Biology 100/101
Lecture 10
Organism Reproduction:
Meiosis and Fertilization
(Print Version)

Announcements &
Assignments

Lecture Objectives

Web Resources

Sex and Reproduction

Meiosis
The Process

Genetic Recombinations
During Meiosis

Cloning

Comparison of
Mitosis and Meiosis

Glossary

Lecture Syllabus

IB 100/101 Home Page

Announcements
Text Readings
in Lewis, et. al.		 Testing Your Knowledge Thinking Scientifically
Chapter 9, Meiosis Page 175, questions 2, 4, 6, 7, and 8 Page 176, questions 1-3

The "Mastering Concepts" boxes are valuable summaries of the main ideas in these sections of the text.

Answers to many of these questions can be found at the Text On-Line Learning Center

You may also ask questions and see answers to your classmates' questions in Web Crossing in the "Talk to Ross and Ed" discussion.

The content of today's lecture will help you answer the questions on this assignment:

Objectives:

After studying this material you should be able to:

  1. Discuss the relationship between sex and reproduction, and compare 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.

  4. Explain the role of meiosis in an organism's sexual life cycle.

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

  6. Indicate where and when in your body meiosis occurs and describe what is produced by the process.

  7. Compare the process of meiosis in human females and males.

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

  9. Compare the timing, location, numbers of cells, numbers of chromosomes, and genetic outcomes of mitosis and meiosis.

Web resources:

Sex - Biologically speaking:

  • What is it?

  • Why is it important?

  • Are sex and reproduction always linked?

Sex, reproduction, and the usefulness of genetic variability

  1. Reproduction and sex are not necessarily linked. Many can reproduce without sex (asexual reproduction):
  2. Asexual reproduction tends to produce genetically identical individuals.
    • A "good idea" in a stable environment

    • Maybe not a good idea if the environment is variable

    • Genetic changes can occur randomly by mutation
  3. Sexual reproduction produces genetic variability within a population.
    • Variability is evolutionarily beneficial in a changing environment, allowing populations to adapt to changes over time (as measured in generations).

The General Sexual Life Cycle


Human Life Cycle (Online Biology Text Book)

Does meiosis occur in your body?

What is meiosis?

  • Human Chromosomes Lewis, et. al. Page 201
    • The problem is to produce a new human being.

    • Both parents have to contribute genetic information.

    • The question is, in what form and how much?
  • Preparing for Meiosis:
  • How Meiosis works:
  • Summary of the basic process of meiosis
    • A process including TWO cell divisions which results in FOUR daughter cells.

    • Meiosis occurs in the sex organs in cells destined to produce gametes.

    • Each daughter cell receives only half the number of chromosomes as the mother cell. That means the cells are described as haploid

    • Each of the resulting 4 cells is genetically unique.

    • If meiosis works properly, each of the 4 cells has one of each type of chromosome.

    • The resulting haploid cells develop into gametes (eggs or sperm).
    • Diploidy is restored when egg and sperm combine at fertilization. Lewis, et. al. pg. 168, fig. 9.11

Genetic Recombinations during Meiosis

The way the chromosomes are assorted 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 type (one of each numbered) chromosome.

    Independent assortment.

    • Independent assortment Lewis, et. al., pg. 188, figure 10.12

    • 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!

    Crossing Over

    • 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 Lewis, et. al., pg. 164, fig. 9.8

Cloning - Another means of asexual reproduction

Comparison of Mitosis and Meiosis:

See Lewis, et. al., pg 166, table 9.2

See Lewis, et. al., pg 167, fig. 9.10

Mitosis Meiosis
One division Two divisions
Homologous Chromosomes line up
independent of each other at metaphase		 Homologous Chromosomes
synapse at Metaphase I
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. half 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

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 division 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 gene locations (Genes for the same traits). One chromosome of each pair comes from each of the parents through the gametes.

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

Sister Chromatids: The two halves of a replicated chromosome. Each chromatid is an identical copy of the DNA of the original chromosome before DNA replication.

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.