Structure of RNA

STRUCTURE OF RNA

There are three major types of RNA that participate in the process of protein synthesis: rRNA, tRNA, and mRNA. Like DNA, these three types of RNA are unbranched polymeric molecules composed of nucleoside monophosphates joined together by 3I →5I - phosphodiester bonds. However, they differ from DNA in several ways. For example, they are considerably smaller than DNA, contain ribose instead of deoxyribose and uracil instead of thymine, and exist as single strands that are capable of folding into complex structures. The three major types of RNA also differ from each other in size, function, and special structural modifications. [Note: In eukaryotes, additional small ncRNA molecules found in the nucleolus (snoRNAs), nucleus (snRNA), and cytoplasm (miRNA) perform specialized functions.]

A. Ribosomal RNA

rRNAs are found in association with several proteins as components of the ribosomes, the complex structures that serve as the sites for protein synthesis. There are three distinct size species of rRNA (23S, 16S, and 5S) in prokaryotic cells (Figure 30.2). In the eukaryotic cytosol, there are four rRNA species (28S, 18S, 5.8S, and 5S, where “S” is the Svedberg unit for sedimentation rate, which is determined by the size and shape of the particle.) Together, rRNAs make up about 80% of the total RNA in the cell. [Note: Some RNAs function as catalysts, for example, an rRNA in protein synthesis. RNA with catalytic activity is termed a “ribozyme.”]

Figure 30.2 Prokaryotic and eukaryotic ribosomal RNAs (rRNAs). S = Svedberg unit.

B. Transfer RNA

tRNAs are the smallest (4S) of the three major types of RNA molecules. There is at least one specific type of tRNA molecule for each of the 20 amino acids commonly found in proteins. Together, tRNAs make up about 15% of the total RNA in the cell. The tRNA molecules contain a high percentage of unusual bases, for example, dihydrouracil (see Figure 22.2) and have extensive intrachain base-pairing (Figure 30.3) that leads to characteristic secondary and tertiary structure. Each tRNA serves as an “adaptor” molecule that carries its specific amino acid, covalently attached to its 3I -end, to the site of protein synthesis. There it recognizes the genetic code sequence on an mRNA, which specifies the addition of its amino acid to the growing peptide chain.

Figure 30.3 A. Characteristic transfer RNA (tRNA) secondary structure (cloverleaf). B. Folded (tertiary) tRNA structure found in cells. D = dihydrouracil; Ψ = pseudouracil; T = thymine; C = cytosine; A = adenine.

C. Messenger RNA

mRNA comprises only about 5% of the RNA in the cell, yet is by far the most heterogeneous type of RNA in size and base sequence. mRNA carries genetic information from DNA for use in protein synthesis. In eukaryotes, this involves transfer of mRNA out of the nucleus and into the cytosol. If the mRNA carries information from more than one gene, it is said to be polycistronic (cistron = gene). Polycistronic mRNA is characteristic of prokaryotes. If the mRNA carries information from just one gene, it is said to be monocistronic and is characteristic of eukaryotes. In addition to the protein-coding regions that can be translated, mRNA contains untranslated regions at its 5I - and 3I -ends (Figure 30.4). Special structural characteristics of eukaryotic (but not prokaryotic) mRNA include a long sequence of adenine nucleotides (a poly-A “tail”) on the 3I -end of the RNA chain, plus a “cap” on the 5I -end consisting of a molecule of 7-methylguanosine attached through an unusual (5I →5I ) triphosphate linkage. The mechanisms for modifying mRNA to create these special structural characteristics are discussed.

Figure 30.4 Structure of eukaryotic messenger RNA.