MeiosisMeiosis

Division of Sex CellsDivision of Sex Cells

MeiosisMeiosis

A process of reduction division in which the A process of reduction division in which the number of chromosomes per cell is cut in half number of chromosomes per cell is cut in half through the separation of homologous through the separation of homologous chromosomes in a diploid cell.chromosomes in a diploid cell.

DiploidDiploid – 2 sets of chromosomes – 2 sets of chromosomes HaploidHaploid – 1 set of chromosomes – 1 set of chromosomes HomologousHomologous – chromosomes that each have a – chromosomes that each have a

corresponding chromosome from the opposite corresponding chromosome from the opposite sex parent sex parent

MeiosisMeiosis

Meiosis StagesMeiosis Stages

Meiosis usually involves 2 distinct stagesMeiosis usually involves 2 distinct stages Meiosis IMeiosis I Meiosis IIMeiosis II

Prophase IProphase I

Each chromosome pairs with its corresponding Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.homologous chromosome to form a tetrad.

There are 4 chromosomes in a tetrad.There are 4 chromosomes in a tetrad. The pairing of homologous chromosomes is The pairing of homologous chromosomes is

the key to understanding meiosis.the key to understanding meiosis. Crossing-over may occur hereCrossing-over may occur here Crossing-over is when chromosomes overlap Crossing-over is when chromosomes overlap

and exchange portions of their chromatids.and exchange portions of their chromatids.

Prophase IProphase I

Metaphase IMetaphase I

Spindle fibers attach to the chromosomesSpindle fibers attach to the chromosomes

Metaphase IMetaphase I

Anaphase IAnaphase I

The fibers pull the homologous The fibers pull the homologous chromosomes toward opposite ends of chromosomes toward opposite ends of the cell.the cell.

Anaphase IAnaphase I

Telophase I & CytokinesisTelophase I & Cytokinesis

Nuclear membranes form.Nuclear membranes form. The cell separates into 2 cells.The cell separates into 2 cells.

Telophase ITelophase I

Prophase IIProphase II

Meiosis I results in two haploid (N) cells.Meiosis I results in two haploid (N) cells. Each cell has half the number of Each cell has half the number of

chromosomes as the original cell.chromosomes as the original cell.

Prophase IIProphase II

Metaphase IIMetaphase II

The chromosomes line up similar to The chromosomes line up similar to metaphase in mitosis.metaphase in mitosis.

Metaphase IIMetaphase II

Anaphase IIAnaphase II

Sister chromatids separate and move to Sister chromatids separate and move to opposite ends of the cell.opposite ends of the cell.

Anaphase IIAnaphase II

Telophase IITelophase II

Meiosis II results in 4 haploid cells.Meiosis II results in 4 haploid cells.

Telophase IITelophase II

Gamete FormationGamete Formation

In males, meiosis results in 4 sperm cellsIn males, meiosis results in 4 sperm cells In females, meiosis results in 1 egg cell In females, meiosis results in 1 egg cell

and three polar bodies, which are not and three polar bodies, which are not used in reproduction.used in reproduction.

Mitosis vs MeiosisMitosis vs Meiosis

MitosisMitosis MeiosisMeiosis

Results inResults in 2 Diploid Cells 2 Diploid Cells (2N)(2N)

4 Haploid Cells 4 Haploid Cells (N)(N)

Cells areCells are Genetically Genetically IdenticalIdentical

Genetically Genetically DifferentDifferent

Occurs inOccurs in Somatic (Body) Somatic (Body) CellsCells

Sex CellsSex Cells