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The cell cycle is a series of complex, ordered, sequential events that control how a single cell divides into two cells, and involves several different phases. The phases include the G1 and G2 phases, DNA replication or S phase, and the actual process of cell division, mitosis or M phase. [1]
Mitosis in an animal cell (phases ordered counter-clockwise), with G 1 labeled at left. The G 1 phase, gap 1 phase, or growth 1 phase, is the first of four phases of the cell cycle that takes place in eukaryotic cell division. In this part of interphase, the cell synthesizes mRNA and proteins in preparation for subsequent steps leading to mitosis.
However, replication does not start at all the different origins at once. Rather, there is a defined temporal order in which these origins fire. Frequently a few adjacent origins open up to duplicate a segment of a chromosome, followed some time later by another group of origins opening up in an adjacent segment.
The different stages of mitosis altogether define the mitotic phase (M phase) of a cell cycle—the division of the mother cell into two daughter cells genetically identical to each other. [3] The process of mitosis is divided into stages corresponding to the completion of one set of activities and the start of the next.
The eukaryotic cell cycle consists of four distinct phases: G 1 phase, S phase (synthesis), G 2 phase (collectively known as interphase) and M phase (mitosis and cytokinesis). M phase is itself composed of two tightly coupled processes: mitosis, in which the cell's nucleus divides, and cytokinesis, in which the cell's cytoplasm and cell membrane divides forming two daughter cells.
Three types of cell division: binary fission (taking place in prokaryotes), mitosis and meiosis (taking place in eukaryotes).. When cells are ready to divide, because cell size is big enough or because they receive the appropriate stimulus, [20] they activate the mechanism to enter into the cell cycle, and they duplicate most organelles during S (synthesis) phase, including their centrosome.
The hyper phosphorylation of Rb is considered the late G1 restriction point, after which the cell cannot go backwards in the cell cycle. At this point, E2F 1-3 proteins bind to DNA and transcribe Cyclin A and Cdc 6. [11] Cyclin-dependent kinase inhibitor 1B (CDKN1B), also known as p27, binds to and prevents the activation of CyclinE:Cdk2 by ...
However, while both clades are capable of performing thermogenesis, the biological processes involved are different. The reason that avians and eutherians both developed the capacity to perform thermogenesis is a subject of ongoing study by evolutionary biologists , and two competing explanations have been proposed to explain why this character ...