Cell Cycle: Interphase

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Chapter: Anatomy and Physiology for Health Professionals: Levels of Organization : Cells

The cell obtains nutrients to grow andduplicate. This is actually the period from cell for mation to cell division. This step may be betterunderstood as being a metabolic or growth phase.

Cell Cycle: Interphase

Cell Cycle

The life cycle of each cell is regulated via stimulation from hormones or growth factors. Disruption of the cycle can affect the health of the body. Most human cells divide from 40 to 60 times before they die. The life cycle of a cell includes the following steps:

Interphase: The cell obtains nutrients to grow andduplicate. This is actually the period from cell for mation to cell division. This step may be betterunderstood as being a metabolic or growth phase.

Cell division (mitosis): The nucleus divides.

Cytoplasmic division (cytokinesis): The cytoplasmdivides.

Differentiation: The cell becomes specialized.


Interphase

A cell must grow and duplicate most of its contents before it can actively divide (FIGURE 3-17). Interphase describes this period of the cell preparing to divide. During interphase, the cell manufactures new living material, duplicating membranes, lysosomes, mito-chondria, and ribosomes. It also replicates its own genetic material. Interphase is divided into an initial growth phase (G1 phase), a synthesis phase (S phase), and a final growth phase (G2 phase). Chromatin is produced only during the S phase.


In the G1 phase, the cell is metabolically active, with rapid protein synthesis and growth. This phase may vary widely in its length. Cells that divide rap-idly may do so in minutes to hours, but cells that divide slowly may last for days to years. Any cell that permanently stops dividing is described as being in the G0 phase. Nearly no cell division activities occur during the G1 phase. As this phase ends, the centrioles begin replication, preparing for cell division.

In the S phase, DNA is replicated, so identical copies of genetic material are received by the two future cells. Chromatin is assembled by the making of new histones (basic alkaline proteins). There can be no correct mitotic phase without the S phase also being correctly­ accomplished. The process of DNA replication involves enzymes attaching to origins or ­replication. The DNA strands separate, with replica-tion bubbles forming. A Y-shaped replication fork onthe end of each bubble is where helical parental DNA strands are unwound. These strands help to make com-plementary DNA strands. However, new nucleotides can only be added to preexisting strands and cannot be manufactured newly. Therefore, a short RNA primer is formed by a primase enzyme. The enzyme DNA poly-merasealigns complementary nucleotides along thestrand and links them together using covalent bonds only in one direction. The leading strand is synthe-sized continuously once it has been primed, according to the movement of the replication fork. Another lag-ging strand is made in segments in the opposite direc-tion. It requires that a primer replicates each segment. Short segments of DNA are spliced together by ligaseenzymes. The primers are then replaced by DNA poly-merases with DNA nucleotides.

In the very brief G2 phase, cell division-­influencing enzymes and other proteins are synthesized and moved to their required sites. Centriole replication finishes and the cell is now ready to divide. The cell has continued to grow through this phase as well as the S phase and also has been functioning normally.

Ultimately, two DNA molecules are formed from the original template strands (DNA helix), which are identical to their parents. Each new mol-ecule has one old and one new nucleotide strand, a process known as semiconservative replication. Histones then work with the DNA to make two new chromatin strands that are joined by a centromere. Via a protein complex called cohesin, they remain together until the anaphase stage of mitotic cell division. Distribution then occurs to the daughter cells, with identical genetic information. If any DNAdamage­ has occurred during the entire process, the cell cycle often stops until the DNA is repaired. How-ever, the cell cycle may also cause a cellular change to occur or even develop into cancer.

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