It is an equational division. Occurs in somatic cells. P rophase, M etaphase, An aphase, T elophase Mnemonic: “ P eople M e et A nd T alk” Prophase: Coiling of chromatin occurs, forming thin long threads. By the end, chromosomes start forming, Nucleolus & nuclear membrane starts disappearing by the end. Spindle fiber formation starts.
And growthIn unicellular organisms, cell division is the means of in multicellular organisms, it is the means of and maintenance. Survival of the depends upon interactions between many cell types, and it is essential that a balanced distribution of types be maintained. This is achieved by the highly regulated process of cell proliferation.
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The growth and division of different cell populations are regulated in different ways, but the basic mechanisms are similar throughout multicellular organisms. Of the materialBefore a cell can divide, it must accurately and completely duplicate the genetic information encoded in its in order for its progeny cells to function and survive. This is a complex problem because of the great length of DNA molecules.
Each consists of a long double spiral, or helix, each strand of which consists of more than 100 million ( see above ).The duplication of DNA is called DNA replication, and it is initiated by complex called DNA polymerases. These progress along the, reading the sequences of nucleotides that are linked together to make DNA chains.
Each strand of the DNA double helix, therefore, acts as a template specifying the nucleotide structure of a new growing chain. After replication, each of the two daughter DNA double helices consists of one parental DNA strand wound around one newly synthesized DNA strand. In order for DNA to replicate, the two strands must be unwound from each other. Enzymes called helicases unwind the two DNA strands, and additional bind to the separated strands to stabilize them and prevent them from pairing again. In addition, a remarkable class of called DNA topoisomerase removes the helical twists by cutting either one or both strands and then resealing the cut.
These enzymes can also untangle and unknot DNA when it is tightly coiled into a chromatin fibre.In the circular DNA of, replication starts at a unique site called the origin of replication and then proceeds in both directions around the molecule until the two processes meet, producing two daughter molecules. In rapidly growing prokaryotes, a second round of replication can start before the first has finished. The situation in eukaryotes is more complicated, as replication moves more slowly than in prokaryotes. At 500 to 5,000 nucleotides per minute (versus 100,000 nucleotides per minute in prokaryotes), it would take a human chromosome about a month to replicate if started at a single site. Actually, replication begins at many sites on the long chromosomes of,.
Distances between initiation sites are not always the same; for example, they are closer in the rapidly dividing embryonic cells of frogs or flies than in adult cells of the same species.Accurate DNA replication is crucial to ensure that daughter cells have exact copies of the genetic information for synthesizing proteins. Accuracy is achieved by a “proofreading” ability of the DNA polymerase itself. It can erase its own errors and then synthesize anew.
There are also repair systems that correct genetic damage to DNA. For example, the incorporation of an incorrect nucleotide, or damage caused by, can be corrected by cutting out a section of the daughter strand and recopying the parental strand. Cell division Mitosis and cytokinesis.
The Cell Cycle & Mitosis Tutorial Tutorial The Cell Cycle & Mitosis Tutorial Mitosis What is (and is not) mitosis?Mitosis is nuclear division plus cytokinesis, and produces two identicaldaughter cells during prophase, prometaphase, metaphase, anaphase, andtelophase. Interphase is often included in discussions of mitosis, but interphase is technically not part of mitosis, but rather encompasses stages G1, S, and G2 of the cell cycle.Interphase & mitosis InterphaseThe cell is engaged in metabolic activity and performing its prepare for mitosis (the next four phases that lead up to and include nuclear division). Chromosomes are not clearly discerned in the nucleus, although a dark spot called the nucleolus may be visible. The cell may contain a pair of centrioles (or microtubule organizing centers in plants) both of which are organizational sites for microtubules.ProphaseChromatin in the nucleus begins to condense and becomes visible in the lightmicroscope as chromosomes. The nucleolus disappears. Centrioles begin movingto opposite ends of the cell and fibers extend from the centromeres.
Some fiberscross the cell to form the mitotic spindle.PrometaphaseThe nuclear membrane dissolves, marking the beginning of prometaphase. Proteinsattach to the centromeres creating the kinetochores. Microtubules attachat the kinetochores and the chromosomes begin moving.MetaphaseSpindle fibers align the chromosomes along the middle of the cell nucleus. Thisline is referred to as the metaphase plate.
This organization helps to ensurethat in the next phase, when the chromosomes are separated, each new nucleuswill receive one copy of each chromosome.AnaphaseThe paired chromosomes separate at the kinetochores and move to opposite sidesof the cell. Motion results from a combination of kinetochore movement alongthe spindle microtubules and through the physical interaction of polar microtubules.TelophaseChromatids arrive at opposite poles of cell, and new membranes form around thedaughter nuclei. The chromosomes disperse and are no longer visible underthe light microscope. The spindle fibers disperse, and cytokinesis or thepartitioning of the cell may also begin during this stage.CytokinesisIn animal cells, cytokinesis results when a fiber ring composed of a proteincalled actin around the center of the cell contracts pinching the cell intotwo daughter cells, each with one nucleus. In plant cells, the rigid wallrequires that a cell plate be synthesized between the two daughter cells.(480 k)ORFlash Tutorial.