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

Text readings in Hoefnagels

Chapter 9, Sexual Reproduction and Meiosis

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Moodle

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Objectives:

The content of this lecture will help you complete this assignment:

Take-Home Assignment #4 - Meiosis and Genetics due at Lecture Wednesday October 21.

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:

• Meiosis from Sumanasinc.com

• Meiosis Tutorial from the Biology Project, University of Arizona

• Meiosis diagramfrom Access Excellence from The National Health Museum, formerly from Genentech

• Genes and Disease (Selected genes and their functions and locations on the chromosomes) from the National Center for Bitechnology Information

• Interactive Chromosome/Gene Poster from Gene Gateway - Exploring Genes and Genetic Disorders

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):

Asexual Reproduction from Kimball's Biology Pages.

• Microbes (bacteria, fungi): "Sex" in bacteria (Lewis, et. al. pg. 166, fig. 10.3

• Insects: Parthenogenesis (asexual reproduction) in aphids from bugguide.net

An egg develops into a new individual without fertilization. Parthenogenesis naturally occurs in some plants, insects, some fishes, frogs, and lizards. It does not normally occur in mammals, but has been artificially stimulated in mice.

• Plants: Dandelion

• But not mammals (and very rarely in other vertebrates)

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 

Does meiosis occur in your body?

What is meiosis?

• Human Chromosomes Hoefnagels Page 158, figure 84.5

• 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?

• You get one of each pair of numbered chromosomes from each parent.

• The paired chromosomes are called homologous pairs.

• Homologous chromosomes carry genes that control the same characteristics (traits).

Homologous chromosomes Hoefnagels. Page 196, fig. 10.1

Genes and Disease (Selected genes and their functions and locations on the chromosomes) from the National Center for Bitechnology Information

Interactive Chromosome/Gene Poster from Gene Gateway - Exploring Genes and Genetic Disorders

• Preparing for Meiosis:

• Cell Cycle Diagram Hoefnagels Page 160, fig. 8.5

• Single or Replicated Chromosomes? Hoefnagels Page 157, figure 8.3

• DNA replication PRODUCES sister chromatids

• How does DNA replicate?  Hoefnagels, pg 144, figure 7.11

• What are chromosomes made of? (Hoefnagels pg 157, figure 8.2

• Visit cellsalive.com and select Cell Biology and then the Cell Cycle.

• Watch for the DNA replication / chromosome replication / formation of sister chromatids during the S phase of the cell cycle.

• How Meiosis works:

• Overview of Meiosis Hoefnagels pg. 179, fig. 9.5

• Details of Meiosis Lewis, et. al. pg. 170-1, fig. 10.7

• Meiosis I Hoefnagels pg. 180, fIg. 9.6.1

• Meiosis II Hoefnagels pg. 180, fIg. 9.6.2

• Meiosis from Sumanasinc.com

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

• Meiosis Tutorial from the University of Arizona

• 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).

Meiosis and the Development of sperm Hoefnagels pg. 188, fig. 9.15

Meiosis and the Development of eggs Lewis, et. al. pg. 180, fig. 10.17

Timing of Development of sperm and eggs Lewis, et. al. pg. 180, fig. 10.17

• Diploidy is restored when egg and sperm combine at fertilization. Lewis, et. al. pg. 174, fig. 10.11

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

• Possible arrangements of homologous Chromosomes during meiosis Hoefnagels pg. 183, figure 9.8

• Independent assortment Hoefnagels pg. 203, figure 10.9

• Independent Assortment and Gamete Diversity from Sumanasinc.com

• Independent Assortment of Alleles from Sumanasinc.com

• 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!

Meiosis and 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 Hoefnagels, pg. 182, fig. 9.7

• Meiosis from Sumanasinc.com

• Gamete variation by Crossing Over depends on the relative location of linked gene loci.

Cloning - Another means of asexual reproduction

• Dolly's pushing up the daisies...

• What is Cloning from the The Genetic Science Learning Center, The Univeristy of Utah
  

• News items dealing with cloning and stem cells from sciencedaily.com
  (Fodder for extra credit?)

Comparison of Mitosis and Meiosis:

Hoefnagels, pg 184, fig. 9.10

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.