One gene codes for one polypeptide chain. Each gene is a stretch of DNA that is several thousand nucleotide pairs long. The DNA in a human cell contains over 3 billion base pairs— enough to code for at least 3 million proteins. Since the average human cell contains less than this amount (30,000 to 150,000 different proteins), it follows that only a fraction of the DNA in each cell is used to code for proteins. The remainder of the DNA may be inactive or redundant. Also, some segments of DNA serve to regulate those regions that do code for proteins.
In order for the genetic code to be translated into the synthesis of specific proteins, the DNA code first must be copied onto a strand of RNA. This is accomplished by DNA-directed RNA synthesis—the process of genetic transcription.
In RNA synthesis, the enzyme RNA polymerase breaks the weak hydrogen bonds between paired DNA bases. This does not occur throughout the length of DNA, but only in the regions that are to be transcribed. There are base sequences that code for "start" and "stop," and there are regions of DNA that function as promoters. Specific regulatory molecules, such as hormones, act as transcription factors by binding to the promoter region of a particular gene and thereby activating the gene. The double-stranded DNA separates in the region to be transcribed, so that the freed bases can pair with the complementary RNA nu-cleotide bases in the nucleoplasm.
This pairing of bases, like that which occurs in DNA replication (described in a later section), follows the law of complementary base pairing: guanine bonds with cytosine (and vice versa), and adenine bonds with uracil (because uracil in RNA is
Chapter Three equivalent to thymine in DNA). Unlike DNA replication, however, only one of the two freed strands of DNA serves as a guide for RNA synthesis (fig. 3.18). Once an RNA molecule has been produced, it detaches from the DNA strand on which it was formed. This process can continue indefinitely, producing many thousands of RNA copies of the DNA strand that is being transcribed. When the gene is no longer to be transcribed, the separated DNA strands can then go back together again.
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