Protein synthesis involves three main types of RNA: rRNA, tRNA, and mRNA. These RNA molecules are similar to DNA in that they are unbranched polymers made up of nucleoside monophosphates joined together by 3′→5′-phosphodiester bonds. However, they differ from DNA in several ways. RNA molecules are smaller, contain ribose instead of deoxyribose, and have uracil instead of thymine. They exist as single strands that can fold into complex structures. Each type of RNA has its own size, function, and unique structural modifications. In eukaryotes, additional small ncRNA molecules perform specialized functions in different cellular compartments.

Ribosomal RNA (rRNA) is found in ribosomes, which are complex structures responsible for protein synthesis. Prokaryotic cells have three distinct size species of rRNA (23S, 16S, and 5S), while eukaryotic cells have four (28S, 18S, 5.8S, and 5S). rRNA makes up approximately 80% of the total RNA in the cell. Some RNA molecules, including rRNA, have catalytic activity and are called ribozymes.

Transfer RNA (tRNA) is the smallest type of RNA molecule. There is a specific type of tRNA for each of the 20 amino acids found in proteins. tRNA makes up about 15% of the total RNA in the cell. These molecules contain modified bases and have intricate base-pairing within their structure. Each tRNA acts as an adaptor molecule, carrying a specific amino acid to the site of protein synthesis. It recognizes the genetic code sequence on mRNA and adds the corresponding amino acid to the growing peptide chain.

Messenger RNA (mRNA) makes up only around 5% of the cell's RNA, but it is highly diverse in size and base sequence. mRNA carries genetic information from DNA for use in protein synthesis. In eukaryotes, mRNA is transported from the nucleus to the cytosol. mRNA can be polycistronic, carrying information from multiple genes, which is characteristic of prokaryotes. In contrast, eukaryotic mRNA is usually monocistronic, carrying information from a single gene. mRNA contains untranslated regions at its 5′ and 3′ ends. Eukaryotic mRNA also has specific structural features, such as a poly-A tail at the 3′ end and a cap consisting of 7-methylguanosine at the 5′ end. These modifications are absent in prokaryotic mRNA. The mechanisms for creating these structural characteristics are discussed in detail on pages 441-442.
There are three major types of RNA that participate in the process of protein synthesis: rRNA, tRNA, and mRNA. Like DNA, these RNA are unbranched polymeric molecules composed of nucleoside monophosphates joined together by 3′→5′-phosphodiester bonds (see p. 412). 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 (snoRNA), nucleus (snRNA), and cytoplasm (microRNA [miRNA]) perform specialized functions as described on pp. 441, 442, and 475.]
A. Ribosomal RNA rRNA are found in association with several proteins as components of the ribosomes, the complex structures that serve as the sites for protein synthesis (see p. 451). Prokaryotic cells contain three distinct size species of rRNA (23S, 16S, and 5S, where S is the Svedberg unit for sedimentation rate that is determined by the size and shape of the particle), as shown in Figure 31.2. Eukaryotic cells contain four rRNA species (28S, 18S, 5.8S, and 5S). Together, rRNA make up ~80% of the total RNA in the cell. [Note: Some RNA function as catalysts, for example, an rRNA in protein synthesis (see p. 455). RNA with catalytic activity is termed a ribozyme.]
B. Transfer RNA tRNA 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, tRNA make up ~15% of the total RNA in the cell. The tRNA molecules contain a high percentage of unusual (modified) bases, for example, dihydrouracil (see Fig. 22.2, p. 292), and have extensive intrachain base-pairing (Fig. 31.3) 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 3′-end, to the site of protein synthesis. There, it recognizes the genetic code sequence on an mRNA, which specifies the addition of that amino acid to the growing peptide chain (see p. 447).
C. Messenger RNA mRNA comprises only ~5% of the RNA in a cell, yet is by far the most heterogeneous type of RNA in size and base sequence. mRNA is coding RNA in that it carries genetic information from DNA for use in protein synthesis. In eukaryotes, this involves transport of mRNA out of the nucleus and into the cytosol. An mRNA carrying information from more than one gene is polycistronic (cistron = gene). Polycistronic mRNA is characteristic of prokaryotes. An mRNA carrying information from only one gene is monocistronic and is characteristic of eukaryotes. In addition to the protein-coding regions that can be translated, mRNA contains untranslated regions at its 5′-and 3′-ends (Fig. 31.4). Special structural characteristics of eukaryotic (but not prokaryotic) mRNA include a long sequence of adenine nucleotides (a poly-A tail) on the 3′-end of the RNA, plus a cap on the 5′-end consisting of a molecule of 7-methylguanosine attached through an unusual (5′→5′) triphosphate linkage. The mechanisms for modifying mRNA to create these special structural characteristics are discussed on pp. 441–442.