65 Meiosis

Key Idea: Meiosis is a reduction division that produces haploid cells for the purposes of sexual reproduction.

What is meiosis?

` Meiosis is a special type of cell division necessary for the production of sex cells (gametes) for the purpose of sexual reproduction. ` DNA replication precedes meiosis.

If genetic mistakes (mutations) occur here, they will be passed on (inherited by the offspring). ` Meiosis involves a single chromosomal duplication followed by two successive nuclear divisions, and it halves the diploid chromosome number. ` An overview of meiosis is shown on the right. Meiosis occurs in the sex organs of plants and animals.

Meiosis produces variation

` During meiosis, a process called crossing over may occur when homologous chromosomes may exchange genes. This further adds to the variation in the gametes. ` Meiosis is an important way of introducing genetic variation. The assortment of chromosomes into the gametes (the proportion from the father or mother) is random and can produce a huge number of possible chromosome combinations. ` Recall that each sister chromatid is one half of a replicated chromosome.

When they are separated, they are once again called chromosomes.

1. (a) What is the purpose of meiosis?

(b) Where does meiosis take place?

(c) Why are the gametes produced by meiosis haploid?

2. Describe how meiosis produces variation in the gametes:

The process of meiosis

Diploid (2N) cell has two complete sets of chromosomes, one from the mother, one from the father. N is the haploid number of chromosomes. Each chromosome is replicated.

Homologous chromosomes pair up. The pairs are separated. Crossing over has occurred.

An intermediate cell forms with a replicated copy of each chromosome.

The chromosomes line up again. This time the chromatids are separated. Chromatid

Haploid (N) gametes form. They have one complete set of chromosomes.