"To think about":
Page 208, questions 1, 3, and 4.

Answers to many of these questions can be found on the "Answers to End-of-Chapter Questions" page at the text website.

For feedback, post possible answers and ideas in the folder "Text 'Review' and 'To Think About' Questions" in the Biology Chat Section of Web Crossing.

Linda Chapman says to check out these links to "Extended Lecture Outlines" from the text book web page.

Web resources:

• Review of the cell cycle and mitosis
  From the On-Line Biology Book

• Meiosis and Sexual Reproduction
  From the On-Line Biology Book

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

• National Institute of Health Gene Testing Glossary

• The Dictionary of Cell Biology

• Access Excellence from Genentech

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.

• 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".
   • Genetic changes can occur randomly by mutation, but this may or may not be adequate to cope with environmental change.

3. Sexual reproduction is an important way to assure genetic variability within a population.
   • Variability is evolutionarily beneficial, allowing species to adapt to changes over time (as measured in generations).

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   The General Sexual Life Cycle

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   Where does meiosis occur in your body?

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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?
     DNA in a Chromosome

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

   • Homologous pairs of chromosomes (homologs)

   • What's a gene?
     A Gene Map of the Human Genome

   • What's an allele?

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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).
     • Virtual Meiosis

     • Meiosis Animations From McGill University

     • Meiosis and Sexual Reproduction

     • Meiosis: an illustration of the meiotic process provided by Access Excellence.
       There is something very wrong with this illustration. Be the first to tell me what it is in the "Talk to Ed" discussion in the Web Crossing Bio chat Folder.

     • 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

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   Genetic Recombinations during Meiosis

   • 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.
     independent assortment 1
     independent assortment 2

   • 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 of homologous chromosomes during Meiosis I

     • Crossing over and recombination of alleles

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

   • Diploidy is restored when opposite gametes combine at fertilization.

   • Review of Human Sexual Life Cycle

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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

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   Glossary of terms relating to reproduction 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.

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   Take me home, please!