Meiosis

Chapter 10, Section 2

Diploid & Haploid Cells

Diploid = a cell with two sets of chromosomes, 2n, somatic (body) cells

muscle, skin, liver, etc.Haploid = a cell with one set of chromosomes, gametes, 1n, sperm and eggsMeiosis forms haploid gametes required for sexual reproductionMeiosis has 2 stages – Meiosis I– Meiosis II

Interphase and stages of mitosis

Chromosomes from metaphase stage of mitosis

Below are chromosomes after they split into chromatids during (telophase) of mitosis

MEIOSIS ISeparation of homologous pairs

Homologous Pair = in a diploid cell the 2 sets of chromosomes pair up

1. Prophase I

DNA coils into chromosomes

Nuclear envelope & nucleolus break down

Synapsis = homologous chromosomes pair up into tetrads

Crossing-over = homologous pairs switch pieces of their chromatids

Genetic Recombination = maternal & paternal genetic info get shuffled during crossing-over

2. Metaphase I

Tetrads line up in the middle of the cell

Spindle fibers attach at the centromeres of each homologous chromosome

3. Anaphase I

Each homologous chromosome moves to the opposite end of the cell

Independent Assortment = random separation of chromosomes, allows for genetic variation

4. Telophase I & Cytokinesis I

Two newly formed cells are haploid

Each chromosome is still made up of two chromatids

MEIOSIS IISeparation of sister chromatids

Sister Chromatids = identical halves of a chromosome that has been duplicated

5. Prophase II

2 newly created cells from Meiosis I start to divide

6. Metaphase II

Chromosomes line up in the center

7. Anaphase II

Sister Chromatids separate

8. Telophase II & Cytokinesis II

4 new cells, each with half the number of chromosomes of the original cell

Development of Gametes

Spermatogenesis = production of sperm cells

Oogenesis = production of mature egg cells

Nondisjunction

Nondisjunction = failure of homologous pairs to separate in Meiosis I

If an organism survives, it may have a genetic disorder

Polyploidy

Polyploid = organisms with more than the usual number of chromosome sets

Occurs frequently in plants, results in plants that are larger than normal