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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.

General structure

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.

  1. Ribose sugar (a pentose sugar), with a pentagonal ring structure

  2. Phosphate, as phosphoric acid

  3. Nitrogenous bases

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 ribonucleoside.

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

Table of Contents

8.0 Introduction
8.1 Packaging of Hereditary Material
8.2 The Structure of DNA
8.3 Replication Of DNA In Eukaryotes
8.4 Replicatin of Pokaryotic Chromosome
8.5 Plasmids
8.6 RNA: Structure and Types
8.7 The Genetic Code
8.8 The Central Theme of Protein Synthesis

Chapter 9





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