficial, allowing species to adapt to changes over time.

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   Chromosomes and their relationship to genes and alleles

   (See Lewis pg. 317, Figure 15.10, and pg. 200, Fig. 10.9)

   • What's a chromosome?

   • How many chromosomes do we have? How many sets?

   • Homologous pairs of chromosomes (homologs)

   • What's a gene?

   • What's an allele?

   Gene: an illustration of the relationship of a gene to the structure of a chromosome

   Crossing over and recombination during meiosis

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   What is meiosis?
   • A two stage process of cell division which results in four daughter cells, each having only half the number of chromosomes as the mother cell (it halves the genetic material).

     • Meiosis Animations From McGill University

     • Meiosis and Sexual Reproduction

     • Meiosis: an illustration of the meiotic process provided by Access Excellence.

     • A simple animation of meiosis from Yale.

     • Hands on Genetics Demo of Meiosis.

     • Meiosis Tutorial from the University of Arizona

   • Summary of Stages:
     • Interphase
       • G1 phase
       • S phase (DNA material is replicated)
       • G2 phase
     • Meiosis I (Reduction Division: Halves chromosome number)
       • Prophase I
       • Metaphase I
       • Anaphase I
       • Telophase I
     • Interphase (DNA material does NOT replicate)
     • Meiosis II (Equational Division: Produces 4 daughter cells from 2)
       • Prophase II
       • Metaphase II
       • Anaphase II
       • Telophase II

   • There is no way to predict which set of chromosomes will end up in which daughter cell. It is only certain that, unless something goes wrong, each daughter cell will have one of each chromosome.

   • This is called independent assortment. In humans, because there are 23 pairs of chromosomes, the number of possible assortments is:
     2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2x2 = 8,388,608!

   • Any one of these assortments can combine with any one of the 8,388,608 combinations of his/her partner!

   • But, during meiosis, chromosomes exchange parts of their genetic material with the corresponding regions on their homologous chromosome. This process is called crossing over and it makes the number of possible combinations nearly unlimited.
     • Crossing over and recombination during meiosis

   • The haploid cells which result from meiosis, in animals, are gametes.

   • Diploidy is restored when opposite gametes combine at fertilization.

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   The General Sexual Life Cycle

   How does the human sexual cycle relate to the general cycle?

   

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   Where does meiosis occur in your body?

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   Class Activity: Turkeys in the Cell

   Objective: To change a diploid cell with six chromosomes (that is, three homologous pairs of chromosomes) into four haploid cells each with three chromosomes while dancing around and trying not to look silly.

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   Comparison of Mitosis and Meiosis:
   See pg 199, table 10.1 of Lewis

   Mitosis	Meiosis
   One division	Two divisions
   Two daughter cells per cycle	Four daughter cells per cycle
   Daughter cells genetically identical	Daughter cells genetically different
   Same chromosome no. as parents	Chromosome no. halfed that of parents
   Occurs in somatic cells	Occurs in germ-line cells
   Throughout life cycle	Completed after sexual maturity
   Used in growth, repair, asexual reproduction	Sexual reproduction, new gene combinations

   Meiosis vs. Mitosis

   Virtual Meiosis, a really neat link to real moving chromosomes!!

   Take me home, please!