Introduction Meiosis is a special type of cell division that occurs in the gonads. Its purpose is to create reproductive cells known as gametes. These.

IntroductionMeiosis is a special

type of cell division that occurs in the gonads.

Its purpose is to create reproductive cells known as gametes.

These gametes are haploid as they only carry one copy of each type of chromosome.

IntroductionLike mitosis, the gonad cells go through Interphase prior to

meiosis.This means that the chromosome number prior to meiosis

doubles.As with mitosis, meiosis only deals with the separation of the

nuclear material, whereas cytokinesis is still needed to split up the organelles and cytoplasm.

IntroductionUnlike mitosis, meiosis

produces 4 non-identical haploid cells.

Their purpose is for reproduction only.

Also meiosis has the same stages as mitosis, however they occur twice in order to reduce the chromosome number from diploid to haploid (reduction division).

The first half of meiosis(Reduction Division)

Prophase INuclear membrane

dissolvesChromosomes are

attached to their copy by a centromere.

Centrioles appear and move to the poles.

All this is similar to mitosis.

Prophase IAll the chromosomes

of the same chromosome number (i.e. all chromosome #1 and their copies), known as homologous chromosomes, join together to form a tetrad or bundle of four chromosomes (2 from each parent).

This is different to mitosis!

Prophase IDuring the tetrad

formation, non-sister chromatids can exchange genes to create unique allele combinations that are not seen in the parent.

This is called crossing over and is one of the two reasons for the uniqueness of the offspring.

Crossing Over

Metaphase IThe tetrads align

themselves at the equator with the maternal pair of chromosomes facing one pole and the paternal chromosome pair facing the other pole.

All tetrads align independent of one another.

This is called independent assortment and is the other reason for the variation seen in offspring.

Metaphase IOnce aligned, the

tetrads are held in place by spindle fibres released from the centrioles at the poles.

The spindle fibres attach to the centromeres of the sister chromatids.

Anaphase IThe spindle fibres

contract and pull apart the tetrad such that one pair of sister chromatids goes to one pole and the other pair goes to the other pole.

Notice that the centromere did not split apart during this phase. It still holds the chromosome copies (sister chromatids) together.

Telophase I and CytokinesisA nuclear membrane

reforms around each new nucleus and the cytoplasm is divided by cytokinesis creating 2 non-identical cells.

These two cells immediately proceed into the next round of meiosis as there is no second round of interphase.

In some cell types, telophase I does not occur and the cell proceeds directly to the second round of meiosis.

The second half of meiosis

Meiosis IIThe stages of meiosis II

are all exactly identical to the happenings from mitosis.

The only difference is that they are occurring in two cells at the same time.

The end product is four, non-identical, haploid cells.

The process of meiosis occurs somewhat differently in males and females.

Prophase IIThe nuclear

membrane dissolves.The chromosomes

become more visible and are still attached to their copy by the centromere.

Centrioles move towards the “poles” of the cell.

Metaphase IIThe sister

chromatids line up at the “equator” of the cell.

Spindle fibres emerge from the centrioles and attach to the centromere of each chromosome pair.

Anaphase IIThe spindle fibres

contract, breaking the centromere, and pull apart the sister chromatids.

One copy of the chromosome pair goes to one pole, while the other copy goes to the other pole.

Telophase IIThe last stage of

meiosis, where the nuclear membrane starts to reform around the chromosomes that have been pulled to each pole and two new nuclei have formed in each cell.

The cell membrane starts to pinch inward at the equator signaling the end of meiosis and the beginning of cytokinesis.

The end product is four, non-identical, haploid cells.

HomeworkPage 174 #1, 2, 10 and the questions on the back of your diagram sheet.