Cell Cycle PDF

The Cell Cycle “The Life of a Cell”  Cells & Cell Reproduction Cells must reproduce else they die. The "life of a cell" is termed the cell cycle. The cell cycle has distinct phases, which are called G1, S, G2, and M. Cells that have temporarily or reversibly stopped divid...

The Cell Cycle “The Life of a Cell”  Cells & Cell Reproduction Cells must reproduce else they die. The "life of a cell" is termed the cell cycle. The cell cycle has distinct phases, which are called G1, S, G2, and M. Cells that have temporarily or reversibly stopped dividing are said to have entered a state of quiescence called G0 phase. 2  Frequency of cell division Frequency of cell division varies by cell type – embryo cell cycle < 20 minute – skin cells divide frequently throughout life 12-24 hours cycle – liver cells retain ability to divide, but keep it in reserve M divide once every year or two metaphase anaphase – mature nerve cells & muscle cells prophase telophase C G2 do not divide at all after maturity permanently in G0 interphase (G1, S, G2 phases) mitosis (M) cytokinesis (C) G1 S  The G1 Phase of the Cell Cycle During this time organelles are reproducing, protein synthesis is occurring for growth and differentiation. Because, transcription is occurring, the DNA is uncoiled. This phase is the most variable, ranging from almost nothing to years. 4  The G1 Phase of the Cell Cycle Most cells that differentiate will do so during this phase. Cells arrested in G1 may no longer have the capability of reproducing and are said to be in G0. Certain cells in G0, however, when given some external or internal cues may revert back to G1 and enter the cell cycle again. Nerve and muscle cells are usually arrested in G0.  S Phase of the Cell Cycle The S or synthesis phase is the second phase of the cell cycle. DNA uncoils DNA replication occurs Additional organelle replication occurs This phase ensures that each emerging daughter cell will have the same genetic content as the mother cell. 6  G2 Phase of the Cell Cycle The G2 or Gap 2 phase occupies the time from the end of S until the onset of mitosis. During this time, the cell prepares for mitosis by making and organizing necessary proteins such as the tubulin needed to construct microtubules which used to make spindle fibers. On the average this phase may take four hours. 7  M Phase or Mitosis During mitosis the nucleus is replicated and the cytoplasm divides to produce two genetically identical daughter cells. Remember that the DNA is replicated in S prior to mitosis. The phases are triggered by the accumulation of cell signals.  The Amount of DNA Varies During the Cell Cycle This graph represents the amount of DNA found in the cell during the cell cycle. What caused the changes? What happens at the end of Mitosis? 9  Internal Controls of the Cell Cycle The control of the cell cycle is dependent on an accumulation of “signal molecules”. Quite often these signal molecules must be phosphorylated in order to be functional. This are simple illustrations. 10  Checkpoint control system 3 major checkpoints: – G1/S can DNA synthesis begin? – G2/M has DNA synthesis been completed correctly? commitment to mitosis – spindle checkpoint are all chromosomes attached to spindle? can sister chromatids separate correctly?  Cyclins vs. Kinases Cyclins are a family of proteins that control the progression of cells through the cell cycle by activating cyclin-dependent kinase (Cdk) enzymes. Only with the cyclin is the Cdk active. Cyclins were originally named because their concentration varies in a cyclical fashion during the cell cycle. A kinase is a type of enzyme that transfers phosphate groups from high-energy donor molecules, such as ATP, to specific substrates, a process referred to as phosphorylation.. 12  Cyclins vs. Kinases Certain cyclins are made at certain times during the cell cycle, and their concentration will rise and fall. Cyclins are also destroyed after they are no longer needed by the cell. CDKs are not destroyed as they are only activated or deactivated. Which cyclin affects which phase of the cycle? 