Mitosis & Meiosis

Meiosis

a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell, as in the production of gametes and plant spores.

Mitosis

cell division in which the nucleus divides into nuclei containing the same number of chromosomes

Chromosomes

Each DNA strand is split into chromosomes• All sexually reproducing animals have pairs of chromosomes. One set of each pair comes from each parent• Humans have 23 pairs

Interphase

Cell is actively synthesising proteins. The chromosomes are not visible prior to mitosis. The DNA replicates

Prophase

The nuclear membrane breaks down. The nucleolus disappears. The DNA is free in the cytoplasm. Chromosomes condense and become shorter. The chromosomes become visible.
Centrioles divide and move to the poles of the cell. Spindle fibres form across the cell.

Metaphase

Chromosomes move to the equator of the cell (they line up in single file).
Chromosomes attach to the spindle at their centromere.

Anaphase

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Spindle fibres contract.

The centromeres holding a pair of chromatids together divide.

Sister chromatids are pulled apart.

1 sister chromatid from each chromosome moves to the opposite poles of the cell.

Telophase

Nuclear membrane forms around each group of chromosomes - the cell has 2 nuclei briefly. The nucleolus reforms.
The chromosomes unwind and become diffuse (decondense) and are no longer visible. The cells split down the middle and two new cell membranes form.

Meiosis

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•This is the process by which sex cells (gametes) are produced.

•Therefore it ONLY occurs in the gonads

•Instead of producing two identical daughter cells, meiosis produces four non-identical daughter cells, each with only half the number of chromosomes (23 instead of 23 pairs in humans)

Interphase

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•As before

•Standard condition of cell

•DNA replicates

•Cell enters reproductive cycle with four copies of each chromosome

Prophase I

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•DNA supercoils and chromosomes become visible

•Nuclear membrane breaks down

•Homologous pairs line up

•Non-sister chromosomes join up and trade sections

•Centrosomes migrate and spindles form

Metaphase I

•Homologous chromosomes line up on the equator of the cell
•Random assortment takes place

Anaphase I

•Homologous chromosomes attach to spindles
•Spindles contract and pull homologous chromosomes apart

Telophase I

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•Nuclear membrane reforms

•Chromosomes disperse

•Cytokinesis begins

Prophase II

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•Chromosomes recondense

•Centrosomes migrate

•Nuclear membranes disperse

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Metaphase II

•Chromosomes migrate to equator
•Spindles form

Anaphase II

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