Tobacco mosaic virus genetic code

Nitrous acid-induced mutations in tobacco mosaic virus (TMV) have contributed to the derivation of the genetic code (see Table 8.1) through amino acid analyses determining replacements in the coat protein (2). Since only C- U, G- A or A->G transitions are possible in the case of a nitrous acid point mutation, the number of possible replacements for a given amino acid is small. Thus, an Ala Val transition corresponds to a change of the codon GCN to GUN, whereas an Ala- Thr transition must be due to a change in the first letter GCN- ACN (See Chapter 8, Table 8.1). 

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