A.2.5. Mitosis and Meiosis


A. Mitosis:
1.
The vast majority of all the cells in our body will die at some moment. Before they do that, they often divide into daughter cells to preserve their function and their DNA!
2.
The process of a cellular division is called mitosis. This occurs million of times in every single minute that you are alive; blood cells, skin cells, liver cells etc.
3.
This process of mitosis is quite complicated and consists of several steps:
a. Interphase
b. Prophase
c. Metaphase
d. Anaphase
e. Telophase
4.
The Interphase is the phase between mitosis, between cell divisions, when the cell is actually doing what it should do. This is the longest period for a cell (days to years depending on the type of cell). At the end of the interphase, when the cell has decided to divide, the chromosomes are duplicated (from 2x24 to 4x24).
process of cellular division: mitosis
5.
The Prophase is the stage when:
a) The membrane of the nucleus gradually disappears
b) The chromosomes become visible. Every chromosome pair is coupled to each other at a centromere.
c) Cellular structures called centrioles (also called centrosomes) appear and move towards the two poles of the cell.
6.
The Metaphase is the phase when the chromosomes move towards a row located in the centre, the equator, of the cell. In addition, spindle fibres connect every chromosome to the centrioles.

7.
In the Anaphase, the centromeres are split and the spindles pull each chromosome towards the centrioles at one side of the cell: 2 x 24 chromosomes to one side, another 2 x 24 to the other side of the cell.
8.
In the Telophase, the spindle fibres disappear, a nucleus membrane is formed around each chromosome cluster and the cell membrane squeezes inside the cell to develop two separate cells.
9.
When the cellular membranes have formed and divided the two cells from each other, both daughter cells now enter their own interphase.


10.
This is the ‘growth’ phase when water flows into the cell, making it bigger and when all the organelles have to multiply their numbers (mitochondria, vesicles, Golgi apparatus etc.) to be able continue their function in the new cells.

B. The names of the mitosis phases:

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1.
The names of the different phases during mitosis may sound difficult and confusing. Let’s try to explain them!
2.
Interphase: this is not so difficult; the phase between two events (= the mitosis!).

3.
Prophase: from the Greek word “Pro”, which means “before”. The cell starts to get ready for the mitosis; the nucleus membrane disappears, the centrioles appear etc.
4.
Metaphase: from the Greek word “Meta” which means “adjacent”. So, in this phase, all the chromosomes are aligned together (‘adjacent’) to each other along the equatorial plane.
5.
Anaphase: from the Greek word “Ana”, which means “Up”. This is the phase when the replicated chromosomes are split and the ‘daughter’ chromosomes are pulled, by the spindle fibres, towards the opposite poles of the cell.
6.
Telophase: from the Greek word “Telo” which means the “end”, as this is the final stage of the mitosis, when the nuclear envelope is re-assembled, the centrioles and spindle fibres disassembled.


C. Meiosis:

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1.
There is however one type of mitosis which is ‘slightly’ different from mitosis but crucial for our survival! This is the cellular division of our ‘sex’ cells; our gametes. These are the sperm cells (in males) and the oocytes (in females).
2.
As you may know/remember, life starts when a sperm cell and an oocyte ‘merge’ together to form an embryo. If their chromosomes where put together into the new child, his (or her!) cells would contain 2 x 2 x 24 chromosomes! That is impossible.
(Why not?) Suppose that the numbers of chromosomes is not halved during meiosis II. Then the daughter cells will each get 2x2x24 chromosomes (total of 96). In the next child, this will become 2x2x2x24 (=192), and in the next ‘generation’ 2x2x2x2x24 (=284), etc etc. This is simply impossible!
3.
Therefore, the number of chromosomes has to be reduced before fertilization occurs. This is (one of) the purposes of meiosis.

4.
Meiosis actually consists of two parts, or two steps, called “meiosis I” and “meiosis II”. Both meiosis contain the same phases as during mitosis: prophase, metaphase, anaphase and telophase.
process of gamete division: meiosis


5.
Meiosis I is very similar to a normal mitosis whereby the number of chromosomes are duplicated. Then, when the cells divide into two daughter cells, the number of chromosomes is still 2 x 24.

6.
The purpose of meiosis I is to exchange the genes (=DNA) across their chromosomes during the actual duplication. This is called “crossing-over” or “recombination”. In this way, the daughter cells are actually genetically different from each other!
7.
After Meiosis I, obviously, Meiosis II will start. Now, in contrast to all other divisions, the chromosomes are NOT duplicated.
8.
So, when the daughter cells are created, each cell, in its nucleus, contains only one set of chromosome: 1 x 24!!
9.
This is SO important that we have names for cells that have one or two sets of chromosomes; “diploid” and “haploid”.

10.
Diploid means two sets of chromosomes while Haploid means one set. Therefore, sperm cells and oocytes are haploid, while all other cells in the body are diploid.

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A.2.5. Mitosis and Meiosis
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