Where is the Shine-Dalgarno sequence located in prokaryotic cells?

The Shine-Dalgarno sequence is a key component in the initiation of translation in prokaryotic cells. It is located upstream from the AUG start codon, which is critical for aligning the ribosome with the mRNA. This sequence is complementary to a region on the 16S rRNA of the ribosomal small subunit, facilitating the proper binding of the ribosome to the mRNA.

When the ribosome assembles at the mRNA, the Shine-Dalgarno sequence helps to position the ribosome so that the AUG codon, which specifies the first amino acid in the protein sequence, is in the P site of the ribosome. This sequence is not found downstream of the AUG codon, within the coding region, or in the 3’ untranslated region (UTR), as these locations do not serve the function of ribosome binding for the initiation of translation. Thus, the correct answer emphasizes the crucial role of the Shine-Dalgarno sequence's position in facilitating the start of protein synthesis in prokaryotes.