Meiosis | Genetics | Biology | FuseSchool

There are two types cell division processes: Mitosis & Meiosis

The simpler one is Mitosis which produces two identical cells with exactly the same genetic information. You can think of them as clones of each other.

The other process, Meiosis, is a much more complicated process creating not two but four cells, with only half the number of chromosomes and crucially all genetically different from each other.

Both mitosis and meiosis include the same phases – prophase, metaphase, anaphase, and telophase.

Except, in Meiosis, they happen twice, so they’re usually referred to as 1 and 2.  

The easiest way to remember these phase names is to remember IPMAT: Interphase, Prophase, Metaphase, Anaphase, Telophase.

So let’s look at meiosis in more detail...

As always, cellular division starts with a process called DNA replication.

This involves making two identical copies of the original DNA molecule.

The cell ends up temporarily with double the normal number of chromosomes.

In prophase I, the duplicated chromosomes join up with the pair from the other parent so the mother’s pair bind with the father’s pair, forming a group of two chromosomes called ‘homologous’ chromosomes.


As each chromosome is linedup next to it’s partner pair, one chromatid from each side gets entangled with the corresponding chromatid from the other side. This is called ‘crossingover’.


During this brief period, the two chromatids swap certain sections of DNA. This is called recombination. The sections that they trade correspond to the same location, so that each chromatid retains the correct number of genes.  

Recombination is really important because it creates variety. The new cells aren’t identical to their parents, and they also are different to one another as well.

There are new genetic combinations.

In fact that’s the whole point of sexual reproduction!!! … to increase genetic variability.

Each chromatid is now different, and as each one will end up in a separate gamete, it means each sex cell is genetically different from all others! This explains why brothers and sisters are different despite having the same parents. Only identical twins have the same genetic makeup as they both originated from the exact same egg and sperm!

Now back to meiosis: next comes Metaphase I as the chromosomes align themselves up in the middle of the cell.


In Anaphase I, the spindle fibres pull the chromosomes apart, to opposite ends.


Then during telophase I and cytokinesis, the cell pinches apart in the middle and the nuclear membrane reforms around the two new daughter cells.

That’s the end of meiosis 1.

We start with our ‘recombined’ daughter cells, each still with 46 chromosomes.

But sperm and eggs cells only have 23 chromosomes, so we need to cut these cells in half...

The process is exactly the same as before, except that there is no DNA replication.

We start straight with Prophase II, with chromatin clumping again to form chromosomes. They align in the middle of the cell during Metaphase II, and chromatids are pulled apart during Anaphase II by the spindle fibres. Telophase and cytokinesis pinch the cells together, with four new granddaughter cells being formed.

The end of meiosis gives us 4 different sex cells, each with only 23 chromosomes. Ready for future fertilisation



CREDITS
Animation & Design: Bing Rijper
Narration: Bing Rijper
Script: Alex Reis

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