Mitosis

Not all cells can keep their ability to divide, the ones that do must undergo the process of the cell cycle. This cycle begins when a cell is produed by cell divison, and it ends with the cell dividing to produce 2 identical cells. The cell cycle consists of:

• interphase (period of cell growth and DNA replication)
• mitosis (cell division)

Interphase can be divided into 3 growth stages. G1, S and G2.

G1, or Gap phase 1: the cell grows and new organelles and proteins are made

S, or Synthesis: the cell replicates its DNA, and is ready to divide by mitosis

G2, or Gap phase 2: the cell keeps growing and proteins needed for cell divison are made

During interphase, the cell can carry out its normal functions, and also prepare to divide. The DNA is unravelled and replicated, doubling its genetic content. The organelles are replicated, so there are some spare, and the ATP content is increased to provide the energy needed for cell divison.  

The 2 types of cell divison are mitosis and meiosis. Mitosis is the form that occurs during the cell cycle. During this a parent cell divides, producing 2 genetically identical daughter cells (containing an exact copy of the DNA of the parent cell).

Mitosis is needed for the growth of multicellular organisms, and for repairing damaged tissues. 

It is one continuous process, but described in a series of division stages, prophase, metaphase, anaphase, and telophase. 

As mitosis begins, the chromosomes are made of 2 strands joined in the middle by a centromere. The strands are called chromatids, 2 strands on the same chromsome are called sister chromatids. There are already 2 strands, because each chromosome already replicated itself during interphase. when mitosis is over, the chromatids end up as single strand chromosomes in the new daughter cells.

1. Prophase

• chromosomes condense, become shorter and fatter
• bundles of protein, centrioles, move to opposite ends of the cell, forming a network of protein fribres across it, called the spindle
• nuclear envelope breaks down, chromosomes are free in the cytoplasm

2. Metaphase

• chromosomes (wiht 2 chromatids) line up along the middle of the cell 
• they attach to the spindle by their centromere

3. Anaphase

• centromeres divide, each pair of sister chrmoatids are separated
• spindles contract, chromatids pulled to opposite poles of the spindle, centromere first
• chromatids appear v-shaped

4. Telophase

• chromatids reach opposite poles on the spindle, they uncoil, becoming long and thin again, now called chromosomes again
• nuclear envelopes form around each group of chromosomes, making 2 nuclei
• cytoplasm divides, 2 identical daughter cells that are genetically identical to the original cell have been formed 
• mitosis is ended, each new cell starts interphase, ready for the next round of mitosis

Mitosis and the cell cycle are controlled by our genes. Normally, when the cells have divided enouhg they will stop. However, a mutation in a gene controlling cell division can cause cells to grow out of control. They continue to divide more and more, forming a tumour. Cancer is a tumour that invades the surrounding cells.

Some of the treatments for cancer involve controlling the rate of cell divison in the tumour cells, done by disrupting the cell cycle, killing the tumour cells. However, these types of treatment dont distinguish between tumour cells and normal cells, meaning they will also kill normal cells that are dividing. Luckily, the treatments are in fact more likely to kill the tumour cells, as they divide much more frequently than normal cells.

the treatments can be used to target specific parts of the cell cycle including:

• G1 (cell growth and protein production)
  • chemical drugs such as chemotherapy prevent the synthesis of enzymes that are needed for DNA replication. If the porduction of these are stopped the cell can't enter the synthesis (S) stage
  • cell cycle is disrupted, the cell kills itself
• S phase (DNA replication) 
  • radition and some drugs can dmage the DNA
  • during the cell cycle the DNA is checked for damage
  • if there is severe damage to the DNA, the cell kills itself, stopping further tumour growth