Mitosis, Meiosis, and Cell Cycle PDF

Topic 1: Cell cycle Interphase is known to be the longest phase of the cell cycle because the cell is actively growing and performing its functions during this phase. During interphase, the cell absorbs nutrients, produces and utilizes proteins and other components, and initiates the cell division p...

Topic 1: Cell cycle Interphase is known to be the longest phase of the cell cycle because the cell is actively growing and performing its functions during this phase. During interphase, the cell absorbs nutrients, produces and utilizes proteins and other components, and initiates the cell division process by reproducing the DNA. Interphase has three major stages, which are Gap 1, Synthesis, and Gap 2. Interphase occurs in all cell types to prepare for cell division, which occurs at different stages of the cell cycle. Aside from the three major stages, Interphase has a special stage called the G0 Phase. The G0 phase is known to be a special stage in interphase because the cell may or may not enter the phase. This will be determined by the cell's intrinsic and extrinsic factors, which include resource availability and nutritional deprivation. Facts about Interphase and its stages: Interphase allows the cell to grow, replicate its DNA, and make final preparations for cell division. Interphase has three major stages (G1, S, and G2) and one special stage (G0). In the G1 stage, or First Gap phase, of the interphase, the cell does not duplicate. Instead, the cell grows in size. The S stage, or Synthesis phase, of the interphase takes up the responsibility of DNA and centrosome replication. In the G2 stage, or Second Gap phase, of the interphase, the cell grows larger and prepares the cell for mitosis.  The special stage of the interphase, also known as the G0 phase, acts as the resting phase. It is called a "special stage" since the cell may or may not enter this phase depending on the cell’s intrinsic and extrinsic factors. Cell division occurs during the M phase, which consists of nuclear division (mitosis) followed by cytoplasmic division (cytokinesis). Depending on the type of cell, the M-phase, or dividing phase, can be either mitosis or meiosis. Mitosis helps in the healing and repairing of cells and tissues and in the growth and development of the body. It occurs in somatic or body cells (skin cells and nerve cells) and in all organisms except viruses. Mitosis involves only one cell division, produces two daughter cells that are genetically similar, and has four stages of cell division: prophase, metaphase, anaphase, and telophase. Meiosis helps produce gametes and ensures genetic diversity. It occurs in reproductive cells (sperm and ovum) and in plants, animals, and fungi. Meiosis involves two successive cell divisions (meiosis I and meiosis II), produces four daughter cells that are genetically different, and has eight stages of cell division (2 rounds): meiosis I (prophase I, metaphase I, anaphase I, and telophase I); meiosis II (prophase II, metaphase II, anaphase II, and telophase II). Topic 1.2 PROPHASE It is the longest phase in mitotic division. It is characterized by the condensation of DNA and proteins contained in the nucleus, known as chromatin, which marks the beginning of mitosis. The main events that occur during prophase are the condensation of chromosomes, the movement of the centrosomes, the formation of the mitotic spindle, and the beginning of nucleoli breaking down. MITOSIS A process by which a cell replicates its chromosomes and then segregates them, producing two identical nuclei in preparation for cell division. MEIOSIS A process where a single cell divides twice to produce four cells containing half the original amount of genetic information. These cells are our sex cells – sperm in males, eggs in females. CELL DIVISION This happens when a parent cell divides into two or more cells called daughter cells. It usually occurs as part of a larger cell cycle. SUBSTAGES IN PROPHASE: Early stage Condensed chromosomes consist of two identical sister chromatids that join at a point called the centromere. Several binding proteins, which include cohesin and condesin, aid the connection formed in the centromere. Cohesin creates rings that hold the sister chromatids together, while condesin creates rings that entwine the chromosomes into highly condensed forms. These are the x-shaped bodies seen under the microscope. Mid stage At this stage, the nucleolus disappears. In the cytoplasm, microtubules that form the mitotic spindle begin to develop. The two spreads onto the opposite side of the cell due to the forces created by the microtubules themselves. Late stage (prometaphase) The nucleoli start to break down, which stops the production of ribosomes. This allows the cell to redirect its energy expenditure from metabolism to cell division. Spindle fibers or mitotic spindle stretches out and reaches the kinetochore of the chromosomes within the cell. Three different types of microtubules fibers that form the mitotic spindle: Kinetochore microtubules Interpolar microtubules Astral microtubules Topic 1.3 METAPHASE The stage in eukaryotic cell division in which the chromosomes align on the metaphase plate in the middle of the cell. The stages of prophase and prometaphase come before metaphase. In those stages of cell division, the chromosomes are condensed, the spindle fibers form, and the nuclear envelope is broken down. During metaphase and late prometaphase, the cell performs a series of checkpoints to ensure that the spindle has formed. Individual chromosomes are typically dispersed throughout the cell nucleus. The chromosomes of the cell condense and move toward one another, aligning in the center of the dividing cell, and the nucleus of the cell disintegrates during metaphase. The chromosomes may now be identified when viewed under a microscope. In karyotyping, a laboratory method for detecting chromosomal abnormalities, metaphase chromosomes are utilized. This phase is referred to the process wherein the duplicated genetic material that is in the parent cells’ nucleus splits into 2 daughter cells that are identical in nature. The chromosomes are aligned at the center of the cell during metaphase by pulling and pushing, a process known as a cellular tug of war. BEFORE METAPHASE: In 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. SPINDLE ANATOMY: 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 bind a chromosome are called kinetochore microtubules. AT METAPHASE: 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). 