Unit 1: Cell & Molecular Biology

Cell Growth

&

Cell Cycle

Learning Intentions• Interphase: G1, S and G2 phases

• Mitosis: the M phase (4 stages)

• Cytokinesis

• Mitotic Index

• Control of the cell cycle.

• Abnormal cell division: cancer cells.

Four Phases of Cell Division

Cell growth andsynthesis of organelles

Cell growth andsynthesis of organelles

DNA replication

Phase G1, S and G2 are collectively known as interphase

M phase = Mitosis (cell division)

Cell Division And The Cell Cycle

Cytokinesis is controlled by actinfibres which split the cytoplasm intwo - stage 6

Stages of MitosisStage Description

Prophase No distinct chromosome. Nuclear envelope intact

Prometaphase Chromosome become visible. Nucleus breaks down

Metaphase Chromosomes line up across the centre of cell (metaphase plate)

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

Interphase

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

Mitosis – Prophase

The 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.

Mitosis – Prometaphase

The nuclear envelope breaks downChromosomes attach to the spindle microtubules via structures called kinetochores and move towards the equator of the cell.

Mitosis – Metaphase

The 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 plate

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 polesA new nuclear envelope reassembles around each set forming two separate daughter nuclei and marking the end of Mitosis.

Nuclear division is controlled by microtubules (tubulin protein) which form the spindle fibres and move chromosomes to opposite poles of the cell during mitosis.

Cytokinesis

• Once the cell completes M phase (mitosis) the division of the cytoplasm produces 2 daughter cells this is known as CYTOKINESIS.

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

• In plant cells a new cell membrane and cell wall are built between the two daughter nuclei and cuts the cytoplasm in half

Mitotic Index

• Mitosis duration and frequency varies greatly between cell types.

• The Mitotic index calculates the percentage of cells undergoing division in a given sample.

• Only cells goinf through M phase are counted (not Interphase)

• This can be the first indication of cancerous cells and a developing tumour.

Mitotic Index = Number of mitotic cells X 100 Total number of cells