Second, the small number of DNA molecules in the cell is often insufficient. Creating many identical RNA molecules that are copies of a single segment of DNA provides the necessary numbers. Third, RNA can be differentially degraded when it is no longer needed, providing an important regulatory mechanism that would be unavailable if there were only one type of nucleic acid.
Promoters
Transcription is initiated at regions of DNA called promoters, which are typically 20 to 150 base pairs long, depending on the organism. The sequence of bases at a promoter is recognized by RNA polymerase, the enzyme that synthesizes RNA.
The RNA polymerases in bacteria, as well as in viruses in bacteria, are able to recognize particular promoter sequences without the help of any other cellular proteins. However, in eukaryotes and Archaea, other proteins, called initiation factors, recognize the promoter sequence, "recruit" RNA polymerase and other proteins, help the RNA polymerase bind to the DNA, and regulate the enzyme's activity.
RNA polymerase is assembled on promoters in a particular orientation (Figure 1A). This allows RNA synthesis to start at a precise location and proceed in only one direction, "downstream" toward the gene (Figure 1B).
This is a free page. This page contains 191 words. This
article contains 1,901 words (approx. 6 pages at 300
words per page).
Read the rest of this Article with our Transcription Access Pass.
