General Biology Cell Cycle PDF

Cell Cycle A cell spends most of its life in interphase, which has three phases: G1, S, and G2. In the G1 phase, the cell grows and takes in nutrients. In the S phase, the cell's DNA is replicated. Each replicated chromosome consists of two sister chromatids connected at the centromere. The G2 phas...

Cell Cycle A cell spends most of its life in interphase, which has three phases: G1, S, and G2. In the G1 phase, the cell grows and takes in nutrients. In the S phase, the cell's DNA is replicated. Each replicated chromosome consists of two sister chromatids connected at the centromere. The G2 phase is another growth phase, Have you ever watched a caterpillar turn into a butterfly? If so, you’re probably familiar with the idea of a life cycle. Butterflies go through some fairly spectacular life cycle transitions—turning from something that looks like a worm into a pupa, and finally into a glorious creature that floats on the breeze. Other organisms, from humans to plants to bacteria, also have a life cycle: a series of developmental steps that an individual goes through from the time it is born until the time it reproduces. The cell cycle can be thought of as the life cycle of a cell. In other words, it is the series of growth and development steps a cell undergoes between its “birth”— formation by the division of a mother cell —and reproduction—division to make two new daughter cells.  Stages of the Cell Cycle To divide, a cell must complete several important tasks: it must grow, copy its genetic material (DNA), and physically split into two daughter cells. Cells perform these tasks in an organized, predictable series of steps that make up the cell cycle. The cell cycle is a cycle, rather than a linear pathway, because at the end of each go-round, the two daughter cells can start the exact same process over again from the beginning.  Stages of the Cell Cycle In eukaryotic cells, or cells with a nucleus, the stages of the cell cycle are divided into two major phases: interphase and the mitotic (M) phase. During interphase, the cell grows and makes a copy of its DNA. During the mitotic (M) phase, the cell separates its DNA into two sets and divides its cytoplasm, forming two new cells.  Interphase Let’s enter the cell cycle just as a cell forms, by division of its mother cell. What must this newborn cell do next if it wants to go on and divide itself? Preparation for division happens in three steps: G1 Phase During G1 phase, also called the first gap phase, the cell grows physically larger, copies organelles, and makes the molecular building blocks it will need in later steps. Preparation for division happens in three steps: S Phase In S phase, the cell synthesizes a complete copy of the DNA in its nucleus. It also duplicates a microtubule-organizing structure called the centrosome. The centrosomes help separate DNA during M phase. Preparation for division happens in three steps: G2 Phase During the second gap phase, or G2 phase, the cell grows more, makes proteins and organelles, and begins to reorganize its contents in preparation for mitosis. G2 phase ends when mitosis begins.  M Phase During the mitotic (M) phase, the cell divides its copied DNA and cytoplasm to make two new cells. M phase involves two distinct division- related processes: mitosis and cytokinesis.  M Phase In mitosis, the nuclear DNA of the cell condenses into visible chromosomes and is pulled apart by the mitotic spindle, a specialized structure made out of microtubules. Mitosis takes place in four stages: prophase (sometimes divided into early prophase and prometaphase), metaphase, anaphase, and telophase.  M Phase In cytokinesis, the cytoplasm of the cell is split in two, making two new cells. Cytokinesis usually begins just as mitosis is ending, with a little overlap. Importantly, cytokinesis takes place differently in animal and plant cells. In animals, cell division occurs when a band of cytoskeletal fibers called the contractile ring contracts inward and pinches the cell in two, a process called contractile cytokinesis. The indentation produced as the ring contracts inward is called the cleavage furrow. Plant cells are much stiffer than animal cells; they’re surrounded by a rigid cell wall and have high internal pressure. Because of this, plant cells divide in two by building a new structure down the middle of the cell. This structure, known as the cell plate, is made up of plasma membrane and cell wall components delivered in vesicles, and it partitions the cell in two.  Cell cycle exit and G0 What happens to the two daughter cells produced in one round of the cell cycle? This depends on what type of cells they are. Some types of cells divide rapidly, and in these cases, the daughter cells may immediately undergo another round of cell division. For instance, many cell types in an early embryo divide rapidly, and so do cells in a tumor.  