CMP Life and Death of Cells PDF

The cell cycle Single-celled organisms = cell cycle can repeat continuously as long as there is sufficient space and nutrients. Multi-cellular organisms = cell cycle is more strictly controlled. Cells with slow cell cycles typically exist in G0. Cells that never divide (fully differentiate...

The cell cycle Single-celled organisms = cell cycle can repeat continuously as long as there is sufficient space and nutrients. Multi-cellular organisms = cell cycle is more strictly controlled. Cells with slow cell cycles typically exist in G0. Cells that never divide (fully differentiated) are called post-mitotic cells. Due to ageing some cells lose the ability to divide (not originally supposed to be post-mitotic) – senescent cells. G1 phase Cell undergoes considerable protein synthesis. Cell duplicates organelles. Cells are highly metabolically active at this stage and require lots of energy. S phase Cell has grown big enough to duplicate its DNA. An extra copy of each chromosome is made and the two copies are joined at the centromere. G2 phase Second rapid period of cell growth (readying for mitosis). DNA checks to see whether it has been copied correctly. Many cancer cells go straight from S phase to mitosis. M phase Nuclear envelope breaks down, mitotic spindle forms, and the chromosomes are separated – cell splits in two (cytokinesis). Several stages in M phase: o Prophase o Prometaphase o Metaphase o Anaphase o Telophase Kinetochore = protein complex that attaches chromosomes to microtubules, leading to the segregation of the chromosomes during mitosis. Cytokinesis = actin filaments align around the middle of the cell, forming a cleavage furrow and eventually splitting the cell. Cell cycle regulation There are critical stages during the cell cycle called commitment points when the cell has to make a decision about whether to proceed to the next stage based on the environmental conditions. Once DNA has been copied, the cell must either complete the cycle or die – a cell with 2x the amount of chromosomes is not viable. Checkpoints: o End of G1 = DNA damage checkpoint, entrance to S is blocked if genome is damaged. o S = DNA damage checkpoint, DNA replication halted if genome is damaged. o End of G2 = entrance to M is blocked if DNA replication is not complete. o Metaphase part of M = anaphase is blocked if chromatids are not properly assembled on the mitotic spindle. Senescence The Hayflick limit = most cells from multicellular organisms have a limited lifespan, typically around 40-60 cell divisions (the limit). Cells that are not actively dividing are in the G0 phase of the cell cycle. Senescent cells are permanently stuck in the G0 phase and cannot enter the cell cycle. To reduce the risk of developing cancer (lose control of our own cells and they proliferate uncontrollably), cells have evolved built-in limits on cell division. Telomere shortening following cell division limits the number of times a cell divides – cancer cells rebuild their telomeres using telomerase. HeLa cells: o Type of cancer cell. o Isolated from a cervical tumour – found to be immortal and could be cultured indefinitely. o Used to create the first human cell line, used to test the first polio vaccine. o Immortal cell lines can now be generated by transforming normal cells with viral genes.  Apoptosis vs necrosis Necrosis = uncontrolled cell death, associated with disease. Apoptosis = programmed cell death / suicide, essential part of normal health and development. Necrosis Apoptosis Morphological features: Morphological features: - Loss of membrane integrity. - Membrane blebbing, but no loss of integrity. - Ends with total cell lysis. - Ends with fragmentation of cell into smaller bodies. Biochemical features; Biochemical features: - No energy requirement (passive). - Tightly regulated process involving activation and enzymatic steps using ATP. Physiological significance: Physiological significance: - Affects groups of cells. - Affects individual cells. - Significant inflammatory response. - No inflammatory response. Importance of apoptosis Embryo development – sculpting tissue, quicker and easier to control than making specific structures initially. Immune system – destroying self-reacting immune cells (B and T cell receptors are randomly made and sometimes are accidentally self-reactive, therefore need to be killed), as well as virus infected cells. Homeostasis = counter-balance to cell division, removes old / damaged cells. Cancer = radiotherapy and most chemotherapy drugs work by inducing apoptosis. Triggering apoptosis - overview Two main pathways for triggering apoptosis: Receptor mediated (extrinsic pathway) o 3 possible types of death-inducing ligands can bind to death receptors. Mitochondria mediated (intrinsic pathway) Caspases: Family of 12 proteases that exist as inactive pro-enzymes in cells – following activation by cleavage they can activate other caspases in a cascade. Two types of apoptotic caspases: o Initiator caspases = activate other caspases. o Effector (executioner) caspases = break down cellular components such as the cytoskeleton and DNA. Triggering apoptosis – DISC assembly DISC = death-inducing signaling complex Ligand binds to receptor Causes trimerisation Platform inside membrane is formed Procaspase 8 is able to bind and cap removed (activated) Activa caspase 8 can then activate procaspase 3 Continues to activate other executioner caspases to bring about apoptosis Role of mitochondria in apoptosis Cytochrome is located in the inner mitochondrial membrane and is an essential component of the electron transport chain. To trigger apoptosis, pores form in the outer mitochondrial membrane – this allows the release of cytochrome C from the cytosol. Cytochrome C binds to other cytosolic proteins to form a multi-protein complex = apoptosome: o Formation requires cytochrome C, protein called Apaf-1, procaspase 9, and ATP. o End result is the cleavage and therefore activation of procaspase 9 into active caspase 9 (initiator caspase). Control of cytochrome C release: o Pro-apoptotic members of the Bcl-2 family (proteins) insert themselves into the mitochondrial surface and promote the formation of large pores = allow cytochrome C release. o Anti-apoptotic members exist in the mitochondrial outer membrane and block the action of the pro-apoptotic members = prevent cytochrome C release. o The balance between the two determines how difficult it is to induce apoptosis.