Cell Division - Meiosis

Meiosis

• Process of cell division that reduces the number of chromosomes in new cells to half the number in the original cell.

• The haploid cells produced are called gametes.

Meiosis

• Cells preparing to divide by meiosis undergo the G1, S and G2 phases of interphase.

Meiosis

• Cells will then begin meiosis with a duplicate set of chromosomes.

• Meiosis is divided into two stages called Meiosis I and Meiosis II.

Meiosis I

• Prophase I• DNA coils tightly into

chromosomes, spindles appear, and the nuclear membrane disassembles.

Prophase I

• Each chromosome lines up next to its homologue. This is called synapsis.

• Each pair of homologous chromosomes is called a tetrad.

Prophase I

• During this stage, crossing-over occurs.

• This is when genetic material breaks off and attaches to the adjacent chromatids.

• The result is genetic recombination.

Metaphase I

• Tetrads line up randomly along the midline of the dividing cell.

• Spindles attach to the centromeres of the homologous chromosomes.

Anaphase I

• Each homologous chromosome moves to an opposite pole.

• The chromosomes are separated randomly. This is called independent assortment.

Telophase I

• Chromosomes reach the opposite ends of the cell and cytokinesis begins

Meiosis II

• Meiosis II is not started by the copying of DNA.

• There may be a short resting stage but usually there is no true interphase.

Prophase II

• Spindle fibers form and begin to move the chromosomes toward the midline of the cell

Metaphase II

• Chromosomes move to the midline of the cell.

Anaphase II

• The chromatids separate and move toward opposite poles of the cell.

Telophase II

• Cytokinesis occurs and produces 4 cells

• A nuclear membrane forms around the chromosomes in each of the 4 new cells.

Formation of Gametes

• In males, meiosis occurs in the testes. This is called spermatogenesis.

• Four sperm cells result from meiosis.

Formation of Gametes

• In females, meiosis occurs in the ovaries. This is called Oogenesis.

Formation of Gametes

• During cytokinesis for females, the cytoplasm is unequally divided.

• This results in only 1 egg cell being formed and 3 polar bodies.

Asexual Reproduction

• Offspring from only one parent

• Offspring are genetically identical

• Occurs from binary fission or mitosis

Sexual Reproduction

• Offspring from two parents

• Offspring are genetically different

• Meiosis