Most protein synthesis occurs in the cytoplasm, but DNA is too large to leave the nucleus. It is necessary, therefore, to make a small RNA copy that can migrate through a nuclear pore into the cytoplasm. Just as we might transcribe (copy) a document, transcription in genetics means the process of copying genetic instructions from DNA to RNA. It is triggered by chemical messengers from the cytoplasm that enter the nucleus and bind to the chromatin at the site of the relevant gene. An enzyme called RNA polymerase (po-LIM-ur-ase) then binds to the DNA at this point and begins making RNA. Certain base sequences (often TATATA or TATAAA) inform the polymerase where to begin.
RNA polymerase opens up the DNA helix about 17 base pairs at a time. It transcribes the bases from one strand of the DNA and makes a corresponding RNA. Where it finds a C on the DNA, it adds a G to the RNA; where it finds an A, it adds a U; and so forth. The enzyme then rewinds the DNA helix behind it. Another RNA poly-merase may follow closely behind the first one; thus, a gene may be transcribed by several polymerase molecules at once, and numerous copies of the same RNA are made. At the end of the gene is a base sequence that serves as a terminator, which signals the polymerase to release the RNA and separate from the DNA.
The RNA produced by transcription is an "immature" form called pre-mRNA. This molecule contains "sense" portions called exons that will be translated into a peptide and "nonsense" portions called introns that must be removed before translation. Enzymes remove the introns and splice the exons together into a functional mRNA molecule.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.