Grade 11 University BiologyMiss MacKay

MeiosisA special type of cell division

that occurs in the gonads Purpose: to create haploid

reproductive cells known as gametes.

Meiosis is name for the process that separates the duplicated nuclear material.

Cytokinesis is needed to split up the organelles and cytoplasm.

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

IntroductionJust like mitosis, Interphase

occurs prior to meiosis, and the chromosomes are duplicated.

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

Unlike mitosis, meiosis produces 4 non-identical haploid cells whose purpose is for reproduction only.

Meiosis I

First Half of MeiosisReduction Division

Prophase IMany similarities to

Prophase in Mitosis. Nuclear membrane

dissolves. Chromosomes are

attached to their copy by a centromere.

Centrioles appear and move to the poles

Prophase IBut also, all the

chromosomes of the same chromosome number, known as homologous chromosomes (i.e. all chromosome #1) and their copies from DNA duplication, 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 reasons for the variation seen in offspring.

Crossing Over

Metaphase IThe tetrads align

themselves at the equator.

All tetrads align independent of one another.

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

Metaphase ISpindle fibres

released from the centrioles at the poles attach to the centromeres of the sister chromatids.

Anaphase IThe spindle fibres

contract and pull one pair of sister chromatids to each pole

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

Telophase and CytokinesisA nuclear membrane

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

These two cells proceed into the next round of meiosis.

There is no second round of interphase.

Meiosis II

Second Half of Meiosis

Meiosis IIThe phases of meiosis II are all exactly identical to

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.

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 II The 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 nuclear membrane

starts to reform around the chromosomes at each pole.

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 (sperm in male, eggs in female).

Spermatogenesis and Oogenesis

Spermatogenesis

Oogenesis

MeiosisMeiosis Animation