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Altius FL 7 BB Section question 57
#1 Posted : Wednesday, September 01, 2021 5:43:44 PM
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Hello! I was hoping to clarify a question from our Altius practice exam #7. For the BB Section Question 57:

If the triplet codon on a coding DNA strand is CTG, the complementary tRNA codon must be which of the following?

a) GUC
b) GAC (my answer)
c) CTG
d) CUG (correct answer according to Altius?)

I am wondering why is the answer to the question is CUG instead of GAC? Is it not that the mRNA transcribed be the same sequence as the coding DNA, meaning that the complementary tRNA is GAC (DNA: CTG -> mRNA: CUG -> tRNA GAC)?
#2 Posted : Friday, September 03, 2021 2:00:33 AM
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Hi Moon!

According to Altius, the correct answer is D but is actually CAG (not CUG as written below). The reasoning is because the triplet codon from the coding strand of the DNA is carried to the cytoplasm by mRNA. The mRNA will be composed of complementary sequences to the DNA strand (with U replacing T). This mRNA is then read by tRNA in the cytoplasm which carries an anticodon for each triplet sequence. If CTG is the original codon in the coding strand of DNA; the mRNA complementary sequence will be CUG; the complementary anticodon in tRNA must therefore be CAG because, by convention, nucleotide sequences are always written from 5' to 3'.

Hope that helps!
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