CHAPTER 13

MEIOSIS AND SEXUAL LIFE CYCLES


OBJECTIVES

After reading this chapter and attending lecture, the student should be able to:

 1.

 Distinguish between asexual and sexual reproduction.

2.

Diagram the human life cycle and indicate where in the human body that mitosis and meiosis occur.

3.

Distinguish between mitotic and meiotic cell division.

4.

List the phases of meiosis I and meiosis II and describe the events characteristic of each phase.

5.

Recognize the phases of meiosis from diagrams or micrographs.

6.

Describe the process of synapsis during prophase I and explain how genetic recombination occurs.

7.

Describe key differences between mitosis and meiosis.

8.

Explain how the end result of meiosis differs from that of mitosis.

9.

Distinguish between mitotic interphase and meiotic interkinesis.

10.

Explain how independent assortment, crossing over and random fertilization contribute to genetic variation in sexually reproducing organisms.

11.

Explain why inheritable variation was crucial to Darwin's theory of evolution.

KEY TERMS

homologous

meiosis

synaptonemal complex

chiasmata

locus

gene

meiosis I

interkinesis

meiosis II

diploid

haploid

synapsis

sister chromatid

asexual reproduction

sexual reproduction

somatic cell

gamete

crossing over

tetrad

fertilization


LECTURE NOTES

I. Offspring acquire genes from parents by inheriting chromosomes

II. Like begets like, more or less: a comparison of asexual versus sexual reproduction

Asexual Reproduction Sexual Reproduction
Single individual is the sole parent. Two parents give rise to offspring.
Single parent passes on all its genes to its offspring. Each parent passes on half its genes, to its offspring.
Offspring are genetically identical to the parent. Offspring have a unique combination of genes inherited from both parents.
Results in a clone, or genetically identical individual. Rarely, genetic differences occur as a result of mutation, a change in DNA. Results in greater genetic variation; offspring vary genetically from their siblings and parents.

III. Fertilization and meiosis alternate in sexual life cycles: an overview

  IV. The Variety of Sexual Life Cycles

 V. Meiosis reduces chromosome number from diploid to haploid: a closer look

A. The Stages of Meiotic Cell Division

Meiosis I: This cell division segregates the two chromosomes of each homologous pair and reduces the chromosome number by one-half. It includes the following four phases:

Meiosis II: This second meiotic division separates sister chromatids of each chromosome.

B. A Comparison of Mitosis and Meiosis

 COMPARISON OF MEIOSIS I AND MITOSIS
  MEIOSIS I MITOSIS
Prophase Synapsis occurs to form tetrads. Chiasmata appear as evidence that crossing over has occurred. Neither synapsis nor crossing over occurs.
Metaphase Homologous pairs (tetrads) align on the metaphase plate. Individual chromosomes align on the metaphase plate.
Anaphase Meiosis I separates pairs of chromosomes. Centromeres do not divide and sister chromatids stay together. Sister chromatids of each chromosome move to the same pole of the cell; only the homologues separate. Mitosis separates sister chromatids of individual chromosomes.

Centromeres divide and sister chromatids move to opposite poles of the cell.

Meiosis II is virtually identical in mechanism to mitosis, separating sister chromatids, except that cells in meiosis two are haploid (possess a single set of chromosomes).

V. Sexual life cycles produce genetic variation among offspring

A. Independent Assortment of Chromosomes

B. Crossing Over

C. Random Fertilization

VI. Evolutionary adaptation depends on a population's genetic variation


REFERENCES

Alberts, B., D. Bray, J. Lewis, M. Raff, K. Roberts and J.D. Watson. Molecular Biology of the Cell. 2nd ed. New York: Garland, 1989.

Becker, W.M., J.B. Reece, and M.F. Puente. The World of the Cell. 3rd. ed. Redwood City, California: Benjamin/Cummings, 1996.

Campbell, N. Biology. 4th ed. Menlo Park, California: Benjamin/Cummings, 1996.

Kleinsmith, L.J. and V.M. Kish. Principles of Cell Biology. New York: Harper and Row, Publ., 1988.