Presentation on theme: "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."— Presentation transcript:
1 The Cell CycleThe 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 cellThe cycle has 4 main stages as follows :-G1 – Cell growth and synthesis of organellesS – DNA replicationG2 - Cell growth and synthesis of organellesM – Cell division
2 Four Phases of Cell Division Cell growth andsynthesis oforganellesCell growth andsynthesis oforganellesDNA replication
3 The stages G1, S and G2 are collectively known as interphase The M phase or cell division phase is known as the mitotic phaseMitosis has a number of stages as shown below:
4 Cell Division And The Cell Cycle Nuclear division is controlled bymicrotubules which form the spindlefibres and move chromosomes - stages 1-5Cytokinesis is controlled by actinfibres which split the cytoplasm intwo - stage 6
5 Stages of Mitosis Stage Description Prophase Prometaphase Metaphase No distinct chromosome. Nuclear envelope intactPrometaphaseChromosome become visible. Nucleus breaks downMetaphaseChromosomes line up across the centre of cell ( equator)AnaphaseChromosomes divide into chromatids which are pulled to opposite poles by spindle fibres. These are made of microtubules and radiate from the centrosome.TelophaseDaughter chromosomes ( chromatids) reach opposite poles and begin to de-condenseCell divides into two by contraction of actin fibresCytokinesis
6 Interphase Follows the M phase and involves cell growth and DNA replication. Made up of G1, S and G2
7 Mitosis – ProphaseThe replicated chromosomes each consisting of two closely associated sister chromatids condenseOutside the nucleus the mitotic spindle assembles between the two centrosomes which have replicated and moved apart.
8 Mitosis – Prometaphase The nuclear envelope suddenly breaks downChromosomes attach to the spindle microtubules via structures known as kinetochoresChromosomes start to actively move
9 Mitosis – MetaphaseThe chromosomes are moved to the equator by the spindle fibresThe kinetochores of all chromosomes align on the equator, midway between the poles at a structure known as the metaphase plateThe paired microtubules attached to each chromosome attach to opposite poles of the spindle
10 Mitosis - AnaphaseThe 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
11 Mitosis - TelophaseThe two sets of daughter chromosomes arrive at the polesA new nuclear envelope reassembles around each set forming to separate daughter nuclei and marking the end of Mitosis
12 CytokinesisIn 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.
13 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 divisionTotal number of cellsTip – Remember cells in interphase don’t countTo practise recongnising the stages of mitosis and to work out the mitotic index of a couple of samples click here