Suppose a gene contain the following double stranded sequence:
5'-ATGTTTAGCGCC 3' 3'-TACAAATCGCGG 5'
If the top strand is the "sense strand" and codes for a mRNA, which of the following would be the sequence of the corresponding segment of "antisense RNA"?

Question

Suppose a gene contain the following double stranded sequence: 
5'-ATGTTTAGCGCC 3' 3'-TACAAATCGCGG 5' 
If the top strand is the "sense strand" and codes for a mRNA, which of the following would be the sequence of the corresponding segment of "antisense RNA"?

Accepted Answer

The correct option is 4: 5' GGCGCUAAACAU 3'

This problem requires a conceptual understanding of gene structure, transcription, and the definition of antisense RNA. The 'sense strand' is given as 5'-ATGTTTAGCGCC 3'. In molecular biology, the sense strand, also called the coding strand, is the DNA strand whose sequence is identical to the messenger RNA (mRNA) transcript (with Uracil (U) replacing Thymine (T)). Therefore, the corresponding mRNA sequence is 5'-AUGUUUAGCGCC 3'. Antisense RNA, by definition, is a non-coding RNA molecule that is perfectly complementary to the target mRNA. Its biological function is to bind to the mRNA, forming a double-stranded hybrid, which prevents translation of the genetic message into a protein—a process called gene silencing. To find the antisense RNA sequence, we must apply the RNA base-pairing rules (A pairs with U, G pairs with C) to the mRNA transcript, ensuring the sequence is antiparallel. The complementary sequence to the mRNA (5'-AUGUUUAGCGCC 3') is 3'-UACAAAUCGCGG 5'. Following the convention of writing nucleic acid sequences in the 5' to 3' direction, we flip this sequence to get 5'-GGCGCUAAACAU 3', which is the correct antisense RNA.

The incorrect option is 1: 5' AUGUUUAGCGCC 3'

This sequence is the **mRNA (messenger RNA)** transcript itself, not the antisense RNA. Since the top strand (5'-ATGTTTAGCGCC 3') is designated as the sense strand, the mRNA sequence is generated by simply replacing all the Thymine (T) bases in the sense strand with Uracil (U) bases. The antisense RNA's function is to bind to and silence this mRNA transcript, meaning the two must be complementary. If this were the antisense RNA, it would be identical to the target mRNA, and perfect binding/hybridization would not occur across the entire length of the sequence, making it ineffective for gene silencing.

The incorrect option is 2: 5' CCGCGAUUUGUA 3'

This sequence is the reverse of the correctly derived mRNA transcript. If you take the mRNA sequence (5'-AUGUUUAGCGCC 3') and read its bases from 3' to 5', you get 3'-CCGCGAUUUGUA 5', which, when written in the 5' to 3' direction, is 5' CCGCGAUUUGUA 3'. Antisense RNA is not a reverse reading of the mRNA; it is a **complementary** sequence. This sequence would not be able to pair with the target mRNA according to the rules of base-pairing, and therefore, it would not be able to perform its function of gene silencing.

The incorrect option is 3: 5' UACAAAUCGCGG 3'

This sequence is derived from transcribing the **template DNA strand** (3'-TACAAATCGCGG 5') into RNA (U replaces T) but incorrectly assuming a 5' to 3' orientation from the left, which ignores the antiparallel nature of transcription. The template DNA strand's sequence is TACAAATCGCGG, which when read left to right and converted to RNA bases is UACAAAUCGCGG. However, this transcription is not what is defined as the antisense RNA. The antisense RNA is defined as the complement of the *mRNA*, not a non-functional copy of the template strand that incorrectly maintains the 5' to 3' convention.