Biology: Meiosis

As in mitosis, the DNA starts off in long strands, not visible with a light microscope.

When the cells gets the order to divide using meiosis, the DNA becomes short and fat in chromosomes, and they duplicate, in a "butterfly shape", joined at the chromatid. Each "wing" of the "butterfly" is an exact copy of the other wing (and so doesn't count as another chromosome, because it is identical).

The chromosomes line up along the centromere, in two rows one chromosome deep either side of the centromere. They then divide into two cells. This mixes up the father's and the mother's chromosomes to introduce genetic variation.

Cover up the answers below and try these questions:

1) What happens first when the cell gets the order to divide?
2) Where are the two arms of the chromosomes joined?
3) Where do the cells line up?
4) How many rows deep are the chromosomes?
5) Why do they divide like this?

Answers:
1) The DNA strands become short and fat, then duplicate.
2) At the chromatid
3) Along the centromere
4) One row deep
5) To introduce genetic variation of the mother's and father's genes.

In the second division, the chromosomes line up (in one line this time) along the centromere in both cells.

Then the chromosomes are pulled apart along the chromatid (like in mitosis) so each of the cells created in the first division forms two cells.

This makes a total of four genetically different cells, each with only 23 chromosomes in them, whereas in mitosis, there are only two cells created, and they are identical.

Cover up the answers below and try these questions:

1) Where do the chromosomes line up?
2) How many cells are made in meiosis?
3) How many chromosomes are there in each cell created by mitosis?
4) What is the difference between cells made in mitosis and meiosis?

Answers:
1) Along the centromere
2) 4
3) 23
4) Cells made in meiosis are not genetically identical

In meiosis, four genetically different cells are created with 23 chromosomes in each.

Meiosis creates gametes (sex cells) and so happens in the ovaries and testes (in humans). Gametes must have 23 chromosomes in animals that reproduce sexually because otherwise the offspring will have 46 chromosomes from each parent, meaning that they have 92. The next generation would then have 92 chomosomes from each parent, and so on. To keep the number of chromosomes stable, we have to only have 23 chromsomes in each gamete.

Meiosis also happens in the sex organs of sexually producing plants, for example in the pollen of flowers.

The first division of meiosis is used to create genetic variation. The chromosomes inherited from the father and mother are mixed up so that each cell has a random selection. Genetic variation is what helps us to survive and makes us all unique.

Cover up the answers below and try these questions:

1) What is are gametes?
2) Where does meiosis take place in humans?
3) Why must gametes only have 23 chromosomes?
4) Why are the chromsomes mixed up in the first stage of meiosis?
5) Name one place where meiosis takes place in a plant.

Answers:
1) Sex cells
2) Ovaries and testes
3) To keep the number of chromosomes in a human stable.
4) To introduce genetic variation.
5) Pollen