Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 135

RNA is similar to DNA in that it is composed of a chain of nucleotides. However, RNA nucleotides contain the sugar ribose instead of deoxyribose. These five-carbon sugars differ in the second carbon, which is attached to a hydroxyl group in ribose rather than the hydrogen found in deoxyribose (see Figure 1.2). Figure 2.1A shows the structure of a ribonucleoside triphosphate (rNTP), which is the form that is used as a precursor during RNA synthesis.

RNA and DNA chains also vary in the bases that are present. Three of the bases—adenine, guanine, and cytosine—are the same, but RNA has uracil instead of the thymine found in DNA (Figure 2.1B). The RNA bases also can be modified after they are incorporated into an RNA chain, as discussed below.

Figure 2.1C shows the basic structure of an RNA polynucleotide chain. As in DNA, RNA nucleotides are held together by phosphates that join the 5′ carbon of one ribose sugar to the 3′ carbon of the next. This arrangement ensures that, as with DNA chains, the two ends of an RNA polynucleotide chain will be different from each other, with the 5′ end terminating in a phosphate group and the 3′ end terminating in a hydroxyl group. The 5′ end of a newly synthesized RNA chain has three phosphates attached to it because transcription initiates with an rNTP. As each new rNTP is added to the growing RNA chain, two phosphate groups are released so that the sugar phosphate backbone alternates between the ribose (to which the base is attached) and a single phosphate group.

According to convention, the sequence of bases in RNA is given from the 5′ end to the 3′ end, which is the direction in which the RNA is synthesized, by addition of the 5′ α-phosphate of an incoming nucleoside triphosphate to the 3′ hydroxyl end of the growing RNA chain. Also by convention, regions in RNA that are closer to the 5′ end in a given sequence are referred to as upstream, and regions that are closer to the 3′ end are referred to as downstream, because RNA is both synthesized and translated in the 5′-to-3′ direction.