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
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
Cytokinesis
Cell divides into two by contraction of actin fibres
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
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