2.4 Making an RNA copy of DNA
You may already know that it is RNA that carries the genetic code to ribosomes with instructions for protein synthesis. Our master DNA tape stays safely away from the action and sends messenger RNA out with the code.
One strand of DNA is copied into RNA using RNA polymerase. In order to copy the genetic code from DNA molecules, we need to copy one of the strands of the DNA into RNA using RNA polymerase and the same base paring principles that we used for DNA synthesis. Again we have the problem of gaining access to a tightly wound coil of DNA. The DNA must be unwound slightly to allow the enzyme to bind and begin copying the sequence.
Control regions are found upstream of the coding region of the gene. The binding only occurs at a region of the gene called the promoter region. This base sequence occurs before the 5' end of the gene (upstream) along with many other sequences that serve to control reading of the gene. Synthesis proceeds with the required base paring (remember adenine now pairs with uracil) using the ribonucleotide triphosphates (containing the sugar ribose) as substrates. The next figure shows a more complete model of the polymerization with an indication of how the unwinding of DNA coils may occur.
Figure 8. Copying the DNA sequence into an RNA is accompanied by local unwinding of the DNA
There are some differences between bacterial messenger RNA (mRNA) and that of higher organisms that have great importance if we are cloning the gene. Bacterial mRNA is used directly without modification. In fact, ribosomes often attach to mRNA of bacteria as it is being synthesized and immediately use the code for new protein synthesis.
The RNA of higher organisms must be processed before it becomes messenger RNA. In higher organisms, there are stretches of RNA that are cleaved out and discarded (called introns). The ends of the remaining RNA that contain the actual code (called exons) are reattached (spliced) to each other to give a shorter mRNA that is now ready to serve as a coding strand for protein synthesis. The mRNA of higher organisms also usually has polyadenylate tail added to the 3' end of each mRNA.
Figure 9. The organization of eukaryotic genes