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. Difference between metaphase of mitosis and metaphase 1 of meiosis: Topic 1.4 ANAPHASE Cell division is essential to life; without it, no species would be able to reproduce and life would cease to exist. Cell division permits new cells to be formed while simultaneously enabling old ones to die, allowing our bodies to grow and evolve. There are a total of four generally recognized stages of cell division, one of which is anaphase. Anaphase is the fourth stage of mitosis, and it is the process by which the duplicated genetic material held in the nucleus of a parent cell is divided into two identical daughter cells. It is the process by which chromosomes are split and moved to opposite poles of the cell. Etymologically, anaphase is derived from two Greek words, "ana" meaning "backward" and "phasis" meaning "appearance." Although we have not personally observed the structures of anaphase using the microscope, the mitotic anaphase comprises four major structures: 1) The centrosome, which creates interpolar and astral microtubules around the chromosomes that aid in the movement of the chromatids; 2) The chiasma, which is the chromosomes that are in an X shape and aligned at the equatorial plane; 3) Centromeres, the specific DNA strings that allow them to connect to the spindle fiber; and 4) Motor proteins, which transport chromatids and other proteins through the cell division phase. Moving to its process, before anaphase begins, the duplicated chromosomes or known as sister chromatids, are oriented on the equatorial plane along the cell's equator. The sister chromatids are two identical copies of DNA that are linked at the centromere. Anaphase starts once the cell has passed the spindle formation checkpoint, which allows chromosomes or chromatids to separate and keeps all chromosomes linked to microtubules and positioned on the metaphase plate. During anaphase, each pair of chromosomes is divided into two identical, independent chromosomes. The chromosomes are then divided by a structure known as the mitotic spindle – which is composed of multiple long proteins known as microtubules that are connected to a chromosome at one end and to the cell pole at the other. The sister chromatids are split at their centromeres at the same time. The spindle then pulls the divided chromosomes to opposite ends of the cell. Anaphase guarantees that each daughter cell obtains the same set of chromosomes, and it is preceded by telophase, the fifth and final phase of mitosis. Additionally, anaphase occurs in both mitosis and meiosis. During anaphase in mitosis, the centromere separates, enabling the sister chromatids to divide. The kinetochore spindle fibers shrink, allowing the 46 newly freed chromatids to be pulled to one end of the cell and the remaining 46 chromatids, which are now referred to as daughter chromosomes, to be dragged to the other end. At the end of anaphase in mitosis, each end of the cell has an identical and full set of 46 chromosomes, or 23 pairs of homologous chromosomes. On the other hand, the anaphase in meiosis has two phases: anaphase 1 and anaphase 2. The main distinction between anaphase 1 and anaphase 2 is the part that moves during the phase. The sister chromatids split in anaphase I to allow mobility. In anaphase 2, these chromatids migrate along the microtubules towards the spindle poles. Topic 1.5 TELOPHASE The fifth and final stage of mitosis is known as telophase. The cell is almost finished dividing and it begins to rebuild its usual structure. Telophase occurs when a parent cell's replicated genetic material is divided into two identical daughter cells. Once the duplicated, paired chromosomes have been pulled to the cell's poles and split, the process known as telophase has begun. TELOPHASE IN ANIMAL CELLS I. During this stage, chromosomes will uncoil or straighten towards the cell poles and will form chromatin, after which they will be enclosed by nuclei. II. The cell will now undergo cytokinesis that will divide the cytoplasm of the original cell into two daughter cells. III. In animal cells, cytokinesis involves the formation of a cleavage furrow which pinches the cell into two. In the process of cleavage furrow, the microfilament ring will contract and contract up until the point of constriction making the cell in half. IV. As a result, two daughter cells are formed through the action of cytokinesis. TELOPHASE IN PLANT CELLS I. Like the process in animal cells, the chromosomes during telophase in a plant cell will also uncoil or straighten towards the cell poles and will form chromatin, after which it will be surrounded by nuclei. II. Cytokinesis begins when vesicles containing cell wall materials formed in the middle and will eventually be the cell plate. III. The cell plate will go outwards until the membrane fuses with the plasma membrane separating the two daughter cells. And then a new cell wall was made.