2. AUG specifying the incorporation of methionine. Be­cause

2. The code is degenerate but unambiguous: To introduce the terms degenerate and ambiguous, consider the English language. English language shows considerable de­generacy meaning that the same concept can be indicated using a number of different words think, e.g.

of lockup, cell, pen, pound, brig, and dungeon. English language also shows ambiguity so that it is only by context that one can tell whether cell means a lockup or the basic unit of life. Like the English language the genetic code shows degeneracy but unlike language the code is unambiguous. 3.

Start and stop codons and the reading frame:The order of the codons in DNA and the amino acid sequence of a protein are co-linear. The start signal for protein synthesis is the codon AUG specifying the incorporation of methionine. Be­cause the genetic code is read in blocks of three. There are three potential reading frames in any mRNA. The codons UAA, UAG and UGA are stop signals for protein synthesis. 4. The code is nearly universal:The code is the one used by organisms as diverse as E. coli and humans for their nuclear-encoded proteins.

It was originally thought that the code would be universal. However, several mito­chondrial genes use UGA to mean tryptophan rather than stop. The nuclear code for some protists uses UAA and UAG to code for glutamine rather than stop.

5. Missense mutations:A mutation that changes the codon from one amino acid to that for another is a missense mutation.


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