Cell cycle exit and G0 Other types of cells divide slowly or not at all. These cells may exit the G1 phase and enter a resting state called G0 phase. In G0, a cell is not actively preparing to divide, it’s just doing its job. For instance, it might conduct signals as a neuron or store carbohydrates as a liver cell. G0 is a permanent state for some cells, while others may re-start division if they get the right signals.  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 Key part of the cell cycle, involves a series of stages that facilitate cell division and genetic information transmission. Centrosomes and microtubules play pivotal roles in orchestrating this complex process, ensuring the successful replication of cells. What do your intestines, the yeast in bread dough, and a developing frog all have in common? Among other things, they all have cells that carry out mitosis, dividing to produce more cells that are genetically identical to themselves. Why do these every different organisms and tissues all need mitosis? Intestinal cells have to be replaced as they wear out; yeast cells need to reproduce to keep their population growing; and a tadpole must make new cells as it grows bigger and more complex. Mitosis is a type of cell division in which one cell (the mother) divides to produce two new cells (the daughters) that are genetically identical to itself. In the context of the cell cycle, mitosis is the part of the division process in which the DNA of the cell's nucleus is split into two equal sets of chromosomes. The great majority of the cell divisions that happen in your body involve mitosis. During development and growth, mitosis populates an organism’s body with cells, and throughout an organism’s life, it replaces old, worn-out cells with new ones. For single-celled eukaryotes like yeast, mitotic divisions are actually a form of reproduction, adding new individuals to the population. In all of these cases, the “goal” of mitosis is to make sure that each daughter cell gets a perfect, full set of chromosomes. Cells with too few or too many chromosomes usually don’t function well: they may not survive, or they may even cause cancer. So, when cells undergo mitosis, they don’t just divide their DNA at random and toss it into piles for the two daughter cells. Instead, they split up their duplicated chromosomes in a carefully organized series of steps.  Phases of Mitosis Mitosis consists of four basic phases: prophase, metaphase, anaphase, and telophase. These phases occur in strict sequential order, and cytokinesis - the process of dividing the cell contents to make two new cells - starts in anaphase or telophase. This cell is in interphase (late G2 phase) and has already copied its DNA, so the chromosomes in the nucleus each consist of two connected copies, called sister chromatids. This animal cell has also made a copy of its centrosome, an organelle that will play a key role in orchestrating mitosis. In early prophase, the cell starts to break down some structures and build others up, setting the stage for division of the chromosomes. The mitotic spindle begins to form. The spindle is a structure made of microtubules, strong fibers that are part of the cell’s “skeleton.” Its job is to organize the chromosomes and move them around during mitosis. The spindle grows between the In late prophase (sometimes also called prometaphase), the mitotic spindle begins to capture and organize the chromosomes. The chromosomes become even more condensed, so they are very compact. The nuclear envelope breaks down, releasing the chromosomes. The mitotic spindle grows more, and some of the microtubules start to “capture” chromosomes. Microtubules can bind to chromosomes at the kinetochore, a patch of protein found on the centromere of each sister chromatid. (Centromeres are the regions of DNA where the sister chromatids are most tightly connected.) Microtubules that don’t bind to kinetochores can grab on to microtubules from the opposite pole, stabilizing the spindle. More microtubules extend from each centrosome towards the edge of the cell, forming a structure called the aster. In metaphase, the spindle has captured all the chromosomes and lined them up at the middle of the cell, ready to divide. All the chromosomes align at the metaphase plate (not a physical structure, just a term for the plane where the chromosomes line up). At this stage, the two kinetochores of each chromosome should Before proceeding to anaphase, the cell will check to make sure that all the chromosomes are at the metaphase plate with their kinetochores correctly attached to microtubules. This is called the spindle checkpoint and helps ensure that