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The Cell Cycle

(Cell Division)

A clearer understanding!

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Cell Division Introduction

New cells are needed for GROWTH and REPAIR

2 types of cell division.

- Non reproductive cells (produce daughtercells) go through MITOSIS (Somatic cells)

- Reproductive cells (can produce sperm & egg)

go through MEIOSIS (Gametes)NOT DOING THIS

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Non Reproductive Cells (Most Cells)

2 sets of chromosomes

Cells DNA = GENOME (Genetic make-up)

EUKARYOTIC CELLS (Numerous molecules) and

PROKARYOTIC CELLS (Single DNA molecule)NOT DOING THIS

EUKARYOTIC CELLS consist of CHROMATIN

(Condenses chromosomes so ready for celldivision)

CHROMOSOMES = Organised structures of DNA

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Interphase

INTERPHASE- Process cell undertakes inpreparation for division.

Made up of 3 phases.

G1 Phase (First gap) S Phase (Synthesis)Where duplication happens

(ONLY HERE)

G2 Phase (Second gap) CELL GROWTH takes place throughout all phases.

Longest part of cell division

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Interphase cont.

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Interphase processes

G1 = first growth stage. Grows to nearly full

size. Produces RNA and synthesize protein.

S = DNA in the nucleus is replicated (each

chromosome now has a sister).

G2 = Cell finishes growing. Two centrosomes

appear in the cytoplasm.

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Mitosis (M)

There are 5 phases of MITOSIS

PROPHASE- Chromosomes CONDENSE byCHROMATIN to form SISTER CHROMATIDS,

mitotic spindle starts to form from centrosomes. PROMETAPHASE- Nucleus disappears after

envelope fragments. MICROTUBULE strands aregrowing from the centrosomes. Attach to

KINETOCHORES which are on either side of thesister chromatids CENTROMERE. These areKINETOCHORE MICROTUBULES.

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Mitosis (M) cont.

METAPHASE- CHROMATIDS line up in the centreof the cell (the SPINDLE). Both CENTROSOMESare now on either side of the spindle pole.

ANAPHASE- Chromatids are separated. (NowDAUGHTER CHROMOSOMES) Kinetochoremicrotubules shorten to do this whilenonkinetochore microtubules rub against each

other to lengthen cell. The daughterchromosomes & centrosomes are now onopposite sides of the cell.

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Mitosis (M) cont.

TELOPHASE- Final stage of mitosis.

Components of new cells appear (nuclear

envelopes reform and nucleolus comes back).

Spindle fibres break up releasing

nonkinetochore microtubules from each

other. Chromosomes return to uncondensed

state.

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Cytokinesis

When CLEAVAGE takes place. A CLEAVAGE

FURROW forms which acts as a contracting

ring of microfilaments. Cell eventually

separates when the cytoplasm divides = two

DAUGHTER CELLS. The organelles are

replicated just before the cell splits.

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Mitosis processes

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Control Systems

Eukaryotic cell cycles are regulated by molecular controlsystem.

Cell cycle action is determined by chemical signals in thecytoplasm.

Control system is like a clock.

Internal and external controls. Checkpoints where cell stops until the go-ahead.

Main checkpoint is in G1 of interphase- passing thisnormally means the cell will complete the cycle.

No go-ahead means entering G0 the non dividing state. An important external control system is DENSITY

DEPENDENT INHIBITION which stops cells dividing whenthere is a crowding.