PinkMonkey Online Study Guide-Biology
8.6 RNA : Structure and Types
Ribonucleic Acid (RNA) is another polynucleotide
which occurs in the cells as non-genetic material, with the exception
of some viruses. RNA is present in the nucleus as well as in the cytoplasm.
RNA molecule is single stranded and consists of nucleotides arranged in a long series. The single strand of RNA may be simple and straight, or it may be variously folded upon itself in certain regions.
Structural components : RNA molecule has three primary components.
Ribose sugar (a pentose sugar), with a pentagonal ring structure
Phosphate, as phosphoric acid
There are four kinds of nitrogenous bases found in RNA.
Of those, two are purines and two are pyrimidines, as follows :
Thus in RNA, uracil is present in place of thymine
found in DNA.
Structure of an RNA strand : The strand is made
up of alternating molecules of ribose sugar and the phosphate. The nitrogen
bases are attached to the sugar molecules in the strand and ístick outí
laterally as in DNA (figure 8.9). A sugar, a N-base and a phosphate together
form a ribonucleotide. A nucleotide without the phosphate is called
RNA being single stranded, the nitrogen bases remain mostly unpaired. However, the strand may be folded upon itself in certain regions. In such folded regions, base pairing occurs between purines and pyrimidines as follows :
Adenine = Uracil (two H-bonds). Guanine = Cytosine (three H-bonds). Nitrogen bases remain unpaired in the unfolded regions of the strand. Because of this variability in base pairing in different regions of the same strand, the total number of purines need not be equal to the total number of pyrimidines in RNA.
Figure 8.9 General structure of RNA