Cell Cycle Lecture Notes PDF

Cell Cycle Learning outcomes: a1. Recognize the structure and function of the cell, molecular composition of the cell, cell cycle, and cell signaling. BY Dr. Bassam Mohamed Ali Abdelwahab What is the cell cycle ? A cell reproduces by car...

Cell Cycle Learning outcomes: a1. Recognize the structure and function of the cell, molecular composition of the cell, cell cycle, and cell signaling. BY Dr. Bassam Mohamed Ali Abdelwahab What is the cell cycle ? A cell reproduces by carrying out a sequence of events in which it duplicates its contents and then divides in two. This cycle of duplication and division, known as the cell cycle, is the essential mechanism by which all living things reproduce. Cell cycle (cell-division cycle): is the series of events that take place in a cell leading to duplication of its DNA (DNA replication) and division of cytoplasm and organelles to produce two daughter cells.  Cell Cycle The alternation between interphase and cell division in all tissues with cellular turnover.  Duration of the cell cycle ▪ The duration of the cell cycle varies greatly from one cell type to another. ▪ In an early frog embryo, cells divide every 30 minutes whereas a mammalian fibroblast in culture divides about once a day. Skin cells ………….. Liver cells ………….. The lengths of the various phases: M phase, for example, is typically much shorter and G1 much longer than shown.  Interphase The stage between two successive cell division. If a cell receives GO signal → it divides. If a cell doesn't receive a signal → it exits the cycle and switches to G0 phase. The eukaryotic cell cycle usually occurs in four phases. S-phase G1-phase G2-phase M phase The cell grows continuously in interphase, which consists of three phases: G1, S, and G2. G1 is the gap between M phase and S phase DNA replication is confined to S phase. G2 is the gap between S phase and M phase. M phase, the nucleus divides in a process called mitosis; then the cytoplasm divides in a process called cytokinesis. G1-Phase 1. Cell growth. 2. RNA and protein synthesis. 3. Cell doubles its organelles. 4. Accumulates raw materials for DNA synthesis (DNA replication). DNA Synthesis phase (S - phase) 1. DNA replication. 2. Proteins associated with DNA are synthesized. 3. Chromosomes enter with 1 chromatid each. 4. Chromosomes leave with 2 identical chromatids each (= Sister chromatids). S-Phase  Between DNA replication and onset of mitosis 1. Cell synthesizes proteins necessary for cell division. 2. The energy of mitosis is stored. 3. The duplication of the centriole is completed forming two pairs of centrioles. G2-Phase ❑ Centrosome: is the primary microtubule-organizing centre (MTOC) in eukaryotic animal cells. is made up of two centrioles (= small set of microtubules). ❑ Microtubules: ▪ components of the cytoskeleton, found in all eukaryotic cells. ▪ involved in mitosis, cell motility, intracellular transport, and maintenance of cell shape. ▪ composed of α- and β-tubulin subunits assembled into linear protofilaments. Cell division is a process that insure growth of the organism, replacement of worn out cells. Cell Division Mitosis Meiosis Somatic Germinal Cells Cells  The two most dramatic events in the cell cycle are when the nucleus divides, a process called mitosis, and when the cell later splits in two, a process called cytokinesis. Mitosis and cytokinesis together constitute the M phase of the cell cycle. In a typical mammalian cell, the whole of M phase takes about an hour, which is only a small fraction of the total cell-cycle time.  PHASES OF MITOSIS ▪ Mitosis involves 5 phases, based on the physical state of chromosomes and spindle. ▪ These phases are: prophase, prometaphase, metaphase, anaphase, and telophase. ▪ Cytokinesis is the final physical cell division that follows telophase, and is therefore sometimes considered a sixth phase of mitosis. ▪ Biochemical understanding of mitotic phases has greatly increased in recent years. Prophase During early prophase: Cell begins breaking down some structures and creating others, preparing for the division of chromosomes. Duplicated chromosomes start to condense, meaning they become compacted and tightly wound.  Nuclear envelope starts to break down. An apparatus known as a mitotic spindle forms on the edges of dividing cell. The spindle is made up of strong proteins called microtubules, which are part of the cell's skeleton and drive the division of the cell through elongation. The spindle gradually lengthens during prophase. Its role is to organize chromosomes and move them during mitosis. The nucleolus (or nucleoli, plural) disappears (this is a sign that the nucleus is getting ready to break down). ❑ The nucleolus → a part of the nucleus, where ribosomes and ribosomal RNA (rRNA) are made. Prometaphase It occurs at the end of the prophase stage. Nuclear envelope breaks down. Microtubules reach from each cell pole to the cell's equator. ??? ✓ Centromere: constricted region of the chromosomes that holds the sister chromatids together. ✓ Kinetochores: are disc shaped protein complexes closely associated with centromere. ✓ Kinetochores attach to a type of microtubules called kinetochore fibers. ✓ Kinetochore fibers interact with the spindle polar fibers that connect kinetochores to polar fibers, causing chromosomes to migrate toward the cell's centre. Metaphase Chromosomes align at metaphase plate at right angles to spindle poles, and are held there by the equal forces of polar fibers exerting pressure on the centromeres of chromosomes. The metaphase plate is not a physical structure – this is simply a term for the plane where the chromosomes line up. Anaphase Sister chromatids separate from each other and are pulled to opposite poles (ends) of the elongated cell. The protein "glue" that holds sister chromatids together breaks down allowing them to separate.  Each sister chromatid is now its own "full" chromosome. They are now called daughter chromosomes. The daughter chromosomes are pulled towards opposite ends of the cell. The microtubules get shorter, which lets the process of cell separation begins. As the chromosomes approach a pole, they migrate centromere first and the kinetochore fibers shorten. To prepare for telophase, the two cell poles move further apart. Upon completion of anaphase, each pole contains a complete collection of chromosomes. At this point, cytokinesis begins. This is the division of the original cell's cytoplasm, and it continues through telophase stage. Telophase In the telophase stage, the cell is nearly done dividing.  The nuclear envelope, which had previously broken down reforms as two new nuclear envelopes around the separated sister chromatids. Nuclei start to form at opposite poles. The mitotic spindle is broken down into its building blocks. Two new nuclei form (one for each set of chromosomes). During this process, nuclear membranes and nucleoli reappear and chromatin fibers of chromosomes open out, returning to their previous string-like form. After telophase, mitosis is almost complete → the genetic contents of one cell have been divided equally into two cells. However, cell division is not complete until cytokinesis takes place. Cytokinesis ❑ Is the division of the cell's cytoplasm to form two new cells. ❑ Each daughter cell is genetically identical, containing its own nucleus and a complete copy of the organism's DNA. ❑ Cytokinesis starts before anaphase ends and completes shortly after the telophase stage of mitosis. ❑ The cytoplasm → the fluid in which all cell components are bathed, is equally divided between the two new daughter cells. ❑ During cytokinesis in animal cells, a ring of proteins called actin and myosin (the same proteins found in muscle) pinch the elongated cell into two brand new cells. ❑ A band of filaments made of a protein called actin is responsible for the pinching, creating a crease called the cleavage furrow.