13 Kinases Phosphorylate Cell Signal Molecules Certain kinases may have two forms (active and inactive). Kinases are enzymes (proteins) that phosphorylate certain molecules or other enzymes. Most cell cycle signals are phosphorylated by kinases. 14  Cyclins Activate Kinases Most cell cycle kinases are activated by molecules called cyclins. A kinase that requires a cyclin for activation is called a cyclin-dependent kinase or Cdk. 15  Cyclins Activate Kinases The cyclin attaches to the Cdk. It is now called a cyclin-Cdk complex. The complex that regulates the M (mitosis) portion of the cell cycle has 3 names (ugh!): the maturation-promoting factor, mitosis-promoting factor or M-Phase promoting factor. Luckily they are all referred to as “MPF”. MPF is activated at the end of G2 by a phosphatase, which removes an inhibitory phosphate group added earlier. The MPF is also called the M phase kinase because of its ability to phosphorylate target proteins at a specific point in the cell cycle and thus control their ability to function.  An example of how MPF initiates mitosis …. MPF promotes the entrance into mitosis (the M phase) from the G2 phase by phosphorylating multiple proteins needed during mitosis. The steps follow: The nuclear lamina depolymerizes causing it to disassemble which in turn causes the nuclear membrane to disassemble Histone H1 binds to the DNA in chromosomes, causing the chromosomes to condense Cytoskeletal proteins allow cytoskeletal filaments to assemble which leads to: – Formation of the mitotic spindle which separates the daughter chromosomes – formation of the cleavage furrow by microfilaments which allows cytokinesis (constricting the cell at the center) to occur resulting in the formation of two new cells Cyclins Activate Kinases Once the CDK phosphory- lates certain signals, the cyclin is destroyed. In the cell, the concentration of cyclins will rise and fall depending on the phase of the cell cycle. When the cyclin is destroyed the Cdk returns to an inactive form (it is NOT destroyed!). 18  Cyclins Activate Kinases Below is an example of how the M cyclin concentration affects MPF or M/CDK activity. 19  Random info …. The term cyclin was coined by R. Tim Hunt who discovered them while studying the cell cycle of sea urchins cells. He said he named it after his hobby of cycling and at the time he did not realize the role of these molecules in the cell cycle. However due the cyclic nature of the concentration of these compounds and their role in the cell cycle, the name stuck. Cyclins (D, E, A, B) are named based on the their protein structure and conserved parts. Older classification of cyclins is based on their role in the cell cycle. Most introductory books use the terms like S cyclin and M cyclin. R. Tim Hunt along with Leland Hartwell and Sir Paul Nurse received the Nobel Prize in Medicine in 2001 for their discovery and research in the role of cyclins and CDKs in the cell cycle. Leland H. Hartwell Tim Hunt Sir Paul Nurse checkpoints Cyclins CDKs 20  Different Types of Cyclins Cyclin/CDK Cyclin Function of Cyclin/CDK Complex Complex G1-CDK Cyclin D Drives the transition G1 S transition G1/S-CDK Cyclin E Cyclins bind to CDK at the end of G1 and commits the cell to DNA replication. S-CDK Cyclin A Cyclins bind the CDK during S and are necessary for the initiation of DNA replication M-CDK Cyclin B Cyclins promote the events of Mitosis 21 Cyclins/CDKs Control the Cell Cycle The cell cycle has a number of several external and internal checkpoints much like a timer or clock. Often, moving past these check points involves chemical signals that have been phosphorylated by cyclin -CDK complexes. G1/S (R point) checkpoint is the primary determining factor for cell division to take place. Growth factors are affecting the cell cycle, and cells are growing. Once the R point is passed the DNA is going to be replicated. If a cell receives a go-ahead signal at this check-point, it will complete the cell cycle and divide. However, if the cell does not receive the go-ahead signal in G1, the switches to a nondividing state called G0. If the cell passes the G1 checkpoint, it is usually committed to cell reproduction. 