Consider the following statements about the Shine-Dalgarno sequence:
A. It is named after the Australian scientists John Shine and Lynn Dalgarno.
B. It is a ribosomal binding site in bacterial and archaeal mRNA.
C. It is located 8 bases upstream of the start codon AUG.
D. It helps in replication.
From the choices given below, select the correct one:
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While prokaryotes utilize the Shine-Dalgarno sequence to recruit ribosomes, eukaryotes use a 5'-cap-dependent scanning mechanism guided by the Kozak consensus sequence.
Step 1: Understanding the Concept:
The Shine-Dalgarno (SD) sequence is a conserved ribosomal binding site present in the $5'$ untranslated region (UTR) of prokaryotic messenger RNA (mRNA) molecules.
It serves to align the ribosome correctly with the start codon to initiate translation. Step 2: Detailed Explanation:
Let us evaluate each statement individually:
- Statement A: The sequence is named after its discoverers, John Shine and Lynn Dalgarno, who were researchers from Australia. This statement is correct.
- Statement B: It acts as a conserved purine-rich sequence (typically AGGAGG) that is complementary to a pyrimidine-rich region at the $3'$ end of the 16S rRNA within the small (30S) ribosomal subunit of bacteria and archaea. This base-pairing interaction is critical for recruiting the ribosomal machinery to the mRNA transcript. This statement is correct.
- Statement C: It is typically positioned 8 bases upstream (towards the $5'$ end) of the translation start codon (AUG). This optimal spacing ensures that when the 16S rRNA binds to the SD sequence, the start codon is positioned directly within the P-site of the ribosome. This statement is correct.
- Statement D: The Shine-Dalgarno sequence is involved in translation initiation (protein synthesis) and has no role in DNA replication. This statement is incorrect. Step 3: Final Answer:
Since statements A, B, and C are correct, the correct option is (B).
