Top Questions What are nucleic acids?

RNA metabolism
RNA provides the link between the genetic information encoded in DNA and the actual workings of the cell. Some RNA molecules such as the rRNAs and the snRNAs (described in the section Types of RNA) become part of complicated ribonucleoprotein structures with specialized roles in the cell. Others such as tRNAs play key roles in protein synthesis, while mRNAs direct the synthesis of proteins by the ribosome. Three distinct phases of RNA metabolism occur. First, selected segments of the genome are copied by transcription to produce the precursor RNAs. Second, these precursors are processed to become functionally mature RNAs ready for use. When these RNAs are mRNAs, they are then used for translation. Third, after use the RNAs are degraded, and the bases are recycled. Thus, transcription is the process where a specific segment of DNA, a gene, is copied into a specific RNA that encodes a single protein or plays a structural or catalytic role. Translation is the decoding of the information within mRNA molecules that takes place on a specialized structure called a ribosome. There are important differences in both transcription and translation between prokaryotic and eukaryotic organisms.
Transcription
Small segments of DNA are transcribed into RNA by the enzyme RNA polymerase, which achieves this copying in a strictly controlled process. The first step is to recognize a specific sequence on DNA called a promoter that signifies the start of the gene. The two strands of DNA become separated at this point, and RNA polymerase begins copying from a specific point on one strand of the DNA using a ribonucleoside 5′-triphosphate to begin the growing chain. Additional ribonucleoside triphosphates are used as the substrate, and, by cleavage of their high-energy phosphate bond, ribonucleoside monophosphates are incorporated into the growing RNA chain. Each successive ribonucleotide is directed by the complementary base pairing rules of DNA. Thus, a C in DNA directs the incorporation of a G into RNA, G is copied into C, T into A, and A into U. Synthesis continues until a termination signal is reached, at which point the RNA polymerase drops off the DNA, and the RNA molecule is released. In some cases this RNA molecule is the final mRNA. In other cases it is a pre-mRNA and requires further processing before it is ready for translation by the ribosome. Ahead of many genes in prokaryotes, there are signals called “operators” where specialized proteins called repressors bind to the DNA just upstream of the start point of transcription and prevent access to the DNA by RNA polymerase. These repressor proteins thus prevent transcription of the gene by physically blocking the action of the RNA polymerase. Typically, repressors are released from their blocking action when they receive signals from other molecules in the cell indicating that the gene needs to be expressed. Ahead of some prokaryotic genes are signals to which activator proteins bind that positively induce transcription.
Transcription in higher organisms is more complicated. First, the RNA polymerase of eukaryotes is a more complicated enzyme than the relatively simple five-subunit enzyme of prokaryotes. In addition, there are many more accessory factors that help to control the efficiency of the individual promoters. These accessory proteins are called transcription factors and typically respond to signals from within the cell that indicate whether transcription is required. In many human genes, several transcription factors may be needed before transcription can proceed efficiently. A transcription factor can cause either repression or activation of gene expression in eukaryotes.
During transcription, only one strand of the DNA is usually copied. This is called the template strand, and the RNA molecules produced are single-stranded. The DNA strand that would correspond to the mRNA is called the coding or sense strand, and it is not unusual for this to change from one gene to the next. In eukaryotes the initial product of transcription is called a pre-mRNA, which is extensively spliced before the mature mRNA is produced, ready for translation by the ribosome.