Mammoth Memory

Meiosis

Produces either sperm eggs or pollen 

Starts with one human cell containing 46 chromosomes

Ends up with 

4 cells each containing 23 chromosomes

 

These 4 cells are either

4 eggs of animal or plants

or

4 sperm of animal or pollen for plants

NOTE:

After Meiosis

  1. If 4 sperm are produced one may join with a human egg
  2. If pollen are produced they may join with plant eggs
  3. If 4 human eggs are produced one may join with a sperm
  4. If plant eggs are produced they may join with pollen

 

Lets follow specifically human meiosis

Meiosis starts with a cell. This is a normal cell containing 46 chromosomes, 23 come from your mother and 23 come from your father.

See parent cell (diploid)

It starts with one human cell containing 46 chromosomes.

Parent cell (Diploid)

46 chromosomes

46 chromosomes

Diploid or Di means two

Diploid - a cell containing two complete

sets of chromosomes one from each parent

The chromosomes are copied and each one remains attached to the chromosome it was copied from to form an X shape. Each chromosome X shape inherited from one parent matches itself up with the corresponding chromosome from the other parent.

See the 46 copied chromosomes

The chromosomes are copied
46 chromosomes being duplicated

When its found its match it then joins the two X shapes. During this phase genes inherited from one parent may be transferred over to the matching chromosome inherited from the other parent in a process known as crossing over (shown by red moving onto blue and vice versa). Crossing over produces genetic variation.

Chromosomes pair up and swap DNA called cross over

All the matched chromosomes form a line down the middle of the cell.

See matched chromosomes in a line

Chromosomes align down the centre of cell.
23 pairs of chromosomes

The cell then splits to form two cells.

See cell split in two

Cell split into two
Cell split in two containing chromosomes

These cells then split again to form 4 cells with 23 chromosomes in each one.

See cell split into 4

Cell now splits again and we have a total of four cells.
Cell split into four containing chromosomes
We have four haploid cells (haploid is half the number of original chromosomes) haploid is a cell having a single set of unpaired chromosomes.

We now have 4 cells containing 23 chromosomes.

If a lady produces meiosis this would be 4 eggs

If a man produces meiosis this would be 4 sperm.

The lady and man can now continue to sexual reproduction joining the mans sperm to the ladies eggs. 

Normally one sperm fertilises one egg only.

Simplified, if we look at just 4 chromosomes, meiosis would be as follows:

Meiosis produces four genetically different haploid cells meiosis is a reduction division

This produces 4 cells which form into either sperm if produced by the male or eggs if produced by the female.

These cells are called sex cells or gametes, and in humans will contain 23 chromosomes.

In humans the 4 cells that are created by meiosis each contain 23 chromosomes as follows:

The 4 cells created through meiosis contain 23 chromosomes

Chromosome 23 will either be an X or Y. Your mothers egg cell can only contain an X chromosome but your fathers sperm cell can provide an X or Y see below:

In males the cells produced through meiosis are inserted into sperm cells in females they take the form of eggs

The probability of inheriting a male or female chromosome can be shown in a punnet square.

The probability of inheriting male or female chromosomes shown in a punnet square

The probability of producing a male or female is 8 out of 16, or 1 out of 2.

Therefore there is an equal chance of their being a girl or a boy.

NOTE

This is usually shown by using only two sperm and two eggs in the punnet square as follows:

The probibility of being male or female is 2 out of 4

Here the probability of producing a male or female is 2 out of 4, or 1 out of 2.

Therefore there is an equal chance of there being a girl or a boy.

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