the sister chromatids will split evenly between the two daughter cells when they separate in the next step. If a chromosome is not properly aligned or attached, the cell will halt division until the problem is fixed. In anaphase, the sister chromatids separate from each other and are pulled towards opposite ends of the cell. The protein “glue” that holds the sister chromatids together is broken down, allowing them to separate. Each is now its own chromosome. Microtubules not attached to chromosomes elongate and push apart, separating the poles and making the cell longer. In telophase, the cell is nearly done dividing, and it starts to re-establish its normal structures as cytokinesis (division of the cell contents) takes place. The mitotic spindle is broken down into its building blocks. Two new nuclei form, one for each set of chromosomes. Nuclear membranes and nucleoli reappear. The chromosomes begin to decondense and return to Cytokinesis, the division of the cytoplasm to form two new cells, overlaps with the final stages of mitosis. It may start in either anaphase or telophase, depending on the cell, and finishes shortly after telophase. When cytokinesis finishes, we end up with two new cells, each with a complete set of chromosomes identical to those of the mother cell. The daughter cells can now begin their own cellular “lives,” and – depending on what they decide to be when they grow up – may undergo mitosis themselves, repeating the cycle. Meiosis  Meiosis Process in which a single cell divides twice to form four haploid daughter cells. These cells are the gametes – sperms in males and egg in females. The process of meiosis is divided into 2 stages. Each stage is subdivided into several phases.  Meiosis Process in which a single cell divides twice to form four haploid daughter cells. These cells are the gametes – sperms in males and egg in females. The process of meiosis is divided into 2 stages. Each stage is subdivided into several phases. Meiosis I: Meiosis II: Prophase I Prophase II Metaphase I Metaphase II Anaphase I Anaphase II Telophase I Telophase II Cytokinesis I Cytokinesis II  Meiosis I Prophase I  The nuclear envelope disintegrates.  Chromosomes begin to condense.  Spindle fibres appear. Prometaphase II  Spindle fibres attach to the chromosomes at  Meiosis I Metaphase I  The homologous chromosomes align at the equatorial plate ensuring genetic diversity among  Meiosis I Anaphase I  The homologous chromosome s are pulled towards the opposite poles.  Meiosis I Telophase I  Spindle fibres disappear.  Nuclear envelope is reformed. Cytokinesis I  The cytoplasm and the cell division result in 2 non-identical haploid daughter cells.  Meiosis II Prophase II  The chromatin condenses into chromosomes.  Nuclear envelope disintegrates.  Centrosomes migrate to either poles.  Spindle fibres are reformed.  Meiosis II Metaphase II  The chromosomes align along the equatorial plate. On the contrary, the chromosomes in metaphase I were in  Meiosis II Anaphase II  Sister chromatids are pulled to the opposite poles.  Meiosis II Telophase II  Nuclear envelope redevelops and the spindle fibres disappear.  Meiosis II Cytokinesis II  The cytoplasm and cell divide producing 4 non-identical haploid  End product of mitosis is 2 daughter cells, whereas meiosis produces 4 daughter cells.  Mitosis forms diploid cells that have the same number of chromosomes as the parent, whereas meiosis forms haploid cells with half the original number of chromosomes.  Mitosis produces somatic cells (all cells except sex cells) while meiosis produces sex cells, ie. for example egg or sperm cells.  Mitosis includes one round of cell division, while meiosis contains two rounds of cell division.  THE MITOTIC PHASE Spindle microtubules that do not engage the chromosomes are called polar microtubules These microtubules overlap each other midway between the two poles and contribute to cell elongation. Astral microtubules are located near the poles, aid in spindle orientation, and are required for the regulation of mitosis.  MEIOSIS Sexual reproduction requires fertilization, the union of two cells from two individual organism. If those two cells each contain one set of chromosomes, then the resulting cell contains two sets of chromosomes. Haploid cells contain one set of chromosomes. Cells containing two sets of chromosomes are called diploid. Transport Movement A cell's plasma membrane is more than just a protective covering that defines the borders of the cell. Given that it is selectively permeable, it is capable of having certain materials pass through it, thus allowing them to enter or leave the cell. Sometimes, however, other materials cannot move as freely through it and may need a special structure first or  PASSIVE TRANSPORT  Most direct forms of membrane transport.  