Week 10 - Life and Death of Cells Lecture PDF

Life and death of cells Lecture outline Mitosis Regulation of the cell cycle Immortality and cancer Introduction to cell death Apoptosis  The Cell Cycle Cell division is important for: Embryo development Growth Homeostasis (...

Life and death of cells Lecture outline Mitosis Regulation of the cell cycle Immortality and cancer Introduction to cell death Apoptosis  The Cell Cycle Cell division is important for: Embryo development Growth Homeostasis (Replacing old or dead cells) Cancer Most cell division results in daughter cells with identical genetic information (DNA) – mitosis. Cell division linked to reproduction is called meiosis. The cell cycle is the highly organised process behind these phenomena. The Cell Cycle The cell cycle is the process through which a cell can produce new cells. Each cell division results in two daughter cells. In single-celled organisms this process can repeat continuously as long as there is sufficient space and nutrients. In multi-cellular organism this process is more strictly controlled.  The Cell Cycle Most cell division in the adult human body occurs in the bone marrow (500 billion new cells per day) Speed of the cell cycle varies depending on cell type Typical eukaryote cell = 24 hr Hepatocyte (liver cell) = 1-2 years Neurones, heart cells = never Bacteria = 20 minutes Cells that never divide are called post-mitotic cells. During ageing some cells lose the ability to divide and are called senescent cells.  The Cell Cycle (1 hr) (3-4 hrs) (4-6 hrs) (10-12 hrs)  G1 phase In order to divide a cell needs to split its contents, including the DNA, between the two daughter cells. To do this it needs to grow (get larger) and produce more cellular contents. In G1 phase the cell undergoes considerable protein synthesis – which means there is also lots of gene transcription and RNA synthesis occurring. In this phase, cells also duplicate their organelles (e.g. mitochondria, ER) in order that each daughter cell will have enough to meet its needs. Cells are highly metabolically active at this stage and require lots of energy.  S phase Chromosomal Chromosomes DNA molecules 1 Centromere Chromosome arm In S phase the cell has grown big enough and begins to duplicate Chromosome duplication (including DNA replication) its DNA. and condensation 2 An extra copy of each chromosome is made and the Sister two copies are joined at the chromatids centromere. Separation of sister chromatids into two chromosomes 3  G2 phase The second gap phase. This is another rapid period of cell growth as the cell readies itself for mitosis. This phase is also where the cell checks its DNA to make sure it has been copied correctly. The phase is not always necessary – many cancer cells go straight from S phase to mitosis.  M phase The fastest stage of the cell cycle. This is where the nuclear envelope breaks down, the mitotic spindle forms and the chromosomes are separated. The cell then splits in two (cytokinesis). There are several stages in M phase: Prophase Prometaphase Metaphase Anaphase Telophase  Metaphase Metaphase Chromosomes are blue plate Microtubules (mitotic spindle) is green. In metaphase, the chromosomes align at the centre of the cell along the metaphase plate. Spindle Centrosome at one spindle pole  The kinetochore The kinetochore is a structure that attaches chromosomes to microtubules, leading to the segregation of the chromosomes.  Cytokinesis (a) Cleavage of an animal cell (SEM) A cell in telophase undergoing cytokinesis. Cleavage furrow 100 m Actin contractile ring (green), microtubules Contractile ring of Daughter cells (red), chromosomes microfilaments (white) Mitosis 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. There are also checkpoints where the cell checks that everything has gone correctly before moving to the next stage of the cycle. Cell cycle checkpoints  The Hayflick limit Most cells from multicellular organisms have a limited lifespan even when all of their nutrients are provided. This lifespan is typically around 40-60 cell divisions. This is known as the Hayflick limit. The cell cycle and senescence Cells that are not actively dividing are in G0 phase of the cell cycle. When more cells are needed cells can leave G0 phase and enter into G1 phase, starting the first phase of mitosis. Senescent cells are permanently stuck in G0 phase and cannot enter the cell cycle. The cell cycle and senescence Why is there a limit on cell division? For multicellular life, cancer is a major problem. Cancer is what happens when we lose control of our own cells and they proliferate without the normal controls leading to the formation of a tumour. To reduce the risk of developing cancer, cells have built-in limits on cell division. These limits are linked to telomeres.  