The cell cycle is a process in which a cell grows and divides to make a copy of itself. Prokaryotes reproduce through binary fission, while eukaryotes have a more complex cycle with interphase and mitosis. The cycle is regulated by proteins, and errors can occur, causing cancer. Proteins can halt the process if there are errors in DNA replication, but cancer can occur if these proteins don’t work properly.
The cell cycle is a process in which a cell grows and divides to make a copy of itself. Some organisms reproduce solely through this process, whereas in complex multicellular life, it allows an organism to grow and replace cells as they wear out. In animals, the cycle takes about 24 hours from start to finish for most cell types, although some, such as those in the skin, go through this cycle constantly, while others may divide infrequently, if at all. Neurons, for example, do not grow or divide once they mature.
Organisms can be divided into two types, each with its own cell cycle. In prokaryotes, the genetic information of the organism consists of a single strand of DNA, known as a chromosome, which is not contained in any special structure. Bacteria are examples of prokaryotes, along with some other single-celled organisms. Eukaryotes consist of all multicellular organisms, such as plants and animals, along with some types of unicellular life forms. They differ from prokaryotes primarily in that they have a nucleus, a self-contained structure that contains genetic material in the form of chromosomes made up of DNA.
Prokaryotes
Reproduction in prokaryotes is known as binary fission. In this process, the DNA, which is floating freely within the cell, replicates itself. The two new DNA strands then migrate to opposite ends of the cell, which divides into two identical copies, each with its own DNA; this division step is known as cytokinesis. The new cells will then grow to a certain size, then, if conditions are right, they will divide again, starting a new cycle. For these primitive organisms, the process is usually very fast: under ideal conditions, some bacteria divide every 20 minutes, leading to very rapid multiplication.
Eukaryotes
Eukaryotic cells are more complex than those of prokaryotes, making division more complicated. In addition to a nucleus, they have a number of other structures, known as organelles, which have specific functions and which must be duplicated during division. The eukaryotic cycle has a number of distinct phases, the main two being interphase and mitosis.
During interphase, which is by far the longest phase, accounting for most of the 24-hour cycle typical of most animal cells, the cell is normally growing and increasing in size. The processes involved in division require energy, and interphase has two periods during which reserves of the energy-storing compound adenosine triphosphate (ATP) build up and growth occurs, separated by a period of DNA duplication in the nucleus. The first period of growth is known as Gap 1 (G1) and occurs in new cells after division. This is followed by the synthesis phase (S), during which new DNA strands identical to the originals are synthesized. Then begins the Gap 2 (G2) stage, before mitosis.
In contrast to the previous stage, mitosis is relatively short, typically taking only an hour. This is the process by which two identical sets of DNA are physically separated from each other, forming two sets of chromosomes, which are then drawn to different ends of the cell to form separate nuclei. Organelles, such as mitochondria in animals and chloroplasts in plants, also divide during this stage. These structures have their own DNA, which replicates in the same way as prokaryotes, leading some scientists to believe that they may once have been independent prokaryotic organisms incorporated into eukaryotic cells in the distant past.
The final stage is cytokinesis. This is when the split into two distinct units actually takes place. In animals, opposing cell walls are attracted towards each other around the midpoint until they meet, forming two units that separate from each other. In plants, a new cell wall is built at the point of division, separating the two new cells.
Regulation and errors
Each part of the cycle is regulated by proteins that tell the cell what to do. These proteins are also used during interphase to confirm that conditions are appropriate for division. If not enough nutrients are present or other problems are identified, these proteins will signal the cell to lie dormant and wait for conditions to improve, rather like a guard going through division.
Errors can and do occur during the process. Sometimes information is not copied exactly during interphase and errors are created in the genome. These errors can prove fatal to the cell or they can be harmless. They can also cause cancer, where a mistake causes repeated replication and uncontrolled division, without controls, forming a tumor.
Fortunately, there are proteins that can halt the process if there are errors in DNA replication. In some cases, the splitting process will be suspended to allow the DNA to repair, after which it may resume. In others, where the DNA is severely damaged, these proteins can cause the cell to die, in order to prevent further replication of the defective DNA. Cancer is often the result of DNA modifications that prevent these proteins from working properly, so that cells with damaged DNA can reproduce.
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