Cell Cycle, DNA Replication, Mitosis and Cancer

The CELL CYCLE is the process that all multicellular organisms body cells use to grow and divide.

Cells are produced through cell division, where a cell divides itself into two identical cells.

The Cell cycle consists of two main phases:
INTERPHASE: When cell growth and DNA replication occurs. This is divided into separate growth stages-
G1, S, G2

MITOSIS: Where cell division occurs, consisting of
Prophase, Metaphase, Anaphase and Telophase

DNA replication can be split into four key parts.

(1) The hydrogen bonds between two polynucleotide strands are broken by the enzyme DNA Helicase. Forming two single strands.

(2) The original strand forms a template for a new strand. Free floating DNA nucleotides join to the unpaired bases according to the specific base pairing.

• C + G *Note how they are both round letters
• A + T *Note how they are both sharp letters

(3) The enzyme DNA polymerase joins the nucleotides onto the new strand. New hydrogen bonds are formed between the original strand and the new strand.

(4) Each DNA molecule has one original strand and one new strand. This is called semi-conservative replication. *Semi, as there is only half the original DNA.

During interphase, the organelles are also replicated, not only for the new cell, but so there is enough ATP produced to fuel division.

Chromosomes also make a copy of themselves, making them two strand chromosomes- each strand is called a chromatid when it condenses and is joined at the centre by the centromere.

Mitosis is where a parent cell divides itself to produce two identical daughter cells. * An exact copy, including DNA
We need it for growth and repair. It also has four key phases.

(1) Prophase- The chromosomes condense making them shorter and fatter. Bundles of proteins called centrioles move to opposite poles in the cells, forming protein fibres across the cell called spindle fibres. The nuclear envelope also breaks down, leaving the chromosomes free in the cytoplasm.

(2) Metaphase- The chromosomes (with two chromatids formed in interphase) line up along the centre, attached to the spindle fibres by their centromere.

(3) Anaphase- Each chromatid separates from the other at the centomere. The chromatids are pulled to different poles of the cell (centromere first) as the spindle fibres contract.

(4) Telophase- When the chromatids reach the opposite poles, they uncoil becoming long and thin, forming chromosomes once again. A nuclear envelope forms around each group forming two nuclei. The cytoplasm divides producing two seperate identical cells.

Cancer is caused by uncontrolled cell division.

Cell division and growth is controlled by genes. The cells usually stop dividing if they have enough new cells. But if there is a mutation in a gene, the cells can keep on dividing, forming a tumour.

Cancer is a tumour which invades surrounding tissue.

Some treatments disrupt the cell cycle, but some can't distinguish between tumour cells and normal cells, so they often kill both. Tumour cells divide more frequently, so the tumour cells are more likely to be killed.

Chemotherapy and other drugs targets G1. It prevents the synthesis of the enzymes needed for DNA replication. This means the cell is unable to reach synthesis, forcing the cell to kill itself.

Radiation Therapy targets S phase, where it damages the DNA. When the cell checks for damaged DNA and kills itself if it is detected, preventing any further tumour growth.

 A chunk of the tumour can be removed through surgery. Any remaining cells have access to remaining nutrients and oxygen, triggering them to enter the cell cycle, making them succeptable to treatment.

• Breaks are used within repeated treatment. This is because along with killing a lot of tumour cells, treatment could kill so many normal cells, treatment could prove fatal.

• The breaks allow normal cells to recover and reproduce. Treatment has to be repeated to control any remaining tumour cells. So breaks have to be short so it doesn't grow out of control.