22 Cyclins/CDKs Control the Cell Cycle The G2 checkpoint represents the commitment for starting the process of mitosis. This checkpoint also ensures that the DNA has been replicated correctly. If the DNA has been damaged, then the cell does not continue to mitosis. Once the CDK and cyclin combine, it is called “maturation promoting factor" or “mitosis promoting factor” or MPF. The M/spindle checkpoint ensures that all the chromosomes are attached to the spindle in preparation of mitosis. The separation of the chromatids is irreversible. Once chromatids are replicated they are held together by a protein substance called cohesin protein. Another protein called seperase can destroy this protein but has two forms active and inactive. 23 External Signals also affect cell division External Signals also affect cell division. Mammalian cells need certain nutrients and regulatory proteins. In addition external growth factors are can determine cell division in mammals. For example, when the skin has been damage (wound), platelets release a substance called platelet-derived growth factor (PDGF). This growth factor stimulate fibroblasts cells to start to reproduce and make scar tissue. 24  Select this paragraph to edit The initiation of the cell cycle occurs with the receipt of a signal (e.g., a growth factor ligand) by a cell in G0 or G1. The signal induces synthesis of G1 and G1/S phase cyclin-CDKs, which then activate transcription of genes encoding DNA synthesis enzymes and S phase cyclin-CDKs. S phase cyclin-CDKs initially are held in check by inhibitors until G1/S phase cyclin-CDKs phosphorylate the inhibitors. The released S phase cyclin-CDKs then phosphorylate regulatory proteins bound to chromosomal replication origins, promoting initiation of DNA synthesis. The synthesis of mitotic cyclin-CDKs increases in S and G2 phases. The activities of these complexes initially are blocked by phosphorylation of CDK subunits, and then are activated later by dephosphorylation. Once activated, mitotic cyclin-CDKs phosphorylate a large number of proteins that control chromosome condensation, retraction of the nuclear envelop, formation of the mitotic spindle, and alignment of chromosomes at the metaphase plate.  Cells in G0 do not synthesize cyclins or CDKs. The transition of quiescent cells from G0 to G1 and resumption of the cell cycle is triggered by growth factors in serum (mitogens). Shortly after binding to receptors, growth factors turn on the transcription of early response genes using TFs that preexist in the cell. Among the early response genes are c-fos, c-jun, and c-myc These genes turn on the transcription of delayed-response genes. Included within the latter are the G1 cyclin-CDKs and a TF called E2F, which is controlled by the Rb gene (next slide). The synthesis of G1 cyclin-CDKs propels the cell into G1. Prior to the START point, the withdrawal of growth factors leads to rapid degradation of G1 cyclin-CDKs and return to G0. At the restriction point, G1 cyclin-CDKs reach irreversibly high levels and cells are committed to enter S phase. After the restriction point, growth factors are no longer needed for completion of the cycle. One role of TGFß is inhibition of G1 cyclin-CDKs.  Rb is the prototype tumor suppressor gene. Inactivation of Rb leads to tumors of the retina in children. Rb also is inactivated in many other tumors. In non-proliferating cells, Rb protein binds to E2F, and the complex activates histone deacetylases leading to gene silencing. When the expression of the G1 cyclin-CDKs (cyclin D-CDK4/6) are turned on by a mitogen, Rb is phosphorylated and active E2F is released. E2F activates transcription of genes needed for passage into S phase, namely genes encoding DNA synthesis enzymes, Cyclins E & A (G1/S phase cyclins), CDK2, and itself. Cyclins E/A-CDK2 (G1/S cyclin-CDKs) also phosphorylate Rb. This occurs even if the mitogen is withdrawn and is the key control allowing the cell to pass through the restriction point. In S, G2, and mitosis, S-phase and mitotic cyclin-CDKs continue to phosphorylate Rb. Only after degradation of mitotic cyclins at the end of mitosis is Rb dephosphorylated. Rb then can inhibit E2F in early G1 and in G0-arrested cells.  Select this paragraph to edit Select this paragraph to edit