The Cell Cycle

Interphase = G1, S & G2

1. G1 : Cells prepare for DNA replication by growing and synthesising new organelles.

2. S : DNA replication occurs.

3. G2 : a relatively short gap before mitosis, where proteins needed for division are made, along with the ATP required.

4. M : mitosis/meiosis

Mitosis = 2 identical daughter cells = used for repair of tissues and asexual reproduction

Chromosomes

• DNA replicates during synthesis phase and separates during nuclear division

Chromosomes are made up of long, linear structures of DNA tightly coiled around histone proteins - contain genes throughout the strand.

The nucleus of all eukaryotic cells there are a set number of chromosomes (dependant on species)

In Humans the chromosomes exist as 23 homologous pairs (maternal and paternal) - the diploid number of chromosomes is the total number of chromosomes in a normal body cell - Humans = 46

Homologous Chromosomes

Homologous Chromosomes are a pair of chromosomes containing the same genes in the same position - the pair will include one maternal and one paternal chromosome

Mitosis

In mitosis a parent cell divides into 2 identical daughter cells

• Mitosis increases the number of cells during growth

• It replaces tissue during repair

• It allows asexual reproduction

Maintains the same chromosome number from one generation to the next.

Stages of Mitosis

Stages :

• Prophase

• Metaphase

• Anaphase

• Telophase

At the end of mitosis the cytoplasm divides to form 2 daughter cells = cytokinesis

Prophase

• DNA supercoils around histones

• Chromosomes condense and become visible

• Nuclear membrane breaks down and centrioles migrate to poles

Metaphase

• Centrioles make the protein strands known as spindles.

• Chromosomes line up on the equator of cell

• The centromere attaches to the spindle fibres

Anaphase

• Centromeres divide

• Separating each pair of sister chromatids

• Spindles contract pulling chromatids to opposite poles by centromeres

Telophase

• Chromatids uncoil and become long and thin again

• They are now indistinct and we refer to them as chromosomes again

• A nuclear envelope forms around each group of chromosomes - now 2 nuclei

• Spindles disintegrate and cytokinesis begins (splitting of cytoplasm)

Mitotic Index

• This is a ratio showing the number of cells undergoing mitosis in proportion to the total number of cells

Mitotic Index = Number of cells undergoing mitosis/Total number of cells

Can be expressed by a percentage (x100)

Cancer

Mitosis is a controlled process

Cancer results from the mutations in the genes that control cell division, causing rapid, uncontrolled growth and division of cells.

This results in the formation of a mass of abnormal cells = tumour - Cancerous tumours do not respond to nerves and signals as normal healthy cells would.

Normally there is a gene that controls how much cells divide and if it mutates it can lead to tumours = benign or malignant

• Benign Tumours - NOT cancerous - grow slowly and don’t invade other tissues

• Malignant Tumours - ARE cancerous - they grow faster and can spread around the body

Treatment of Cancer

The treatment of cancer often involved blocking part of the cell cycle to control the rate of cell division - Drugs used to treat cancer can disrupt the cell cycle by:

• Preventing DNA from replicating

• Preventing spindle formation or disrupting another stage of mitosis

Cancer drugs are often disrupting the cell cycle of other healthy rapid division cells (hair cells, etc) as they are treating those with high rates of division which causes those side effects of hair loss, etc. Due to cancer cells being even more rapid than healthy cells it should have a larger impact rather than damaging too much healthy tissue.