Meiosis

Meiosis

Meiosis is used to form gametes (Sex cells). It is known as reduction division as the number of chromosomes is reduced by half. It will produce 4 haploid daughter cells that are genetically different. At AS level you need to know the reasons for variation in meiosis and an overview of the stages. At A2 level you need to know the AS material and the individual stages.

AS Level.

meiosisMeiosis diagram supplied by Alila Medical Media. To see more cell diagrams click on the link.

 

 

 

 

 

 

As you can see Meiosis has two divisions. One of the most confusing aspects of meiosis is the chromosome number so lets go through that. A typical human cell has 46 chromosomes arranged in 23 pairs. Lets say that each chromosome has a mass of 1 unit. This means that there are 46 mass units in total.

In the early stages of Meiosis the DNA will replicate. However this causes a lot of confusion.

chromosome replicating

As you can see from the replication that actually its more like it grows another chromatid. This means that when the DNA replicates the number of chromosome will not double. However, the mass of DNA will.

As a result of the first division where the pairs of chromosomes separate. So each daughter cell will have half the number of chromosomes as the original cell, but the mass of DNA after the first division will be equal to the mass of DNA before the DNA replicated.

During the 2nd division the actual chromosome splits into two. This means that the mass of DNA in each cell will halve again. However, the actual number of chromosomes at the end of the second stage will be the same as the number of chromosomes at the end of the first stage. Lets imagine that 23 chromosomes split, this will form 46 chromatids in total. Take the 46 chromatids and share them between two cells. This will result in each daughter cell receiving 23 chromosomes.

Reasons for variation in Meiosis:

1. Mutation, a mutation is random variation in DNA.

2. Crossing over of chromosomes causes chiasmata to form.

3. Random assortment and independent segregation. This is a rather complex idea, but it means that the daughter cells receive a mixture of chromosomes from the parent cell.

 

A2 Meiosis.

Meiosis has two clear divisions Meiosis I and Meiosis II.

Meiosis I

This is made up of 4 stages Prophase, Metaphase, Anaphase and Telophase. The cell starts off like this:

Meiosis 1

© Dreamy Girl – Fotolia.com

 

 

 

 

 

 

Prophase I

During prophase 1 the homologous chromosomes will form a bivalent. Crossing over of sister chromatids will cause chiastmata to form. The chromosomes will shorten and thicken. Nuclear envelope will start to break down and spindle fibres will start to form.

meiosis 2

© Dreamy Girl – Fotolia.com

 

 

 

 

 

 

Metaphase I

The bivalents arrange themselves on the equator of the cell. The chromosomes are attached to the spindle fibres by their centromeres.

meiosis 3

© Dreamy Girl – Fotolia.com

 

 

 

 

 

 

Anaphase I

The spindle fibres shorten and the chromosomes separate going to opposite poles of the cell.

meiosis 4

 

© Dreamy Girl – Fotolia.com

 

 

 

 

 

Telophase I

Nuclear envelope forms around each set of chromosomes and the chromosomes start to uncoil. Spindle fibres start to break down.

meiosis 5

 

© Dreamy Girl – Fotolia.com

 

 

 

 

 

Metaphase II

The stages of Meiosis II are actually very similiar to Mitosis.

Prophase II

Two cells were formed at the end of Meiosis I. During prophase II the nuclear envelope breaks down, spindle fibres form and the chromosomes shorten and thicken.

Meiosis6

© Dreamy Girl – Fotolia.com

 

 

 

Metaphase II

The chromosomes align themselves along the equator. They are attached to the spindle fibres by their centromeres.

meiosis7

© Dreamy Girl – Fotolia.com

 

 

 

Anaphase II

Spindle fibres shorten separating the chromatids. This pulls the chromatids to opposite poles of the cell.

meiosis 8

 

Telophase II

The nuclear envelope forms around each set of chromosomes. Chromosomes uncoil. Also as a result of cytokinesis, the cells will split to form 4 daughter cells.

Meiosis 9

© Dreamy Girl – Fotolia.com