Practice Problem 4

Assume that the DNA chain that codes for the synthesis of a particular protein contains the triplet A-G-T (reading from the 3 to the 5 end). Predict the sequence of nucleotides in the triplet, or codon, that would be built in the messenger RNA constructed on this DNA template. Then predict the amino acid that would be incorporated at this point in the protein.


Solution

The messenger RNA synthesized when this DNA chain is read is a complement of the DNA. Every time a G appears, a C is placed on the complementary chain, and vice versa. Each time a T appears in the DNA message, an A is incorporated into the mRNA chain. When the DNA contains an A, the complementary RNA chain contains a U.

DNA:   3   ...A-G-T...   5
mRNA:   5   ...U-C-A...   3

According to the genetic code table, the mRNA chain is read from the 5' to the 3' end. We therefore look up the U-C-A codon in the table and find that this triplet codes for the amino acid serine.