After studying this chapter, you should be able to:
- Understand that the genetic code is a three-letter nucleotide code, which is encoded in the linear array of the exon DNA (composed of triplets of A, G, C, and T) of protein coding genes, and that this three-letter code is translated into mRNA (composed of triplets of A, G, C, and U) to specify the linear order of amino acid addition during protein synthesis via the process of translation.
- Appreciate that the universal genetic code is degenerate, unambiguous, non-overlapping, and punctuation free.
- Explain that the genetic code is composed of 64 codons, 61 of which encode amino acids while 3 induce the termination of protein synthesis.
- Explain how the transfer RNAs (tRNAs) serve as the ultimate informational agents that decode the genetic code of mRNAs.
- Understand the mechanism of the energy-intensive process of protein synthesis that occurs on RNA-protein complexes termed ribosomes.
- Appreciate that protein synthesis, like DNA replication and transcription, is precisely controlled through the action of multiple accessory factors that are responsive to multiple extra- and intracellular regulatory signaling inputs.
The letters A, G, T, and C correspond to the nucleotides found in DNA. Within the protein-coding genes, these nucleotides are organized into three-letter code words called codons, and the collection of these codons makes up the genetic code. It was impossible to understand protein synthesis—or to explain mutations—before the genetic code was elucidated. The code provides a foundation for explaining the way in which protein defects may cause genetic disease and for the diagnosis and perhaps the treatment of these disorders. In addition, the pathophysiology of many viral infections is related to the ability of these infectious agents to disrupt host cell protein synthesis. Many antibacterial drugs are effective because they selectively disrupt protein synthesis in the invading bacterial cell but do not affect protein synthesis in eukaryotic cells.
The genetic information within the nucleotide sequence of DNA is transcribed in the nucleus into the specific nucleotide sequence of an RNA molecule. The sequence of nucleotides in the RNA transcript is complementary to the nucleotide sequence of the template strand of its gene in accordance with the base-pairing rules. Several different classes of RNA combine to direct the synthesis of proteins.
In prokaryotes there is a linear correspondence between the gene, the messenger RNA (mRNA) transcribed from the gene, and the polypeptide product. The situation is more complicated in higher eukaryotic cells, in which the primary transcript is much larger than the mature mRNA. The large mRNA precursors contain coding regions (exons) that will form the mature mRNA and long intervening sequences (introns) that separate the exons. The mRNA is processed within the nucleus, and the introns, which make up much more of this RNA than the exons, are removed. Exons are spliced together to form mature mRNA, which is transported to the cytoplasm, where it is translated into protein.