Telomeres and telomerase Telomere shortening following cell division limits the number of times a cell divides. Tumour cells are effectively immortal and can rebuild their telomeres using the enzyme telomerase. without telomerase with telomerase (normal) (stem cells & cancer) HeLa cells HeLa cells Isolated from Henrietta Lacks in 1951 from a cervical tumour. Some of the tumour cells that were isolated were immortal and could be cultured indefinitely. First human cell line and still one of the most widely used cell models today. Were used to test the first polio vaccine in the 1950s. Can contaminate other cell cultures.  Immortalised cell lines PHE and ATCC are worldwide repositories for cell lines More than 4,000 human cell lines Cell lines from over 150 species Immortal cell lines can now be generated by transforming normal cells with viral genes, such as those from the SV40 (simian virus 40) virus.  Summary The cell cycle in mitosis is the process by which cells duplicate proteins, key organelles and their chromosomes. The cytoskeleton and the nucleus break down, leading to alignment of chromatids on the mitotic spindle (formed from microtubules) and separation of chromosomes to opposite ends of the cell. The actin cytoskeleton cleaves the cell into two daughter cells. This process contains several commitment points and checkpoints to ensure correct functioning of the cell cycle. There are a limited number of cell divisions for most cells in multicellular organism to help prevent cancer. Cell death  Types of cell death There are two main ways in which cells can die: 1) Necrosis: (uncontrolled cell death). Associated with disease. 2) Apoptosis (programmed cell death or cell suicide). Apoptosis is an essential part of normal health and development. In an average adult between 50 and 70 billion cells die through apoptosis per day. In a year your entire body weight of cells will have died through apoptosis. Apoptotic Macrophage Apoptotic cell cell Necrotic cell Apoptosis vs necrosis  Apoptosis in action Time-lapse microscopy of apoptosis over a 4h period  Importance of apoptosis Embryo development – sculpting tissue Immune system – destroying self-reacting immune cells. Immune system – destroying virus infected cells. Homeostasis – as a counter-balance to cell division and removal of old or damaged cells. Cancer – radiotherapy and most chemotherapy drugs work by inducing apoptosis. Many cancer cells have defects in the apoptosis pathways that make them resistant to apoptosis. How is apoptosis triggered? Two main pathways for triggering apoptosis: 1) Receptor mediated (extrinsic pathway) 2) Mitochondria mediated (intrinsic pathway) The intrinsic pathway can be activated via the mitochondria by a variety of cell stresses such as free radical damage, DNA damage, viral infection, or loss of survival signals.  Death receptors Two main pathways for triggering apoptosis: 1) Receptor mediated (extrinsic pathway) 2) Mitochondria mediated (intrinsic pathway) Caspases – chief executioners of apoptosis A family of 12 proteases that exist as inactive pro-enzymes in cells. Following activation by cleavage they can activate other caspases in a cascade. There are two types of apoptotic caspases: initiator caspases and effector (executioner) caspases. Initiator caspases activate other caspases. Effector caspases break down cellular components such as the cytoskeleton and DNA. Caspases – chief executioners of apoptosis  Role of mitochondria in apoptosis Cytochrome C 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 allowing release of cytochrome C into the cytosol. Cytochrome C then binds to other cytosolic proteins to form a multi-protein complex called the apoptosome. The apoptosome – the “wheel of death” The formation of the apoptosome requires cytochrome C, a protein called Apaf-1, pro- caspase 9 and ATP. There are 7 molecules of each protein in the complete apoptosome with a combined molecular weight of 700 KDa. The end result is the cleavage and therefore activation of pro-caspase 9 into active caspase 9, an initiator caspase.  What controls the release of cytochrome C from the mitochondria ? Bcl-2 proteins and apoptosis Pro-apoptotic members of the Bcl-2 family are thought to work by inserting themselves into the mitochondrial surface and promoting the formation of large pores in the outer membrane that leads to the release of Cytochrome C. Anti-apoptotic members are thought to exist in the mitochondrial outer membrane and act to block the action of the pro-apoptotic members. Bcl-2 proteins and apoptosis Apoptosis - summary  Apoptosis - summary Programmed cell death (apoptosis) is essential for normal health and development. The mitochondria play a central role in the regulation of apoptosis and can be regarded as one of the main checkpoints in the process. Bcl-2 proteins act on the mitochondria to regulate the formation of pores in the outer mitochondrial membrane. The result depends on the balance between pro-apoptotic and anti-apoptotic Bcl-2 proteins. Release of cytochrome C from the inner mitochondrial membrane to the cytosol, leads to the formation of the apoptosome and activation of the caspase cascade. The caspases are responsible for the destruction of the cell.  Further reading Chapter 12 – The Cell Cycle Chapter 11 - DNA Chapter 17 - Cell cycle Chapter 13 – Cell communication Replication and Cell Division Chapter 18 - Cell death