If mRNA is complementary to the DNA template strand and the DNA template stand is complementary to the DNA non-template strand, why are base sequences of mRNA and the DNA non-template strand not identical? Could they ever be? a. No, they cannot be identical because the T nucleotide in DNA is replaced with U nucleotide in RNA and AUG is the start codon. b. No, they cannot be identical because the T nucleotide in RNA is replaced with U nucleotide in DNA. c. They can be identical if methylation of the U nucleotide in RNA occurs and gives T nucleotide. d. They can be identical if de-methylation of the U nucleotide in RNA occurs and gives T nucleotide
The Correct Answer and Explanation is:
Correct Answer: a. No, they cannot be identical because the T nucleotide in DNA is replaced with U nucleotide in RNA and AUG is the start codon.
Explanation:
Messenger RNA (mRNA) is synthesized through a process called transcription, in which one strand of DNA—the template strand—is used to generate a complementary RNA molecule. The base-pairing rules govern the formation of the mRNA strand: adenine (A) in DNA pairs with uracil (U) in RNA, cytosine (C) with guanine (G), guanine (G) with cytosine (C), and thymine (T) in DNA pairs with adenine (A) in RNA. Unlike DNA, RNA contains uracil (U) instead of thymine (T).
This leads to a fundamental distinction between the DNA non-template strand (also called the coding strand) and the mRNA strand. Although the coding strand and mRNA strand appear similar in sequence, they are not identical due to the presence of uracil in RNA and thymine in DNA. For example, if the coding strand of DNA contains the sequence 5’-ATGCGT-3’, the corresponding mRNA sequence would be 5’-AUGCGU-3’, where each T is replaced by U. Additionally, the mRNA molecule begins with a start codon—typically AUG—which codes for methionine and signals the initiation of translation.
Because of the consistent substitution of uracil for thymine and the transcriptional processes that introduce structural and chemical differences, the mRNA sequence can never be truly identical to the DNA non-template strand. Even though the sequences are complementary in function, they are chemically distinct due to nucleotide differences. Options involving methylation or demethylation (choices c and d) are unrelated to the normal transcription process and do not cause RNA to have thymine instead of uracil. Therefore, the correct explanation aligns with answer choice a.
