MeiosisMeiosis

The process of creating sex cells

Ie: Eggs (female)

Sperm (males)

What is MeiosisMeiosis? Meiosis involves two successive

nuclear divisions that produce four haploid cells. The first division (meiosis I) is the reduction division; the second division (meiosis II) separates the chromatids.

Advantages of Meiosis

Mitotic cell division produces new cells genetically identical to the parent cell.

VS Meiosis increases genetic variation in

the population. – exchange of information can occur during meiosis I while mutations can also occur

Differences

Mitosis happens in somatic cells Somatic: any cell in the body that is not a sex cell Location? __everywhere except gonadal cells_

MeiosisMeiosis happens in sex cells Sex cells: the reproductive cells of living things_ Location?__ovaries and testicle of mammals…plants

ovum and anther as pollen_

Interphase

Interphase is an important stage preceding meiosis. Without this stage meiosis would not occur.

During this stage, each individual chromatid replicates, similar to mitosis.

B replicates B and b replicates b At this stage, the chromosomes are long and stringy

and are not visible. **Remember: All somatic cells are diploid in number

(2n), therefore for each chromatid there also exists its homolog, which also replicates during interphase.

The graphical representation here shows that each chromotid pair BB is connected by a centromere (the gold structure). Each pair of the chromosome are referred to as a sister chromatids

Prophase I Prophase I is one of the most important stages of meiosis.

The chromatids twist and condense, creating chromosomal structures which are visible to the microscope.

Each chromosome then actively seeks out its homologous chromosome (Double stranded chromosomes that code for the same factors: but one was from your mom and one was from your dad originally)

After the homologous chromosomes pair, the structure is referred to as a tetrad (four chromatids)BB and bb. The point at which two non-sister chromatids intertwine is known as a chiasma.

Sometimes a process known as crossing over occurs at this point. This is where two non-sister chromatids exchange genetic material. This exchange does not become evident, however, until the two homologous pairs separate.

NOTE: A pair of non-sister chromatids are chromosomes that code for the same factors

Prophase I

Cell Photographs of Prophase I

Metaphase I At metaphase, each chromosome has

reached its maximum density (condensing). The homologous pairs and their sister

chromatids also prepare for separation They interact with spindle fibers which form

from either side of the nuclear envelope of the cell.

The spindle fibers are produced from a structure called a centriole.

There is a centriole at opposite ends of the cell, which is referred to as poles. Analogous to North and South or East and West

Metaphase I During metaphase, the chromosomes BB and bb are lined by the spindle fibers at what is known as the metaphase plate which is located in the middle of the cell

As in the cell picture, it looks like a line through the center of the cell.

Metaphase I •During this time information (sequences of ATCG) from BB can switch with sequences of ATCG from bb).

•This chiasma and crossing over is what gives the gametes their variations.

•This is why you will nevernever look exactly like your siblings.

Chiasma and Crossing Over

Anaphase I

Anaphase I pulls apart the tetrad, separating each homologous chromosome. BB goes to one pole and bb goes to the other pole

The affects of crossing over are evident when anaphase I occurs.

Telophase I Telophase I varies from species to species. Sometimes Telophase I is skipped and meiosis starts

its second division immediately. In general, however, two nuclear envelopes begin to

surround the separate chromosomes and cytokinesis (splitting of the cytoplasm into two separate entities) will sometimes occur. Then a phase called interkinesis will follow, which essentially is a resting period from Telophase I to Prophase II.

This differs from mitosis because DNA replication of the newly divided genetic information does not occur.

Telophase I

Now cells are 1n or n meaning that B and b, chromosomes that code for the same factor have been separated from each other.

1n or n B and b separated, NOT 2n Bb

Prophase II

During Prophase II, each dyad (1/2 a tetrad) is composed of a pair of sister chromatids and they are connected by a centromere. BB and bb now in separate cells

The centrioles (replicated during Telophase I) which produce the spindle fibers also start to move toward the poles of the cell.

Spindle Fibres:___________________________________________________________________

Prophase II

Evidence of crossover

Between BB and bb

Metaphase II Metaphase II is similar to Metaphase I in

that the dyads are lined up at a metaphase plate by the spindle fibers.

In this image BB only

Anaphase II Anaphase II separates the dyads into

individual chromatids. Each sister chromatid ends up on one

side of the cell.

Telophase II

At the end of Telophase II, the nuclear envelopes forms around each set of DNA and the cytoplasm divides once again.

As a result, four haploid cells have formed from one diploid cell.

The chromosomal content of a haploid cell in one-half the chromosomal content of a diploid cell ( n as opposed to 2n ) B B

b b

                

B B

b b

Telophase II

The chromosomal material has be halved.

This happens in a Human reproductive cell (sperm or egg)

2n = 46 chromosomes 1n = 23 chromosomes

Telophase II summary

Meiosis I Flowchart

Meiosis II Flowchart

Males: all 4 hapoid cells become sperm

Females: in oogenesis, only 1 of the haploid cells becomes an egg, and the other 3 are reabsorbed by the body.