The Cell Cycle The cell cycle, or mitosis, is the process by which new cells are produced for growth. All cells resulting from this process are identical to each other and the parent cell The cycle has 4 main stages as follows :- G1 – Cell growth and synthesis of organelles S – DNA replication G2 - Cell growth and synthesis of organelles M – Cell division

Four Phases of Cell Division Cell growth and synthesis of organelles Cell growth and synthesis of organelles DNA replication

The stages G1, S and G2 are collectively known as interphase The M phase or cell division phase is known as the mitotic phase Mitosis has a number of stages as shown below:

Cell Division And The Cell Cycle Nuclear division is controlled by microtubules which form the spindle fibres and move chromosomes - stages 1-5 Cytokinesis is controlled by actin fibres which split the cytoplasm in two - stage 6

Stages of Mitosis Stage Description Prophase Prometaphase Metaphase No distinct chromosome. Nuclear envelope intact Prometaphase Chromosome become visible. Nucleus breaks down Metaphase Chromosomes line up across the centre of cell ( equator) Anaphase Chromosomes divide into chromatids which are pulled to opposite poles by spindle fibres. These are made of microtubules and radiate from the centrosome. Telophase Daughter chromosomes ( chromatids) reach opposite poles and begin to de-condense Cell divides into two by contraction of actin fibres Cytokinesis

Interphase Follows the M phase and involves cell growth and DNA replication. Made up of G1, S and G2

Mitosis – Prophase The replicated chromosomes each consisting of two closely associated sister chromatids condense Outside the nucleus the mitotic spindle assembles between the two centrosomes which have replicated and moved apart.

Mitosis – Prometaphase The nuclear envelope suddenly breaks down Chromosomes attach to the spindle microtubules via structures known as kinetochores Chromosomes start to actively move

Mitosis – Metaphase The chromosomes are moved to the equator by the spindle fibres The kinetochores of all chromosomes align on the equator, midway between the poles at a structure known as the metaphase plate The paired microtubules attached to each chromosome attach to opposite poles of the spindle

Mitosis - Anaphase The paired chromatids from each chromosome separate to form two sister chromatids. Daughter chromosomes are pulled to opposite poles by the simultaneous shortening and lengthening of microtubules

Mitosis - Telophase The two sets of daughter chromosomes arrive at the poles A new nuclear envelope reassembles around each set forming to separate daughter nuclei and marking the end of Mitosis

Cytokinesis In animal cells the cytoplasm is divided into two by a contractile ring of actin and myosin which pinches in the cell to create two daughter cells.

The number of cells actively dividing in a sample of tissue is used to calculate the mitotic index Mitotic Index = Number of cells undergoing mitotic division Total number of cells Tip – Remember cells in interphase don’t count To practise recongnising the stages of mitosis and to work out the mitotic index of a couple of samples click here