Cell Cycle and Cell Death PDF

36 Cell cycle and Cell death Cell cycle, or cell-division cycle: Is the series of events that take place in a cell leading to its division and replication to produce two daughter cells. Phases of cell cycle A. Interphase: during which the cell grows, accumulating...

36 Cell cycle and Cell death Cell cycle, or cell-division cycle: Is the series of events that take place in a cell leading to its division and replication to produce two daughter cells. Phases of cell cycle A. Interphase: during which the cell grows, accumulating nutrients needed for mitosis and duplicating its DNA. It is further divided into G1 , S ,and G2 phases. B. The mitosis (M) phase: Cell splits itself into two distinct daughter cells. A. Interphase: During G1 phase, the cell grows in size and synthesizes mRNA and proteins (known as histones) that are required for DNA synthesis. Once the required proteins and growth are complete, the cell enters the next phase of the cell cycle, S phase. The duration of each phase, including the G1 phase, is different in different types of cells. In human somatic cells, the cell cycle lasts about 18 hours, and the G1 phase takes up about 1/3 of that time. 1. G1 phase is particularly important in cell cycle because it determines whether a cell commits to division or to leaving cell cycle. If a cell is signaled to remain undivided, instead of moving onto S phase, it will leave G1 phase and move into a state of dormancy called G0 phase. Most non proliferating vertebrate cells will enter G0 phase 2. S Phase: DNA synthesis and Histone synthesis are the most important events. 3. G2 phase Assemble of cytoplasmic materials necessary for mitosis and cytokinesis. It is a period of rapid cell growth and protein synthesis during which the cell prepares itself for mitosis. G2 phase is not a necessary part of the cell cycle, as some cell types as cancer proceed directly from DNA replication to mitosis. B. M Phase: A nuclear division (mitosis) followed by a cell division (cytokinesis), the time interval between 2 mitotic divisions that is, G1, S and G2 is known as interphase.  37 Stages of mitosis:- 1-Prophase  Disappearance of nuclear envelope by fragmentation.  Chromosomes condensed and visible as their DNA duplicated during previous (S) phase. Each chromosome is formed of two chromatid joined at centromere region by kinetochore protein complex.  Cenrtosomes move to the opposite pole of the cell. 2- Metaphase Chromosomes arranged in the equatorial of the cell and attached to the dynamic microtubules of the spindle. 3- Anaphase  The spindle splits the chromatids, each chromatid migrates to the opposite pole of the cell and forms future chromosome.  Due to dynamic movement of the spindle microtubules two sister chromatids are further apart. 4- Telophase  Two sets of chromosomes are at spindle pole and assume their uncoiled state.  Spindle microtubules depolymerized and start to disappear.  Formation of constriction ring (f –actin & myosin) split the cell into two. Prophas Metapha Anaph Telophase Early mitotic e & cytokinesis Spindle Metaphase Plateau ase Cleavage Furrow Nuclear Envelope Daughter Chromosome Two sister Chromatid  38 G0 (G zero): A cell may leave the cell cycle, temporarily or permanently at G1 and enters a stage designated G0. A G0 cell is usually terminally differentiated and will never reenter the cycle. They are busy conducting their functions in the organism until they die e.g. secretion, attacking pathogens. Some differentiated cells, such as those of the liver, re-enter the cycle under certain conditions. While others such as muscle and nerve cells, are terminally differentiated. Cell death (apoptosis and necrosis) Apoptosis is a naturally occurring programmed and targeted cause of cellular death by which redundant or defective cells are rapidly eliminated in a manner that does not provoke a local inflammatory reaction in the tissue. Necrosis is a form of cell injury which results in the premature death of cells in living tissue by autolysis, Necrosis is caused by factors external to the cell or tissue, such as infection, or trauma which result in the unregulated digestion of cell components While apoptosis often provides beneficial effects to the organism, necrosis is Almost always detrimental and can be fatal. Morphological features of apoptosis 1. Chromatin aggregation 2. Nuclear and cytoplasmic condensation 3. Partition of cytoplasm and nucleus into membrane bound vesicles (apoptotic bodies) which contain ribosomes, morphologically intact mitochondria, and nuclear material. 4. No inflammatory response is elicited. Morphological features of necrosis 1. Cell lysis: Cell swell and rupture. 2. Cell membrane: Breakdown 3. Inflammatory response: In vivo, necrotic cell death is often associated with extensive tissue damage resulting in an intense inflammatory response. Necrosis Apoptosis