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The study of the cell cycle focuses on mechanisms that regulate the timing and frequency of DNA duplication and cell division. As a biological concept, the cell cycle is defined as the period between successive divisions of a cell. During this period, the contents of the cell must be accurately replicated. Microscopists had known about cell division for more than one hundred years, but not until the 1950s, through the pioneering work of Alma Howard and Stephen Pelc, did they become aware that DNA replication took place only at a specific phase of the cell cycle and that this phase was clearly separated from mitosis. Howard and Pelc's work in the broad bean, Vicia faba, revealed that the cell goes through many discrete phases before and after cell division. From this understanding, scientists then identified the four characteristic phases of the cell cycle: mitosis (M), gap 1 (G1), DNA synthesis (S), and gap 2 (G2). The study of these phases, the proteins that regulate them, and the complex biochemical interactions that stop or start DNA replication and cell division (cytokinesis) are the primary concerns of cell cycle biologists.
The most significant progress in this research field came with the demonstration that specific protein complexes involving cyclins were critical for regulating the passage of cells through the cell cycle. These early observations came from biological studies of the cells of rapidly dividing fertilized frog eggs as well as mutant yeast cells that could not divide. The observations suggested that regulation of the cell cycle is conserved throughout eukaryotes, which has since proved to be the case. The mechanism of division in bacteria differs from that of eukaryotes, and the control of their cell cycle is also somewhat different, although again it is linked with DNA replication.
Although the cell cycle is a highly integrated process, distinct areas of interest within this field of study have emerged. For instance, many genes and proteins that influence the passage from one phase of the cell cycle to another have been identified. When their expression is altered by mutation or aberrant regulation, they are usually classed as oncogenes. Other proteins act to hold the cell at distinct points in the cycle (checkpoints) and are known as tumor suppressor genes. Apart from those with a clearly regulatory role, many proteins have important functions in other aspects of the cell cycle; one is replication of DNA and organelles, which is a fascinating process that includes its own repair mechanisms and self-editing. Other fields focus primarily on the mechanical processes of cell cleavage into two daughter cells at the end of mitosis and on the condensation and decondensation of chromatin.
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<span>Which word best completes the sentence? Pack some snacks for Connie and __him___ to eat later.
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The rate or speed at which the play’s action moves is known as pace. The term pace, or also called rhythm or tempo, denotes the movement or sweep of the play as it progresses.The tempo of an entire theatrical performance is very important.
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A. The message that the author wants to convey.
Explanation:
A theme of a story is what the author wants to convey.
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ovel It by Stephen King and answer the question that follows.Richie had felt a mad, exhilarating kind of energy growing in the room. . . . He thought he recognized the feeling from his childhood, when he felt it everyday and had come to take it merely as a matter of course. He supposed that, if he had ever thought about that deep-running aquifer of energy as a kid (he could not recall that he ever had), he would have simply dismissed it as a fact of life, something that would always be there, like the color of his eyes . . . .Well, that hadn't turned out to be true. The energy you drew on so extravagantly when you were a kid, the energy you thought would never exhaust itself—that slipped away somewhere between eighteen and twenty-four, to be replaced by something much duller . . . purpose, maybe, or goals . . . .Source: King, Stephen. It. New York: Penguin, 1987. Print.Which theme would be advanced by the tone in the above passage best?A. Despite age and experience, some people never grow up.B. Childhood has a magical quality that slips away.C. Don't take childhood for granted.D. Children should be given the chance to expand their vast energy.
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