Cell Division Cell Increase and Decrease Cell division

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Cell Division

Cell Division

Cell Increase and Decrease • Cell division increases the number of somatic cells –

Cell Increase and Decrease • Cell division increases the number of somatic cells – Consists of mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm) • Apoptosis, programmed cell death, decreases the number of cells

The Cell Cycle • Cell cycle: orderly set of stages that take place between

The Cell Cycle • Cell cycle: orderly set of stages that take place between the time a cell divides and the time the resulting cells also divide.

The Stages of Interphase • Most of the cell cycle is spent in interphase

The Stages of Interphase • Most of the cell cycle is spent in interphase – In between episodes of mitosis – Cell carries on its usual functions – Prepares to divide (grows larger, number of organelles doubles, amount of DNA doubles) – Lasts 20 hours in mammals

The Stages of Interphase • 3 stages – G 1 occurs before DNA synthesis

The Stages of Interphase • 3 stages – G 1 occurs before DNA synthesis – S includes DNA synthesis – G 2 occurs after DNA synthesis • G = growth • S = synthesis • M = mitosis

The Stages of Interphase • G 1 stage – Cell doubles its organelles –

The Stages of Interphase • G 1 stage – Cell doubles its organelles – Accumulates materials needed for DNA synthesis

 • S stage The Stages of Interphase – DNA replication occurs – At

• S stage The Stages of Interphase – DNA replication occurs – At the beginning of stage, each chromosome is composed of one DNA molecule (chromatid). At the end, each chromosome consists of two identical DNA molecules (sister chromatids) – DNA replication has resulted in duplicated chromosomes

The Stages of Interphase • G 2 stage – Cell synthesizes proteins needed for

The Stages of Interphase • G 2 stage – Cell synthesizes proteins needed for cell division • G 0 stage – “Resting stage” – Cell is neither dividing nor preparing to divide • Ex. nerve cells

The Mitotic Stage • Sister chromatids of each chromosome separate, becoming daughter chromosomes that

The Mitotic Stage • Sister chromatids of each chromosome separate, becoming daughter chromosomes that are distributed to two daughter nuclei • After cytokinesis, two daughter cells are left • About 4 hours in mammal cells

Control of the Cell Cycle • Controlled by internal and external signals. • Growth

Control of the Cell Cycle • Controlled by internal and external signals. • Growth factors: hormones that act as external signals that regulate mitosis

Control of the Cell Cycle • The stop signs represent checkpoints when the cell

Control of the Cell Cycle • The stop signs represent checkpoints when the cell cycle can possibly stop

Control of the Cell Cycle • Cyclin: protein that control the progression of cells

Control of the Cell Cycle • Cyclin: protein that control the progression of cells through the cell cycle • Must be present from G 2 to M, and G 1 to S

Control of the Cell Cycle • G 1 checkpoint: cell cycle will stop if

Control of the Cell Cycle • G 1 checkpoint: cell cycle will stop if DNA is damaged • In mammals, protein p 53 stops cell cycle at G 1. – If p 53 cannot repair DNA, will bring about apoptosis

Control of the Cell Cycle • G 2 checkpoint: Cell cycle stops if DNA

Control of the Cell Cycle • G 2 checkpoint: Cell cycle stops if DNA has not finished replicating • If DNA is damaged, stopping the cell cycle allows time for damage to be repaired

Control of the Cell Cycle • M checkpoint: Mitosis stops if chromosomes not properly

Control of the Cell Cycle • M checkpoint: Mitosis stops if chromosomes not properly aligned

Apoptosis • Programmed cell death • Cell fragments engulfed by white blood cells or

Apoptosis • Programmed cell death • Cell fragments engulfed by white blood cells or neighbouring cells

Apoptosis • Cells harbour the enzymes that bring about apoptosis • Held in check

Apoptosis • Cells harbour the enzymes that bring about apoptosis • Held in check by inhibitors • Unleashed by internal or external signals • “Initiators” receive signal to activate “executioners” which activate enzymes that dismantle cell – ex. executioners turn on enzymes that tear apart the cytoskeleton and enzymes that chop up DNA.