DNA molecules as a rule are very large, containing the specifications for
thousands of proteins. Segments of the entire DNA sequence are therefore
transcribed into separate mRNA molecules, with each segment coding for a
different protein. A gene is defined as the segment of DNA sequence
corresponding to a single protein (or to a single catalytic or structural RNA
molecule for those genes that produce RNA but not protein).
In all cells, the
manufacturing its full repertoire of possible proteins at full tilt all the time, the
cell adjusts the rate of transcription and translation of different genes
independently, according to need. Stretches of
interspersed among the segments that code for protein, and these noncoding
regions bind to special protein molecules that control the local rate of
transcription (Figure 1-11). Other noncoding DNA is also present, some of it
serving, for example, as punctuation, defining where the information for an
individual protein begins and ends. The quantity and organization of the
regulatory and other noncoding DNA vary widely from one class of organisms
to another, but the basic strategy is universal. In this way, the genome of the
cell that is, the total of its genetic information as embodied in its complete
DNA sequence dictates not only the nature of the cell's proteins, but also
when and where they are to be made.
EmoticonEmoticon