AS OCR Biology Mitosis

Relevant to the OCR spec for biology, about mitosis :)


1. Mitosis, definition and background

Mitosis- The process of nuclear division where two genetically identical nuclei are formed from one parent cell nucleus.

Why make new cells?

  • Growth- multicellular organisms grow by producing more cells, the cells are genetically identical to the parent cells so they can perform the same functions
  • Repair- damaged cells need to be replaced by new ones that perform the same functions, so must be identical
  • Replacement- Red blood cells and skin cells are replaced with new ones.
  • Asexual reproduction- Single celled organisms divide to produce two daughter cells that are separate organisms.
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2. Mitosis photo

Diagramatic Mitosis (

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3. Prophase

During interphase each chromosome has been replicated.

The chromosomes supercoil (shorten and thicken) and consist of a pair of sister chromatids.

The nuclear envelope deteriorates and a centriole divides into two. Each daughter centriole moves to opposite poles of the cell to form a spindle.

The spindle is made from protein threads.

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4. Metaphase & Anaphase

The chromosomes move to the central region of the spindle and each chromosome becomes attatched to the spindle thread by it's centromere.


The replicated sister chromatids are separated from each other when the centromere joining them splits, so the sisters are an individual chromosome. Identical to the original chromosome in the parent cell.

The spindle fibres shorten, and pull the sister chromatids further from eachother towards the poles, they make a V shape because of the centromeres attached to the spindle fibres lead.

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5. Telophase

As the separated sister chromatids reach the poles of a cell, a new nuclear envelope forms around each set.

The spindle deteriorates and the chromosomes uncoil, so you cannot see them in a microscope.

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6. Cytokinesis

The whole cell splits to form two new cells, and each one has a full set of chromosomes identical to the ones in the original parent cell. This splitting in two is called cytokinesis.

So each daughter cell can do what the parent cell did [carry out same functions], with identical information :)

In animals, most cells can carry out mitosis and cytokinesis, but in plant cells only meristem cells can do this.

Plant cells don't have centrioles

In animal cells cytokinesis starts from the outside, but in plant cells it starts with the forming of a cell plate where the spindle equator was and new cell membrane/wall material is laid down the cell plate.

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7. Questions!

Some questions below, answers on the next card :)

1. In an onion cell nucleus there are usually 12 chromosomes- but in which stages of mitosis are there 24 chromosomes?

2. Give a description in the stages of mitosis that allows each daughter cell to have a full copy of each chromosome.

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8. Answers

1. The chromotids separate during anaphase, so during anaphase and teleophase there are 24 chromosomes in the cell.

2. Replicated sister chromatids stay joined together at the centromere and attatch to the spindle and line up down the middle of the cell. When the centromere splits the sister chromatids are pulled in opposite directions towards opposite poles of the cells.

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good stuff :)



I find this really useful thank you so much!



I don't mean to be rude/ignorant, however, do you think your notes are good enough for an high A, or an equivalent of an A*, in AS biology? (to use them alone)

tali grozin.


Katie wrote:

good stuff :)

 awwhh yeah

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