Cell-cycle-and-Cell-Division.pdf

Cell Cycle and Cell Division In 1858 the pathologist Rudolph Virchow coined the cell doctrine which states that "When a cell arises, there must have been a previous cell, just as animals can only arise from animals and plants from plants." This doctrin...

Cell Cycle and Cell Division In 1858 the pathologist Rudolph Virchow coined the cell doctrine which states that "When a cell arises, there must have been a previous cell, just as animals can only arise from animals and plants from plants." This doctrine is founded on the understanding that whether one is examining a single-celled organism or an animal as complex as man, the product is a result of repeated rounds of cell growth and division. Most eukaryotic cells will proceed through an ordered series of events in which the cell duplicates its contents and then divides into two cells. This cycle of duplication and division is called the cell cycle. In order to maintain the fidelity of the developing organism this process of cell division in multicellular organisms must be highly ordered and tightly regulated. The loss of control will lead to abnormal development and is the cause of cancer. Why do cell divides? In unicellular organisms, cell division reproduces an entire organism (e.g. paramecium). In multicellular organisms, cell division is the basis for: growth and development from the fertilized egg. replacement of damaged or dead cells The Cell Cycle The cell cycle is a series of events that lead to cell division. It consists of four phases: G1, S, G2 and M, where "G" stands for "gap", "S" represents "synthesis" and "M" means "mitosis". A newly divided cell may either enter into another round of cell division or remain in the resting state for a long period of time. In the latter case, the cell is said to be in the G0 phase. Upon specific stimulation, cells in the G0 phase may re-enter the G1 phase. The process of cell division is more straightforward in lower, single-celled (prokaryotes) organisms than higher multicellular (eukaryotes) ones. The reason is that a prokaryotic cell has a relatively simpler cell organization with a single circular chromosome, absence of a nucleus, and few cell organelles. In contrast, eukaryotic cells have multiple chromosomes present within a nucleus and various organelles. A eukaryotic cell cycle consists of two main parts: 1) interphase and 2) mitotic (M) phase, and an alternative part called the G0-phase. Phases of Cell Cycle Interphase G1 phase  During G1, the cell reviews the cellular environment and the cell size to ensure that the conditions are appropriate to support DNA replication. Not until the cell is ready does it leave G1. If all is not ready to undergo DNA replication, cells can pause during G1 and enter a phase called G0.  Depending on a cell's preparedness to continue in the cell cycle, G0 can last days, weeks, or even years. When the cell has reached an appropriate size and is in a supportive environment for DNA replication, it will exit either G1 or G0 and enter the next phase of interphase called S phase.  The cell grows in preparation for DNA replication, and certain intracellular components, such as the centrosomes undergo replication. Before a cell begins DNA replication, it must ensure that it is biologically ready to take on such a process. There is a major check-point in a normal cell cycle that is critical for ensuring that all is well for the cell to enter S-phase. S phase  During S phase a number of events additional to chromosome replication take place. Cell growth continues through S phase, as does the rate of synthesis of a number of proteins and enzymes that are involved in DNA synthesis.  Once DNA replication is complete the cell contains twice its normal number of chromosomes and becomes ready to enter the phase called G2 G2 phase  Similar to G1, G2 is an intermediate phase, a time for the cell to ensure that it is ready to proceed in the cell cycle.  Occurring between the end of DNA replication in S phase and the beginning of cell division in mitosis, G2 can be thought of as a safety gap during which a cell can check to make sure that the entirety of its DNA and other intracellular components have been properly duplicated.  In addition to acting as a checkpoint along the cell cycle, G2 also represents the cell's final chance to grow before it is split into two independent cells during mitosis. G0 Phase  Not all cells adhere to the classic cell cycle pattern in which a newly-formed daughter cell immediately enters the preparatory phases of interphase, closely followed by the mitotic phase.  Cells in G0 phase are not actively preparing to divide. The cell is in a quiescent (inactive) stage that occurs when cells exit the cell cycle.  