Meiosis

Meiosis is referred to as reduction division because meiosis results in haploid cells called gametes (sex cells).

Male and female gametes fuse duringfertilization (syngamy) to form a zygote.

The zygote divides by mitosis to form an embryo.

Human embryo develops into a a fetus.

• Meiosis may be thought of as a “double mitosis.”

• Characteristics of meiotic phases are similar to the characteristics of mitotic phases.

• If you know what happens during mitosis, then you know most of what happens during meiosis (with a few differences).

Meiosis occurs in two parts: Meiosis I and then Meiosis II.

Meiosis I –(Starts with a diploid cell-2N) Phases: prophase I, metaphase I, anaphase I,

and telophase I, then cytokinesis which results in two haploid cells.

PROPHASE I• During Prophase I pairs of homologous

chromosomes called tetrads align themselves closely and exchanges pieces.

• When chromosomes of the tetrad are near enough next to each other, it is called synapsis.

• During Prophase I, as chromosomes of a tetrad are in synapsis, and exchange of genes can occur.

• This exchanges of genetic material is called “crossing-over.”

• The point at which crossing-over occurs is referred to as a chiasmata

What is the advantage of crossing-over?

Crossing-over increases genetic variety in organisms, by “shuffling” the genes for different combinations.

• Meiosis I then proceeds through metaphase I, and then anaphase I.

• All phases are similar to the mitotic phases. (spindle formation, tetrads move toward center, tetrads line-up along the center, tetrads separate and chromosomes move to opposite poles)

• Cytokinesis occurs and the 2N cell has now become 2 haploid daughter cells.

Two Haploid cells result

These cells go directly into prophase II --- the first phase of Meiosis II.

Meiosis I leads to Meiosis II• Meiosis I leads directly to Meiosis II, with no interphase

in between.• During meiosis II phases are similar to mitosis phases:

(chromatid pairs move toward the center, sister chromatids separate and then cytokinesis)

Finally – 4 haploid cells result

Meiosis

Meiosis

FAST MEIOSIS

If the gametes are to be male, then 4 individual sperm cells

form.

The process of forming sperm cells is called spermato-genesis

If the gametes are to be female, then the result is 1 egg cell and 3 polar bodies.

The process of producing

the egg cell is called oogenesis.

The egg cell gets most of the

cytoplasm and the much smaller polar bodies disintegrate.

Egg cell – OvumEgg cells - Ova

Relative Sizes of Gametes