Passing on the Code
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Passing on the Code
The genetic code contained in our chromosomes is of no real use unless it can be used to make new cells.
The code is passed on to the new cells using either of two processes, mitosis or meiosis.
Mitosis is the process used during growth to make new cells within a plant or animal. It is also used during asexual reproduction, in which an individual can clone itself to produce identical offspring.
Humans don't make clones of themselves naturally like plants can.
For example: strawberry plants send out runners.
However our body often has to make new cells to replace damaged ones or as we grow. So human cells also go through mitosis in the same way as animal and plant cells but is for growth and repair.
The offspring cells have the same number of chromosomes as the parent cells, therefore they are diploid.
The process of mitosis is shown below:
The DNA starts off as long strands in a soup-like mush in the nucleus.
The chromosomes become clear as the DNA twists up. Each double arm (chromatid) is a copy of each other.
Chromosomes line up along the centre line and begin to be pulled apart by fibres.
Membranes form about the separated chromosomes. These become nuclei of daughter cells.
The chromosomes unwind back into loose threads within the daughter cells.
The single-armed chromosomes within the daughter cells replicate themselves to create the double arms (chromatids) again.
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Bacterial cells also use mitosis to divide and multiply. That's why it all happens so quickly and your immune system has to struggle to cope.
In the notes above, we have explained mitosis - but what if you could magically memorise all this for your exams easily?
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Meiosis is a slightly different process. It is used to create the gametes,these are the sperm or eggs, used in sexual reproduction. The offspring produced during sexual reproduction have characteristics, selected from those of the parents.
The main difference in meiosis as compared to mitosis is that the new cells have half the number of chromosomes as the diploid 'parent' cell. One chromosome comes from each homologous pair of chromosomes. So these offspring cells are haploid not diploid.
The following list summarises the slightly more complicated events of meiosis
- Meiosis starts with all the chromosomes lining up in their (homologous) pairs.
- One chromosome from each pair enters a new nucleus so that 2 daughter cells are fomed.
- The 2 daughter cells now have half the number of chromosomes each compared to the 'parent' cell. They are haploid.
- The chromatids that make up each chromosome are pulled apart by fibres.
- Each single chromatid joins the others in another new nucleus. These are the gamete cells. These cells are haploid too.
- The chromatids in the gamete cells are replicated to re-create the X-shaped chromosome. The gametes are haploid.
Finally then you end up with 4 gamete cells from each parent cell that splits up. These gamete cells, whether eggs or sperm, can then go on to fertilisation.
The beauty of meiosis is that it mixes up all the chromosomes. No gamete is ever the same as the next as the chromosomes get shuffle about so much.
So when an egg and sperm 'get it together' and you pop up 9 months later you are unique. Different, but definitely unique!