Analyze the meiotic maintenance of a constant chromosome number from one generation to the next.

Meiosis is division of the sex cells.

Meiosis involves one DNA replication and two cell divisions.

Interphase – DNA replicates

Stages of Meiosis

Meiosis I

Prophase I    Homologous chromosomes join to form a tetrad in a process synapsis. The

                    chromatids of the tetrad exchange genes in a process called crossing-over.

                    Crossing-over increases genetic diversity.

Metaphase I

Anaphase I

Telophase I  End result is 2 diploid cells that are genetically different.

                    These two cells are genetically different because of crossing-over and the

                    homologs have been separated.

Interkinesis  DNA does not replicate during interkinesis.

Meiosis II

The two daughter cells of meiosis I undergo meiosis II.  Meiosis II is just like mitosis.

Prophase II

Metaphase II

Anaphase II

Telophase II

End result is 4 haploid cells.

The chromosome number of a species must be maintained or genetic problems or death may occur.  Sexual reproduction is advantageous because it increases genetic diversity or variation. Increased variation of individuals within a population increases the likelihood of some individuals surviving environmental changes and thus helping the species survive. Sexual reproduction involves the union of two gametes. If these two gametes remained diploid

the offspring would have twice as many chromosomes as the parents. By replicating the DNA once and

                dividing the cell twice the gametes (sperm and egg) have the chromosome number cut in half making the cells haploid. When the sperm fertilizes the egg the resulting zygote has the original diploid number of chromosomes of the parents. Meiosis also increases genetic diversity through crossing over, something that would not be achieved by simply dividing a cell once without replicating DNA.