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Lecture notes - Genetic code of translation
Biology (C100)
University of Salford
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Lecture notes - Genetic code of translation
The final step in the expression of protein-coding genes is translation. Protein synthesis is called translation because it is a decoding process. The information encoded in the language of nucleic acids must be rewritten in the language of proteins. During translation, the sequence of nucleotides is “read” in discrete sets of three nucleotides, each set being a codon.
Each codon codes for a single amino acid. The sequence of codons is “read” in only one way-the reading frame to give rise to the amino acid sequence of a polypeptide. Deciphering the genetic code was one of the great achievements of the twentieth century. Here we examine the nature of the genetic code.
Reading Frame
During transcription, An mRNA complementary to the template strand of DNA is synthesized. The nucleotides in the mRNA are organized into groups of three, each group being a codon. The first codon translated into protein is the start codon. It establishes the reading frame and therefore the sequence of amino acids in the polypeptide chain that is made from the mRNA.
The genetic code, presented in RNA form. Close inspection of the code reveals several features that are related not only to the way cells use DNA to store information but also to why it is valuable for storing data, as described in the chapter opening story. One feature is that the code words (codons) are three letters (bases) long; thus one small “word” conveys a significant amount of information. Each codon is recognized by an anticodon present on a tRNA molecule.
The Genetic Code
Another feature is that the code has “punctuation:’ One codon, AUG, is almost always the first codon in the protein-coding portion of mRNA molecules. It is called the start codon because it serves as the start site for translation by coding for the initiator tRNA. Three other codons (UGA, UAG, and UAA) are involved in termination of translation and are called stop or nonsense codons.
These codons don’t encode an amino acid and thus don’t have a tRNA bearing their anticodon. Thus only 61 of the 64 codons in the code, the sense codons, direct amino acid incorporation into protein. Finally, the genetic code exhibits code degeneracy; that is, there are up to six different codons for a given amino acid.
Wobble and Coding
Despite the existence of 61 sense codons, there are fewer than 61 different tRNAs. It follows that not all codons have a corresponding tRNA. Cells can successfully translate mRNA using fewer tRNAs because loose pairing between the 5 base in the anticodon and the 3 base′ ′ of the codon is tolerated. Thus as long as the first and second bases in the codon correctly base pair with an anticodon, the tRNA bearing the correct amino acid will bind to the mRNA during translation. This is evident on inspection of the code.
Note that the codons for a particular amino acid most often differ at the third position. This somewhat loose base pairing is known as wobble, and it relieves cells of the need to synthesize so many tRNAs. Wobble also decreases the effects of some mutations.
Lecture notes - Genetic code of translation
Module: Biology (C100)
University: University of Salford
