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Deciphering The Genetic Code

There are two steps in the path from genes to proteins. In the first step, called transcription, the region of the double-stranded DNA corresponding to a specific gene is copied into an RNA molecule, called messenger RNA (mRNA), by an enzyme called RNA polymerase. In the second step, called translation, the mRNA directs the assembly of amino acids in a specific sequence to form a chain of amino acids called a polypeptide. This process is accomplished by ribosomes, special amino acid-bearing RNA molecules called transfer RNAs (tRNAs), and other translation factors. The newly synthesized polypeptides form proteins, which have functional and structural roles in cells. All proteins are synthesized by this process.

The precise order of amino acids assembled during translation is determined by the order of nucleotides in the mRNA. These nucleotides are a direct copy of the linear sequence of the nucleotides in one of the two complementary DNA strands, which have been transcribed using a code in which every three bases of the RNA specify an amino acid. DNA and RNA molecules both have directionality, which is indicated by reference to either the 5′ ("five prime") end or the 3′ ("three prime") end.

The code is always read in the 5′ to 3′ direction, using adjacent, non-overlapping three-base units called codons. Since there are four different nucleotides (also called bases) in RNA (abbreviated A, C, G, and U), there are sixty-four (43) different codons, and each codon specifies a particular amino acid. There are only twenty different amino acids, however, so many of the amino acids can be specified by more than one codon, a circumstance that is known as degeneracy. The list of mRNA codons specific for a given The steps of translation. Initiation: RNA binds to the small ribosomal subunit, a tRNA binds to the RNA, and the large subunit attaches to the small subunit. Elongation: Successive tRNAs bind to the A site, form a peptide bond to the growing amino acid chain, and then move to the P site. The spent tRNA is ejected. Termination: A termination or stop codon binds a release factor, which dissociates two ribosomal subunits. The peptide (amino acid chain) is released. Adapted from Curtis and Barnes, 1994. Genetic code. amino acid is called the genetic code. The start signal, or initiation codon, for translating the mRNA is usually specified by an AUG, which codes for the amino acid methionine. Three codons (UAA, UGA, and UAG) do not specify an amino acid. Instead, these codons serve as stop signals to indicate that the end of the gene has been reached. During the translation process, they signal that no further amino acids are to be assembled.

The process of translation is carried out by ribosomes, which bind the mRNA and conduct a catalytic activity, called peptide bond formation, for joining the amino acids. The amino acids are carried to the ribosome by the tRNAs. Each tRNA has a specific amino acid attached to it and contains a nucleotide triplet called an anticodon. The anticodon recognizes a specific codon on the mRNA by pairing with it, using base-pairing rules like those used by DNA: A pairs with U and G pairs with C. For example, a tRNA with a UUU anticodon recognizes the AAA codon. The amino acid lysine is attached to this tRNA, so every time the ribosome "reads" an AAA codon, the lysine-bearing tRNA is brought in, base pairs via its anticodon to the codon, and delivers a lysine to the growing protein chain.

Mutations arise when one or more bases in the DNA is changed. When the mutated DNA is transcribed, the resulting mRNA will carry the same mutation. Then, when the mRNA is translated, the amino acid sequence of the resulting protein will be different from the original, or wild-type, sequence because the codons affected by the mutation will recruit the wrong amino acids. The resulting mutant protein may have neutral, harmful, or even beneficial effects on the individual. These changes are the basis for evolution.

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Medicine EncyclopediaGenetics in Medicine - Part 4Translation - Deciphering The Genetic Code, Stages Of Translation, Recognition Of Initiation Codons