GCSE Biology Unit 2 - Cell Division

DNA - Deoxyribose Nucleic Acid
Chromosome - long molecules of DNA
Gene - section of DNA
Gene - DNA - chromosomes

Gene Codes:
The 'rungs' in the double-helix of DNA are made of 4 different bases. Sets of 3 bases form a code for amino acids. Different orders make different amino acids. The amino acids then make proteins.

DNA fingerprinting:
DNA is cut up into different sized sections and then ordered by size. Everyone has their own individual genetic fingerprint. This is used in forensic science and for paternity tests. 

All body cells have 2 copies of each chromosome, giving them 23 pairs.

'Mitosis is when a cell reproduces itself by splitting to form two identical offspring.'

Mitosis is used in order to grow, or to replace damaged cells. It is also how some organisms reproduce asexually.

Mitosis:

1. All the chromosomes are duplicated.

2. The chromosomes line up along the centre, and then get pulled apart.

3. Membranes form around each set of chromosomes, making nuclei, and then the cytoplasm divides.

4. There are now 2 identical cells, both with 23 pairs of chromosomes. 

Gametes in humans (sex cells - sperm/eggs) have only 23 chromosomes, so that when they combine they make 23 pairs.

'Meiosis produces cells which have half the normal number of chromsomes.'

Meiosis:

1. All the chromosomes are duplicated.

2. The pairs line up in the centre of the cell, and then are pulled apart, forming two new, but not identical cells with a full set of chromosomes.

3. The cells then divide again, but the chromosomes don't duplicate first.

This makes four cells with half the normal amount of chromosomes, none of them identical.

Most cells are specialised for a particular job.
Differentiation is when a cell changes to become specialised. In most animal cells, once differentiated they can't change again.

Stem cells aren't differentiated and they can differentiate into any type of cell.

Stem Cells can be found in two places - in humn embryos, and in bone marrow. Those in bone marrow can't differentiate to as many different types of cells as embryonic stem cells.

Bone marrow transplants help those with blood disease by making new red blood cells.

Embryonic stem cells can be extracted and then grown into tissues, to replace faulty cells in different places in the body. In order to grow cultures of one type of cell, the differentiation needs to be controlled.

This is very difficult, and doesn't always work.

Pros and Cons:

- Embryos are potential lives, and everyone should have the chance to live (c)

- Can cure lots of people who are suffering - this could be considered more important than embryo rights (p)

- Embryos used in research usually come from fertility clinics, and would otherwise be destroyed (p) but people who want embryo rights would still say it's wrong

In some countries Stem Cell Research is banned, but in the UK it's allowed as long as researchers follow certain laws and guidelines.