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The cell cycle consists of interphase and cell division (mitosis or meiosis). During interphase, in which a cell typically spends most of its life, a cell duplicates its DNA, synthesises proteins, increases the volume of its cytoplasm (resulting in the cell growing to about double the original size) and its mitochondria and plastids divide. Interphase consists of three stages: G1, S and G2. During the S stage, a cell duplicates its genetic material via semiconservative replication – a cell forms DNA molecules out of one by breaking hydrogen bonds between DNA strands in the original DNA molecule (one DNA molecule consists of two antiparallel strands of DNA – this means that the strands are bonded so that on one end of a DNA molecule there are 3’ (hydroxyl) end of one strand and 5’ (phosphoryl) end of the other and vice versa), then synthesising new strands along the existing two, so that the result of the syntesis are two molecules of DNA, each of which contains one of the original strands of DNA and one of the new (syntesised). All the other activities characteristic for the interphase of a cell are performed during the G (gap) stages. Certain cells enter a stage called G0 (gap zero), which is either a stage separate from interphase or an extended G1. Such cells don’t divide often or ever and are usually terminally specialised (quiescent, but continue to perform their main function – such as neurons and heart muscle cells) or “await” a stimulus to...
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