Transcription, the synthesis of RNA using DNA as a template, and translation, the synthesis of proteins using RNA as a template, are highly complex. A number of components participate: most prominently, messenger RNA, transfer RNA, ribosomes, several types of enzymes, and a storehouse of raw materials. After first examining each of these components, we shall see how they come together in the assembly line of the cell.
RNAs: Tools in the Cell’s Assembly Line
Ribonucleic acid (RNA) is an encoded molecule like DNA, but its general structure is different in several ways:
1. It is a single-stranded molecule that can assume secondary and tertiary structure due to bonds within the molecule, leading to specialized forms of RNA (mRNA, tRNA, and rRNA— figure 1).
2. RNA contains uracil, instead of thymine, as the complementary base-pairing mate for adenine. This does not change the inherent DNA code in any way because the uracil still follows the pairing rules.
3. Although RNA, like DNA, is structured with a backbone of alternating sugar and phosphate molecules, the sugar in RNA is ribose rather than deoxyribose.
Fig1. Characteristics of messenger and transfer RNA.
The many functional types of RNA range from small regulatory pieces to large structural ones. All types of RNA are formed through transcription of a DNA gene, but only mRNA is further translated into protein. See table 1 for a comparison of types of RNA and their functions.
Table1. Types of Ribonucleic Acid
Messenger RNA: Carrying DNA’s Message
Messenger RNA (mRNA) is a transcribed version of a structural gene or genes in DNA. It is synthesized by a process similar to syn thesis of the leading strand during DNA replication, and the complementary base-pairing rules ensure that the code will be faithfully copied in the mRNA transcript. The message of this transcribed strand is later read as a series of triplets called codons (figure 1a), and the length of the mRNA molecule varies from about 100 nucleotides to several thousand. The details of transcription and the function of mRNA in translation are covered later in this section.
Transfer RNA: The Key to Translation
Transfer RNAs (tRNAs) are also complementary copies of specific regions of DNA; however, they differ from mRNA. Transfer RNAs typically range from 75 to 95 nucleotides in length, and they contain sequences of bases that form hydrogen bonds with complementary sections in the same tRNA strand. At these points, the molecule bends back upon itself into several hairpin loops, giving the molecule a secondary cloverleaf structure that folds even further into a complex, three-dimensional helix (figure 1b). This compact molecule acts as a translator that converts RNA language into protein language. The bottom loop of the cloverleaf contains three nucleotides called the anticodon that both designates the specificity of the tRNA and complements mRNA’s codons. At the opposite end of the molecule is a binding site for the amino acid that is specific for that tRNA’s anticodon. For each of the 20 amino acids, there is at least one specialized type of tRNA to carry it. Binding of an amino acid to its specific tRNA requires a specific enzyme that can correctly match each tRNA with its amino acid.
The Ribosome: A Mobile Molecular Factory for Translation
The prokaryotic (70S) ribosome is a particle composed of tightly packaged ribosomal RNA (rRNA) and protein. The rRNA component of the ribosome forms complex three-dimensional figures that contribute to the structure and function of ribosomes. The interactions of proteins and rRNA create the two subunits of the ribosome that engage in final translation of the genetic code (see figure 2). A metabolically active bacterial cell may contain 20,000 ribosomes— all actively engaged in reading the genetic program, taking in raw materials, and producing proteins at an impressive rate.
Fig2. The “players” in translation. A ribosome serves as the stage for protein synthesis. Assembly of the small and large subunits results in specific sites for placement of the mRNA (small subunit) and two tRNAs with their amino acids (P and A sites).
