Cell Cycle and Apoptosis

Realise the importance of the cell cycle.

The cell cycle basically involves the processes of growth and replication. I'm sure you can agree that those processes are pretty important without me having to explain why. Also growth and replication have to be coordinated so that you don't end up with a massive cell that hasn't divided yet or lots of tiny cells with not enough stuff in them.

Understand the key steps in mitosis.

See one of my earlier posts on cell structure and mitosis.

Know chromosome structure and how chromosomes behave during the eukaryotic cell cycle.

I described chromosome structure in my first post on eukaryotic gene regulation. In the G1 phase of the cell cycle (i.e. the growth phase before DNA replication), cells have the full complement of chromosomes. In humans, this is 46 chromosomes (23 pairs). Each chromosome has one DNA double helix. In the S phase, the DNA replicates (aside from at the centromere), and so each chromosome has two DNA double helices joined together at the centromere. Each double helix is called a sister chromatid. During cell division, the centromere also replicates and the chromatids are pulled apart.

Remember, one centromere = one chromosome. Despite post S-phase chromosomes having two chromatids, there are still only 46 chromosomes until the centromeres are replicated and the chromosomes are pulled apart during mitosis.

Understand the key steps in meiosis.
Know the differences between mitosis & meiosis.

Both of these are covered in a previous post on reproduction.

Understand the need for, and process of, apoptosis.

Apoptosis is the controlled death of cells. Apoptosis has several different functions. During embryonic development, for example, cells that are needed for earlier development, but not for later development, may undergo apoptosis. A more specific example of this is the webbing between our fingers. As embryos we have far more webbing until some of the cells in the webbing die off. Other reasons for apoptosis may be to kill off diseased cells.

Caspases are enzymes that drive the process of apoptosis. Before they are signalled, they exist in a harmless, inactive state known as a zymogen. After receiving a signal, which can be extracellular or intracellular (I did read up a bit more on these two pathways, but I don't remember them and I think they're beyond the scope of this unit anyway), initiator caspases are cleaved. Initiator caspases then cleave executioner caspases, which in turn cleave other target proteins in the cell so that apoptosis can take place.

At the beginning of apoptosis, executioner caspases cleave a protein that normally holds DNA endonuclease in the cytoplasm. When this protein is cleaved, DNA endonuclease enters the nucleus and eats up the DNA. Executioner caspases then activate an actin-cleaving protein, resulting in degradation of the cytoskeleton and a loss of cell shape. The cell breaks up into small fragments called apoptotic bodies. These apoptotic bodies are later phagocytosed (eaten up) by other cells in the body. Om nom nom. (Sorry, just had to put that there.)