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Cell division

Topic 1
...
6

Biology SL

➢ The interface is a very reactive period
○

Metabolic reactions, growth, DNA replication

○

Protein synthesis occurs at a rapid rate

➢ Exception Gap 0 (G0)
○

No growth takes place
■

Many cells spend more or less time in G0
●

For example, nerve cells spend a lot of time in G0

➢ The progression through the stages are controlled by a group of proteins called

cyclins
○ Bind to cyclin-dependent protein kinases (CDKs) enabling them to act as
enzymes
○

These activated enzymes cause the cell to move from G1 to S and from G2 to
M

➢ The points where the cyclin-activated CDKs function are called checkpoints in the
cell cycle
DNA condensation
➢ Starts in G2
➢ DNA is condensed by supercoiling
○

Chromatin (DNA + histones) begins to condense

○

DNA wraps around histones to produce nucleosomes

○

The nucleosomes are further wrapped into a solenoid

○

Solenoids group together in looped domains

○

Looped domains coil into chromosomes

Chromosomes
➢ After replication the chromosome includes 2 molecules of DNA
➢ Thes 2 identical molecules are held together by the centromere and each molecule
is referred to as a chromatid
○

Together they are called sister chromatids

○

The centromere is a kind of protein

➢ The outer side of the centromere is called kinetochore
Mitosis
➢ Mitosis is divided in 4 phases
○

Prophase

2

Cell division

Topic 1
...
6

Biology SL

Cytokinesis
➢ The stages of mitosis occur along a continuum and not in discrete stages
➢ Cytokinesis in animal cells involves an inward pinching of the plasma membrane
to form cleavage furrows
➢ In plant cells, a cell plate is formed, which occurs midway between the two poles of
the cell and moves outwards towards the sides of the cell
Cancer
➢ Cancer occurs when a cell cycle gets out of control
○

This results in a mass of abnormal cells referred to as a tumour

➢ A tumour that occurs at the original site of a cancer is called a primary tumour
➢ A secondary tumour or metastasis is a cancerous tumour that has spread from the
original location to another part of the organism
➢ Most organisms have sections of genes that mutate or are expressed at abnormally
high levels
○

Are called oncogenes and contribute to converting a normal cell into a
cancer cell

➢ The oncogenes go through mutation, because they are triggered by an outside agent
referred to as a mutagen
➢ When mutated oncogenes are expressed, the cell can start dividing without control

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