Some cells enter G0 temporarily until an external signal triggers the onset of G1. Other cells that never or rarely divide, such as mature cardiac muscle and nerve cells, remain in G0 permanently. Checkpoints and Cell Cycle Regulation A checkpoint is a stage in the eukaryotic cell cycle at which the cell examines internal and external cues and "decides" whether or not to move forward with division.  The cell cycle is controlled at three checkpoints. The integrity of the DNA is assessed at the G1 checkpoint. Proper chromosome duplication is assessed at the G2 checkpoint. Attachment of each kinetochore to a spindle fiber is assessed at the M checkpoint. G1-checkpoint - Present just before the entry into S-phase, it makes the critical decision whether the cell will enter the S-phase At the G1 checkpoint, a cell checks whether internal and external conditions are right for division. Here are some of the factors a cell might assess: Size. Is the cell large enough to divide? Nutrients. Does the cell have enough energy reserves or available nutrients to divide? Molecular signals. Is the cell receiving positive cues (such as growth factors) from neighbors? DNA integrity. Is any of the DNA damaged? G2-checkpoint DNA integrity. Is any of the DNA damaged? DNA replication. Was the DNA completely copied during S phase? M checkpoint The M checkpoint is also known as the spindle checkpoint: here, the cell examines whether all the sister chromatids are correctly attached to the spindle microtubules. Because the separation of the sister chromatids during anaphase is an irreversible step, the cycle will not proceed until all the chromosomes are firmly attached to at least two spindle fibers from opposite poles of the cell. How does this checkpoint work? It seems that cells don't actually scan the metaphase plate to confirm that all of the chromosomes are there. Instead, they look for "straggler" chromosomes that are in the wrong place (e.g., floating around in the cytoplasm) If a chromosome is misplaced, the cell will pause mitosis, allowing time for the spindle to capture the stray chromosome. How do the checkpoints actually work? Internal and external cues trigger signaling pathways inside the cell that activate, or inactivate, a set of core proteins that move the cell cycle forward. Regulator molecules may act individually, or they can influence the activity or production of other regulatory proteins. Therefore, the failure of a single regulator may have almost no effect on the cell cycle, especially if more than one mechanism controls the same event. Conversely, the effect of a deficient or non-functioning regulator can be wide- ranging and possibly fatal to the cell if multiple processes are affected. Control of the cell cycle is necessary for a couple of reasons. First, if the cell cycle were not regulated, cells could constantly undergo cell division. While this may be beneficial to certain cells, on the whole constant reproduction without cause would be biologically wasteful. Second, internal regulation of the cell cycle is necessary to signal passage from one phase to the next at appropriate times. This regulation is not achieved through strict time constraints, but rather with feedback from the cell.  Positive Regulation - promote progress of the cell to the next phase Two groups of proteins, called cyclins and cyclin-dependent kinases (Cdks), are responsible for the progress of the cell through the various checkpoints. The levels of the four cyclin proteins fluctuate throughout the cell cycle in a predictable pattern. Increases in the concentration of cyclin proteins are triggered by both external and internal signals. After the cell moves to the next stage of the cell cycle, the cyclins that were active in the previous stage are degraded. DNA replication and mitosis are dependent on the activity of cyclin-dependent protein kinase (CDK) enzymes, which are heterodimers of a catalytic subunit with a cyclin subunit. Cyclin-Dependent Protein Kinase (Cdks)  A Cdks is an enzyme that adds negatively charged phosphate groups to other molecules in a process called phosphorylation. Through phosphorylation, Cdks signal the cell that it is ready to pass into the next stage of the cell cycle. As their name suggests, Cyclin-Dependent Protein Kinases are dependent on cyclins, another class of regulatory proteins. Cyclins bind to Cdks, activating the Cdks to phosphorylate other molecules. Cyclins  Cyclins are named such because they undergo a constant cycle of synthesis and degradation during cell division. When cyclins are synthesized, they act as an activating protein and bind to Cdks forming a cyclin-Cdk complex. This complex then acts as a signal to the cell to pass to the next cell cycle phase. Eventually, the cyclin degrades, deactivating the Cdk, thus signaling exit from a particular phase. There are two classes of cyclins: mitotic cyclins and G1 cyclins.  