Naturally occurring phenomenon.  Does not require cell to exert any of its energy to accomplish the movement.  Substances move from an area of higher  Selective Permeability  Ability of a membrane to allow some substances to pass through while blocking others.  Maintain a cell's internal environment and regulate conditions like pH, osmotic pressure, and ion concentration.  In animal cells, lipid-soluble materials, hydrocarbons, and oxygen are examples of molecules the cell allows to pass through. Selective Permeability  Diffusion  Process of movement of molecules under a  concentration Substance tendgradient. to move from an area to another until the concentration is equal across the space.  Facilitated Transport  Molecules move from the region of higher concentration to the region of lower concentration  assisted by a carrier. In living systems, the lipid based membrane creates compartments which allow the transport of a selective concentration of water-soluble  substances. The ions, small molecules, proteins, and other solutes have different concentration across the membranes. Hydrophilic, polar or charged molecules cannot cross the membrane. Facilitated Diffusion  Factors Affecting Facilitated Diffusion Brownian motion is the force behind the diffusion of fluids. The main factors affecting the process of facilitated diffusion are:  Temperature- As the temperature increases, the movement of the molecules increases due to an increase in energy.  Concentration- The movement of the molecules takes place from the region of higher concentration to lower concentration.  Diffusion Distance- The diffusion rate is faster through smaller distance than through the larger distance. For eg., gas diffuses much faster through  Osmosis  Movement of water through a semipermeable membrane according to the concentration gradient of water across the membrane.  Most important ways to achieve homeostasis.  A regulated osmotic environment is essential in animal cells to avoid Osmosis  Tonicity  Describes how an extracellular solution can change the volume of cell by affecting osmosis.  Osmolarity describe the total solute concentration of the solution.  The lower the osmolarity, the greater number of water molecules; the Higher the osmolarity the fewer the water molecules respect to solute particles.  ACTIVE TRANSPORT  Requires use of the cell’s energy, in the form of ATP  If substance move against the concentration gradient, the cell must use energy to move.  PRIMARY ACTIVE TRANSPORT In this process of transportation, the energy is utilized by the breakdown of the ATP – Adenosine triphosphate to transport molecules across the membrane against a concentration gradient. Therefore, all the groups of ATP powered pumps contain one or more binding sites for the ATP molecules, which are present on the cytosolic face of the membrane. Basically, the primary  Process of Membrane Pumps  With the enzyme oriented toward the interior of the cell, the carrier has a high affinity for sodium ions. Three ions bind to the protein.  ATP is hydrolyzed by the protein carrier and a low- energy phosphate group attaches to it.  As a result, the carrier changes shape and reorients itself toward the exterior of the membrane.  Process of Membrane Pumps  The shape change increases the carrier's affinity for potassium ions, and two such ions attach to the protein. Subsequently, the low-energy phosphate group detaches from the carrier.  Process of Membrane Pumps  With the phosphate group removed and potassium ions attached, the carrier protein repositions itself toward the interior of the cell.  The carrier protein, in its new configuration, has a decreased affinity for potassium, and the two ions are released into the cytoplasm. The  SECONDARY ACTIVE TRANSPORT  This secondary process is also used to store high-energy hydrogen ions in the mitochondria of plant and animal cells for the production of ATP.  The potential energy that accumulates in the stored hydrogen ions is translated into kinetic energy as the ions surge through the channel protein ATP SECONDARY ACTIVE TRANSPORT  BULK TRANSPORT ENDOCYTOSIS Process by which cells ingest external fluid, macromolecules or other large particles. Plasma membrane of the cell invaginates, forming a pocket around the target particle. The pocket pinches off, resulting in the particle being contained in a newly created intracellular vesicle formed from the plasma  BULK TRANSPORT EXOCYTOSIS Process by which a substance is released from a cell through a vesicle that transports it to the cell surface and fuses with the cell Waste material is membrane. enveloped in a membrane and fuses with the interior of the plasma membrane. This fusion opens the membranous envelope on the exterior of the cell, and the waste RONEL T. AGUSTIN, LPT