Lecture  5   Meiosis and Sexual Reproduction

I. Introduction

Life Cycle:  generation  to generation, from conception to production of its
                     own offspring.

Sexual Reproduction:


Genetic variation:

  Meiosis: special kind of cell division.  Behavior of the chromosomes...Chromosome number is reduced to half in the gametes, and some exchange of the chromosome parts occurs during the formation of the gametes.

II. Meiosis

A.  Useful terms:

Somatic cells:  diploid cells, contain two sets of the chromosome.

Homologous chromosomes:  a pair of chromosome during meiosis, same length, and control
                                                the same inheritated characters.

Sex chromosomes:  XY,   XX


Gametes:  a single set of the 22 autosomes plus X   or   Y  for human

Haploid cell:



 Diploid cells:  zygote and all other somatic cells.
                        2n = 46, 22 pairs of autosomes, plus a pair of sex chromosomes XX for female
                        or XY for male for Humans.

B. Stages of Meiosis:

1.  Interphase I  :   similar to the interphase in mitosis

 Meiosis  I

a. Prophase I:

1) chromatin begins to condense to form chromosome.
2) Exchange of choromosme parts.

b. Metaphase  I:
The chromosomes are now arranged on the metaphase plate, still in homologous pairs.

c. Anaphase I:
 Homologous chromosome separate, and move to the opposite end of the cell.

d. Telophase I and cytokinesis:
Each pole has a haploid chromosome set, but each chromosome still has two sister chromotids.  Cytokinesis form two daughter cells.

*   No interphase, no further replication of the genetic
     material prior to the second division of meiosis.
Meiosis II is just like another round of mitosis.

2. Meiosis  II

a. Prophase II

b. Metaphase  II

c. Anaphase  II:
Sister chromotids of each homologous chromosome are separated, and move to the opposite end of the cell.

d. Telophase II and Cytokinesis:
Nuclei form at opposite poles of the cell, and cytokinesis occurs, four daughter cells, each with the haploid number of chromosomes are formed.

III. Life Cycles

The timing of meiosis and fertilization in different group of species varies.

Alternation of Generations:  A special kind of life cycle, include both diploid and haploid multicellular stages.

Sporophyte:  The multicellular diploid stage.

Gametophyte:  The multicellular haploid stage.

A.  Animal life cycle:  Only has diploid multicellular organism.

B. Most fungi and some algae:  Only has haploid multicellular organism.

C. Plants and some algae:  has both haploid multicellular organism and diploid multicellular

IV.  Advantages and Disadvantages of Sexual

Advantages                                         Disadvantages