10.2 Meiosis

Diploid & Haploid Cells Chromosomes in body cells occur in pairs. Diploid – a cell with two of each kind of chromosome (2N) Gametes have single chromosomes. Haploid – a cell with one of each kind of chromosome (N)

Homologous Chromosomes Paired chromosomes with genes for the same traits are called homologous. Genes are arranged in the same order on homologous chromosomes.

Why Meiosis? Human somatic cells (body cells) are diploid. Gametes (sperm and egg) are haploid Meiosis – a type of cell division that produces four gametes, each with half the number of chromosomes as a body cell.

Meiosis Meiosis occurs in two separate divisions, meiosis I and meiosis II. Meiosis starts with one 2N cell and ends with four N cells. Meiosis is necessary for sexual reproduction.

Meiosis cont.

Whiteboards What are homologous chromosomes? What does diploid mean? What does haploid mean? What is the difference between gametes and somatic cells? What would happen if gametes were diploid instead of haploid?

Meiosis I Crossing Over – non-sister chromatids of homologous chromosomes exchange genetic material. Homologous chromosomes are separated into two new cells.

Meiosis II Sister chromatids are separated Results in four cells, each with one copy of each chromosome.

Meiosis Provides Variation Meiosis makes cells that are not identical. Meiosis results in a variety of different gametes. Gametes are randomly fertilized, producing very large numbers of possible unique offspring. Crossing over in Meiosis I adds more variation.

Mistakes in Meiosis Nondisjunction – failure of homologous chromosomes to separate. Results in gametes that are missing chromosomes or that have extra chromosomes.

Mistakes in Meiosis cont. Trisomy – organism has three chromosomes instead of a pair.

Mistakes in Meiosis cont. Monosomy – one chromosome instead of a pair. Triploidy – three copies of ALL chromosomes, resulting from complete nondisjunction.

Whiteboards What is crossing over? How does crossing over generate variation? At the end of Meiosis how many cells are produced, and are they identical or different? What is non disjunction?