Cell Cycle Apoptosis

Cell cycle Apoptosis  Programmed cell death; cells activate an intracellular death program and kill themselves in a controlled way.  Mechanism: o Cells shrink and condense; cytoskeleton collapses and nuclear envelope disassembles. o Nuclear chromatin condenses and breaks into fragments. o Cell surface often blebs and break into membrane-enclosed fragments called apoptotic bodies.   o Apoptotic bodies can be engulfed by a neighbouring cell or macrophage before they spill their contents. Cell death is neat and rapidly cleared away (no inflammation). Application of apoptosis – formation of digits: o Interdigital cell death has eliminated the tissue between developing digits. Balance of division and death    Research in rats: o Remove parts of liver  cells divide to replace it. o Treat rat with drug that stimulates liver cell division  liver enlarges via cell division. o Stop treatment  apoptosis increases until liver returns to normal size. Apoptosis occurs rapidly in bone marrow. Animal cells can recognise damage in their organelles (e.g. DNA damage) and destroy themselves if they can’t repair it. Apoptosis and disease  Heart attacks and strokes (majority of cells die of necrosis, some less affected die from apoptosis).   If too few cells undergo apoptosis, lymphocytes can accumulate in spleen and liver, leading to an autoimmune disease. Reduced apoptosis leads to cancer. The ageing cell  Chronologically ageing: o Damage accumulates in non-dividing cells. o In the external medium, ethanol initially accumulates and is converted to acetic acid, which induces an apoptotic-like response and cell death. o Inside, damaged mitochondria and oxidised proteins also accumulate and probably contribute to chronological senescence.  Replicatively ageing: o Damage is assymetrically inherited by the mother cell and removed from the daughter cell. o Nuclear extrachromosomal ribosomal DNA circles, cytoplasmic oxidised proteins and damaged mitochondria contribute to replicative senescence. o In very old mother cells, asymmetry breaks down and the daughter cell can inherit sufficient damage to become prematurely aged. End replication problem  During DNA replication: o Direction of base addition always occurs from 5’ to 3’. o The leading strand has no problem; it will continue adding along the sequence. o Lagging strand must add bases in fragments (Okazaki fragments).  Since the machinery of DNA replication cannot perfectly space out the position of fragments, it is not always possible to fully replicate the lagging strand.  In the end, there will be a complete set of chromatid (from leading strand) and an incomplete set of chromatid (from lagging strand).  In response, enzymes will digest parts of the leading strand in order to maintain symmetry.   3’ end longer than the 5’ end. The single stranded end loops back and tucks into the DNA; t-loop.   May protect chromosome from degradation. Distinguishes them from broken DNA molecules. Regulation of length  As new generations are made, chromosomes will become shorter as a result of what happens during end replication.  Replicative cell senescence: after enough divisions, the descendent cells inherit defective chromosomes and cease dividing.  Telomeres thought to provide cell with a “cell division” counting mechanism. o Somatic cells are born with a full complement of telomeres. o Stem cells have telomerase activity and retain full length of telomeres. o In other cells, telomerase is “turned down” so the chromosomes lose 100 – 200 bases each division. Telomeres  Problem with replicating linear DNA – no place to put the RNA primer.  Telomeres are specialised sequences at ends of chromosomes; TTAGGG, repeated about 1000 times.  Sequence recognised by enzyme telomerase.  RNA template of telomerase is part of enzyme – used to synthesise DNA. o Telomerase adds on telomere sequence to the 3’ end of the parental lagging strand.  Incomplete strand can now be fully synthesised since there is space for primer. Summary: end replication  Most cells lose small amounts of DNA from the end of their chromosomes after a round of DNA replication.  Rapidly dividing cells and stem cells have an enzyme (telomerase) that overcomes the end replication problem.  The ends of all our chromosomes are made of telomeres (TTAGGG repeat elements). Hayflick limit  Human cells can only divide a finite number of times; after a certain number of times of cell division, they stop dividing.  Three phases of cell culture: o Phase 1: cells multiply and cover the surface of the culture flask. o Phase 2: cells divide in culture. o Phase 3: cells stop dividing.   Human fibroblasts normally divide about 50 times before senescence. Hayflick limit is also known as replicative senescence. Senescence  Most senescent cells are not dead or dying.  They continue to respond to hormones and other stimuli, but can’t proliferate.  Cellular senescence also triggers important changes in gene expression.  Certain genes control cell proliferation (e.g. c-fos) and others control senescence (e.g. p53). Crisis figure      Pre-senescent cells can be induced to bypass senescence by altering expression of proliferative or senescence genes. Once cells bypass senescence they continue to divide and experience further shortening of their telomeres. Cells eventually reach a second proliferative block often referred to as crisis. o Crisis = genomic instability and massive cell death. Rare variants, appearing at a frequency of less than one cell in 107, emerge from a population of cells in crisis. These variants have acquired the ability to maintain their telomeres at a stable length and spawn a clone of immortalised descendants. Telomeres and disease  Cardiovascular disease:   o Short telomeres  increased risk of cardiovascular disease in the presence of hypertension. Pessimism is linked with short telomeres. Stressful experiences (mothers of chronically ill children) result in shortened telomeres. Cancer  Usually requires a number of changes in gene expression (i.e. multiple mutations) for the cell to bypass senescence.  If they do so, immortalised cells (i.e. cancer) forms. Werner syndrome    Begins teens – 20s. Results in the appearance of old age by 30 – 40 years of age. Clinical features: o Wrinkled skin, baldness. o Cataracts. o Muscular atrophy. o Tendency for diabetes mellitus and cancer.  Cells from WS patients have a shorter lifespan in culture (~20 doublings). Telomere dysfunction is associated with the ageing phenotype.    Genetics: o Autosomal recessive trait. o Gene is RECQL2 on 8p12-p11.2 encoding the RecQ helicase WRN (responsible for proof-reading). Pathophysiology: o WRN functionally interacts with DNA polyermase delta, the major DNA polymerase required for chromosomal DNA replication and repair. o Cells lacking WRN exhibit deletion of telomeres from single sister chromatids. o There is propensity to develop chromosomal abberations, including translocations, inversions and deletions. o Only telomeres replicated by lagging strand synthesis were affected. o Telomere loss could be counteracted by telomerase activity.