Lesson OverviewLesson Overview11.4 Meiosis11.4 Meiosis

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Chromosome Number

Homologous chromosomes – for each chromosome in normal body cells, there is one that comes from the mother and one that comes from the father

Ex.

Diploid vs. Haploid Cells

DIPLOID CELLS (somatic, or normal body cells)

• A cell that contains both sets of homologous chromosomes is diploid, meaning “two sets.”

• Represented by the symbol 2n

Diploid vs. Haploid Cells

HAPLOID CELLS (gametes, or sex cells)

– Some cells contain only a single set of chromosomes, and therefore a single set of genes; such cells are haploid, meaning “one set.”

– Gametes of sexually reproducing organisms are HAPLOID

– can be written as n

Phases of Meiosis

– Meiosis: process in which the number of chromosomes per cell is cut in half

– Meiosis usually involves two distinct divisions, meiosis I and meiosis II.

– By the end of meiosis II, the ONE diploid cell becomes FOUR haploid cells

Meiosis I

– Before meiosis I, the cell goes through INTERPHASE

– Each replicated chromosome consists of two identical chromatids joined at the center

PROPHASE I– Chromosomes pair up forming a tetrad, which

contains four chromatids

– They undergo a process called crossing-over

– Crossing-over is important because it is a source of genetic variation

METAPHASE I / ANAPHASE I

• Paired homologous chromosomes line up across the center of the cell (metaphase)

• Homologous chromosome pair pulled toward opposite ends of the cell (anaphase)

TELOPHASE I / CYTOKINESIS

• Nuclear membrane forms around each cluster of chromosomes (telophase)

• Two new cells formed (cytokinesis)

MEIOSIS I – IMPORTANT EVENTS

1. In prophase I of meiosis, crossing over

occurs

2. During metaphase I of meiosis independent

assortment occurs

Meiosis II

– The two cells produced by meiosis I now enter a second meiotic division (MEIOSIS II)

– NEITHER CELL GOES THROUGH INTERPHASE (no replicating of DNA)

MEIOSIS II

• Basically same as phases as mitosis

• By the end of meiosis II, the diploid cell becomes four haploid cells (GAMETES)

• Male gamete = SPERM

• Female gamete = EGG

Gametes to Zygotes

Fertilization—the fusion of male and female gametes—generates new combinations of alleles in a zygote

Comparing Meiosis and Mitosis

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