AQA GCSE Combined Biology - Cell Division

DNA exists in a cell's nucleus within structures called chromosomes. Each section of a chromosome that contains the code for the production of a specific protein is called a gene.

Each chromosome is made from a single molecule of DNA, but when a cell is ready to devide, the DNA copies itself, then coils and condenses to form the chromosomes we see in micrographs.

Each human body cell contains 46 chromosomes which can be arranged in 23 pairs.

Each chromosome pair carries the same types of genes. The 23^rd pair are the sex chromosomes:

• In females, the two chromosomes are identical in shape. There are 2 X chromosomes and therefore females are refered to **.
• In males, the two chromosomes are not identical in shape. There is an X chromesome and a Y chromosome, and therefore males are refered to XY.

Mitosis - A type of cell division in which the daughter cells are identical to the parent. 

Cells devide when:

• an organism grows
• an organism becomes damaged and needs to produce new cells

It is nessacery that any new daughter cells produced contain genetically identical information as its parent cell. A growing and dividing cell goes through a series of stages called the cell cycle.

The stages of the cell cycle are:

1. Cell growth and DNA synthesis

2. Further growth and error check in DNA

3.Mitosis 

4. Cytoplasm seperates - two cells are produced

The stages of the cell cycle are:

1. Cell growth

2. DNA sythesis - the chromosomes are now double stranded

3. Further growth occurs and the DNA is checked for any errors

4. Mitosis

5. The cytoplasm seperates - two cells are formed

6. Temprorary cell resting period or the cell stops dividing

The first stages of the cell cycle involve cell growth, then the synthesis of DNA. The single strand of DNA that makes up each chromosome, produces an exact copy of itself. 

Once DNA synthesis takes place, the cell undergoes a type of cell division called mitosis.

 In mitosis, two cells called daughter cells are produced, which are identical to the parent cell.

Stages of Mitosis:

1. The cells begin to divide

2. The DNA replicates to form two copies of each chromosome

3. The nuclear membrane breaks down. The chromosomes line up across the center of the cell

4. One set of chromosomes is pulled to each end of the cell and the nucleus divides.

5. The cytoplasm and cell membranes divide to form two identical daughter cells

When looking at cells with a microscope, the length of different stages of the cell cycle can be estimated using the formula:

Stem cells - cells which have not undergone differentiation (a cell which is not yet speciallised)

An embryo develops from a fertilised egg. Cells at the early stages in the development of the embryo are stem cells.

If embryonic stem cells are removed, they can be differentiated into any cell type.

Adult stem cells are found in limited places in the body. 

They can be found in - the brain, eyes, blood, heart, liver, bone marrow, skin and muscle.

Adult stem cells can differentiate into related cell types only - eg. bone marrow cells can differentiate into blood cells and cells of the immune system, but no other cell type apart from these. 

Cell division in plants occurs in regions called meristems.

Cells of the meristem can differentiate to produce all types of plant cells at any time during the life of the plant.

The main meristems are close to the tip of the shoot, and the tip of the root.

In a growing shoot, the cells are being produced continuosly near the tip.

As the cells become older, and become further away from the tip - they become differentiated as they enlarge and develop vacuoles. 

Stem cells have the potential to be transplanted into patients, to treat medical conditions and disease. They could replace damaged or destroyed cells,through diabetes or spinal cord or brain injury for example.

Both embryonic and adult stem cells can be used.

Embryonic stem cells can differentiate into a wider range of cell types, but are difficult to obtain and their use raise many ethical challenges. The best source is the 5 day old embryo.

Adult stem cells will differentiate into a narrower range of cell types.Bone marrow transplants are an example of adult stem cell transplant. Bone marrow cells can differentiate into different types of blood cells and can be used:

• in cases of blood cell cancer, such as leukaemia and lymphoma
• when blood cells have been destroyed by cancer treatement