Genes and Reproduction
Genes have two important jobs to do in the cell: they have to carry the codes for proteins and they have to be able to make copies of themselves when the cell containing them reproduces. We already know that cells can reproduce in two different ways. The nucleus, which contains the DNA and therefore the genes, divides in one of two ways - depending on if the new cells will be used for sexual or asexual reproduction.
This is the process that a cell undertakes if it is reproducing asexually. In other words, mitosis creates exact copies of the parent (unless there is a mutation). Mitosis has five main stages:
- All the DNA in the nucleus makes a second copy of itself then starts to coil up to a much more compact shape. This twisted-up version of DNA is called a chromosome. In Humans there will be 46 separate chromosomes in the nucleus.
- The wall of the nucleus starts to break down. Some extremely thin, extremely small strings (called microtubules) start to form from two different sides of the cell.
- The free ends of the microtubules find the chromosomes and their copies.
- The microtubules pull the chromosomes away from the copies to opposite ends of the cell.
- A new membrane starts to form around the two separate sets of chromosomes and the microtubules break down.
The two sets of chromosomes are identical to each other. The cell will then put the new nucleus into a new cell - and the process will start all over again.
Meiosis is the production of sperm and egg cells (or gametes). It is important that these gametes have half the number of chromosomes than the rest of the body's cells. During fertilisation, when the sperm fuses with the egg, the chromosomes in the sperm join with the egg chromosomes to make a fertilised egg with the correct number of chromosomes.
Meiosis is a more complicated process than mitosis - these are the main stages:
- The DNA in the cell makes a copy of itself.
- The chromosomes (made of DNA) in the cell pair up. A piece of DNA that came from the father pairs with a piece from the mother. Some parts of the DNA from one chromosome swap (or cross over) with the same bit on the other chromosome.
- The microtubules pull the pairs apart to different ends of the cell.
- The cell divides to make 2 daughter cells.
- Microtubules in the daughter cells pull the chromosome pairs apart again - without the DNA having made a copy of itself
- The daughter cells divide again - each contain half the chromosomes that the original parent had.
So, in meiosis the original cell divides twice to produce four gamete cells. The gametes are sperm if you are male or eggs if you're female. The sperm and eggs have half the chromosomes of the parent. But because of the 'mixing up' that happens during crossing over the sperms are all slightly different to each other (and so are the eggs).
The process that makes a new cell during asexual reproduction is called mitosis. During mitosis the genes are kept the same in the child cell as they are in the parent. During meiosis, however, the genes are mixed up when eggs and sperm are created. They are even further mixed up when eggs and sperm join together during fertilisation. This explains why sexual reproduction leads to many differences in a species - or variation.