Starter question If a bacterium is 6m long

Starter question If a bacterium is 6µm long and a virus is 150 nm, calculate how many times larger the bacterium is than the virus… 6µm = 6000 nm 6000÷ 150=40 40 times

CELL DIVISION - MITOSIS

Why is cell division important? • Growth - start life as single cell and become millions of cells • Repair - replace worn out/damaged cells Today we are learning about mitosis - this occurs in eukaryotic cells.

Mitosis At GCSE: 1) Parent cell 2) Chromosomes make identical copies of themselves 3) They line up along the centre and they move apart 4) Two daughter cells form with identical chromosomes to the parent cell

The Cell Cycle Not all cells retain ability to divide Those that can divide, go through a regular pattern of division called the cell cycle. -interphase -nuclear division (mitosis) -division of cytoplasm (cytokinesis) Length of cycle varies Mammalian cell ~24 hours 90% interphase

The cell cycle cytokinesis Interphase is the phase between divisions It is NOT a resting phase. G 1 and G 2 phases • Cell growth and DNA replication all occur in this phase It takes up approx 90% of the cell cycle S phase • DNA replication • Amount of DNA DOUBLES • New proteins synthesised • New organelles formed

CHROMOSOMES Chromosomes are made of strands of DNA tightly coiled around proteins called histones During interphase, chromosomes replicate and become sister chromatids, joined together at a centromere.

INTERPHASE Chromosomes are not visible. Chromosomes replicate Each chromosome is now made up of two identical sister chromatids joined at a centromere. Centrioles replicate. Nuclear envelope is still present. Nucleolus is visible.

PROPHASE Chromosomes condense (becoming visible). Nucleolus disappears. Nuclear envelope begins to break down. Centrioles move to opposite ends and form spindle fibres.

METAPHASE Chromosomes align in the equator of the cell Each chromosome is attached to a spindle fibre by its centromere.

ANAPHASE Centromeres divide and spindle fibres contract Sister chromatids pulled to opposite poles (ends) of the spindle Chromatids look V-shaped

TELOPHASE Nuclear membranes reform around each group of newly divided chromosomes Nucleolus reforms. Spindles disappear. Chromosomes extend, becoming invisible.

CYTOKINESIS Cytoplasm divides to create two identical daughter cells

Mitosis in brief: 4 phases: PMAT (In total IPMAT) Prophase-replicated chromosomes supercoil (thicken & shorten) Metaphase-Replicated chromosomes line up down the middle of the cell Anaphase-The replicas of the chromosomes are pulled apart from each other towards opposite poles of the cell Telophase-two new nuclei are formed

Describe the behaviour of chromosomes during mitosis and explain how this results in the production of two genetically identical cells. (5)

1 chromosomes shorten/thicken/supercoiling; 2 chromosomes (each) two identical chromatids/strands/copies (due to replication); 3 chromosomes/chromatids move to equator/middle of the spindle/cell; 4 attach to individual spindle fibres; 5 spindle fibres contract / centromeres divide / repel; 6 (sister) chromatids/chromosomes (separate) move to opposite poles/ends of the spindle; 7 each pole/end receives all genetic information/ identical copies of each chromosome; 8 nuclear envelope forms around each group of chromosomes/ chromatids/at each pole; 5 max