The Cell Cycle

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Interphase

Very active phase of the cell cycle with many processes occurring in the nucleus and cytoplasm

Consists of 3 phases:

  • G1 - Cellular contents, apart from chromosomes are duplicated 
  • S - Each of the chromosomes are duplicated
  • G2

Some cells do not go past G1 because they are never going to divide so dont need to prepare for mitosis. They enter a phase called G0 which may be temporary or permanant.

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Prophase

The chromosomes become shorter and fatter by coiling.

To become short enough they have to coil repeatedly.

This is called supercoiling.

The nucleolus breaks down.

Microtubules grow from structures called microtubule organising centres (MTOC) to form a spindle-shaped array that links the poles of the cell.

At the endof prophase the nucleur membrane breaks down.

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Metaphase

Microtubues continue to grow and attach to the centromeres on each chromosome.

The two attachment points on opposite sides of centromere allow the chromatids of a chromosome to attach to microtubules from different poles.

The microtubules are all put under tension to test if attachment is correct.

This happens by shortening of the microtubules at the centromere.

If attachment is correct the chromosomes remain on the equator of the cell.

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Anaphase

At the the start of anaphase, each centromere divides, allowing the pairs of sister chromatids to seperate. 

The spindle microtubules pull them rapidly towards the poles of the cell.

Mitosis produces two genetically identical nuclei because sister chromatids are pulled to opposite poles.

This js ensured by the way that spindle microtubules were attached in metaphase.

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Telophase

The chromatids have reached the poles and are now called chromosomes.

At each pole the chromosomes are pulled into a tight group near the MTOC and a nucleur membrane reforms around them.

The chromosomes uncoil and a nucleolus is formed.

By this stage of mitosis the cell is usually already dividing and the two daughter cells enter interphase again.

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