Cyclins activate cyclin dependent kinases (CDKs), which control cell cycle processes through phosphorylation  When a cyclin and CDK form a complex, the complex will bind to a target protein and modify it via phosphorylation  The phosphorylated target protein will trigger some specific event within the cell cycle (e.g. centrosome duplication, etc.)  After the event has occurred, the cyclin is degraded and the CDK is rendered inactive again Figure 1. Cyclins  Figure 2. Mechanism of Cyclins Negative Regulation - halt the cycle Retinoblastoma Protein (Rb), p53, and p21 Retinoblastoma proteins are a group of tumor-suppressor proteins common in many cells. The 53 and 21 designations refer to the functional molecular masses of the proteins (p) in kilodaltons. Much of what is known about cell cycle regulation comes from research conducted with cells that have lost regulatory control. All three of these regulatory proteins were discovered to be damaged or non-functional in cells that had begun to replicate uncontrollably (became cancerous). In each case, the main cause of the unchecked progress through the cell cycle was a faulty copy of the regulatory protein. Rb, p53, and p21 act primarily at the G1 checkpoint. p53 is a multi-functional protein that has a major impact on the commitment of a cell to division because it acts when there is damaged DNA in cells that are undergoing the preparatory processes during G1. If damaged DNA is detected, p53 halts the cell cycle and recruits enzymes to repair the DNA. If the DNA cannot be repaired, p53 can trigger apoptosis, or cell suicide, to prevent the duplication of damaged chromosomes. As p53 levels rise, the production of p21 is triggered. p21 enforces the halt in the cycle dictated by p53 by binding to and inhibiting the activity of the Cdk/cyclin complexes. As a cell is exposed to more stress, higher levels of p53 and p21 accumulate, making it less likely that the cell will move into the S phase. Cells must be able to deal with this damage, fixing it if possible and preventing cell division if not. Key to the DNA damage response is a protein called p53, a famous tumor suppressor often described as “the guardian of the genome.”  Figure 3. p53 halts the cell cycle Simplified diagram of how p53 halts the cell cycle at the G1/S checkpoint. p53 is activated by DNA damage and causes production of a Cdk inhibitor, which binds to the Cdk-G1/S cyclin complex and inactivates it. This halts the cell in G1 and prevents it from entering S phase, allowing time for the DNA damage to be fixed. By ensuring that cells don't divide when their DNA is damaged, p53 prevents mutations (changes in DNA) from being passed on to daughter cells. When p53 is defective or missing, mutations can accumulate quickly, potentially leading to cancer. As cells move through the cell cycle, do they breeze through from one phase to the next? If they're cancer cells, the answer might be yes. Normal cells, however, move through the cell cycle in a regulated way. They use information about their own internal state and cues from the environment around them to decide whether to proceed with cell division. This regulation makes sure that cells don't divide under unfavorable conditions (for instance, when their DNA is damaged, or when there isn't room for more cells in a tissue or organ). How long does the cell cycle take? Different cells take different lengths of time to complete the cell cycle. A typical human cell might take about 24 hours to divide, but fast-cycling mammalian cells, like the ones that line the intestine, can complete a cycle every 9-10 hours when they're grown in culture MITOSIS Mitosis is a fundamental process for life. During mitosis, a cell duplicates all of its contents, including its chromosomes, and splits to form two identical daughter cells. Because this process is so critical, the steps of mitosis are carefully controlled by a number of genes. When mitosis is not regulated correctly, health problems such as cancer can result. Mitosis is divided into the following four stages: Prophase  Chromosomal material condenses to form compact mitotic chromosomes. Chromosomes are seen to be composed of two chromatids attached together at the centromere.  Initiation of the assembly of mitotic spindle, the microtubules, the proteinaceous components of the cell cytoplasm help in the process. Metaphase  Spindle fibres attach to kinetochores of chromosomes.  Chromosomes are moved to spindle equator and get aligned along metaphase plate through spindle fibres to both poles Anaphase  Centromeres split and chromatids separate.  Chromatids move to opposite poles. Telophase  Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements.  Nuclear envelope assembles around the chromosome clusters.  Nucleolus, golgi complex and ER reform.  Cytokineses  Cytokinesis is the process by which the cell divides into two daughter cells.  In animal cells, a contractile ring of actin and myosin filaments forms around the cell, contracting and pinching the cell membrane until the cell is divided into two separate cells.  In plant cells, a structure called the cell plate forms along the equator of the cell, eventually dividing the cell into two separate daughter cells. MEIOSIS Meiosis reduces the number of chromosome sets from diploid to haploid Meiosis takes place in two sets of divisions – Meiosis I reduces the number of chromosomes from diploid to haploid – Meiosis II produces four haploid daughter cells Meiosis involves the same four phases seen in mitosis prophase metaphase anaphase telophase They are repeated during both meiosis I and meiosis II. The period of time between meiosis I and meiosis II is called interkinesis. No replication of DNA occurs during interkinesis because the DNA is already duplicated. MEIOSIS Prophase I Chromosomes begin to condense In synapsis, the 2 members of each homologous pair of chromosomes line up side-by-side, aligned gene by gene, to form a tetrad consisting of 4 chromatids During synapsis, sometimes there is an exchange of homologous parts between non- sister chromatids. This exchange is called crossing over Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred Metaphase I  At metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole  Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad  Microtubules from the other pole are attached to the kinetochore of the other chromosome Anaphase I In anaphase I, pairs of homologous chromosomes separate One chromosome moves toward each pole, guided by the spindle apparatus Sister chromatids remain attached at the centromere and move as one unit toward the pole Telophase I During telophase I, the chromosomes are enclosed in nuclei. The cell now undergoes a process called cytokinesis that divides the cytoplasm of the original cell into two daughter cells. Each daughter cell is haploid and has only one set of chromosomes, or half the total number of chromosomes of the original cell. Prophase II Meiosis II is very similar to mitosis In prophase II, a spindle apparatus forms In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate Metapase II At metaphase II, the sister chromatids are at the metaphase plate Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical The kinetochores of sister chromatids attach to microtubules extending from opposite poles Anaphase II At anaphase II, the sister chromatids separate The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles  Telophase II In telophase II, the chromosomes arrive at opposite poles Nuclei form, and the chromosomes begin decondensing Cytokinesis separates the cytoplasm At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes Each daughter cell is genetically distinct from the others and from the parent cell The advantage of meiotic division and sexual reproduction is that it promotes genetic variation in offspring The three main sources of genetic variation arising from sexual reproduction are: Crossing over (in prophase I) Crossing over involves the exchange of segments of DNA between homologous chromosomes during prophase I  The exchange of genetic material occurs between non-sister chromatids at points called chiasmata As a consequence of this recombination, all four chromatids that comprise the bivalent will be genetically different  Chromatids that consist of a combination of DNA derived from both homologous chromosomes are called recombinants  Offspring with recombinant chromosomes will have unique gene combinations that are not present in either parent Random assortment of chromosomes (in metaphase I) When homologous chromosomes line up in metaphase I, their orientation towards the opposing poles is random The orientation of each bivalent occurs independently, meaning different combinations of maternal / paternal chromosomes can be inherited when bivalents separate in anaphase I  The total number of combinations that can occur in gametes is 2 n – where n = haploid number of chromosomes  Humans have 46 chromosomes (n = 23) and thus can produce 8,388,608 different gametes (2 23) by random orientation  If crossing over also occurs, the number of different gamete combinations becomes immeasurable Random fusion of gametes from different parents The fusion of two haploid gametes results in the formation of a diploid zygote  This zygote can then divide by mitosis and differentiate to form a developing embryo As meiosis results in genetically distinct gametes, random fertilization by egg and sperm will always generate different zygotes  Identical twins are formed after fertilization, by the complete fission of the zygote into two separate cell masses