In terms of the polycistronic composition of mRNAs and the presence or absence of Shine–Dalgarno sequences, compare and contrast bacterial, archaeal, and eukaryotic mRNAs.

Key Concepts

Polycistronic mRNA

Polycistronic mRNA is a type of messenger RNA that encodes multiple proteins within a single transcript. This is common in prokaryotes, such as bacteria and archaea, allowing for the coordinated expression of genes that are functionally related. In contrast, eukaryotic mRNAs are typically monocistronic, meaning they encode only one protein per transcript, which allows for more complex regulation of gene expression.

Shine-Dalgarno Sequence

The Shine-Dalgarno sequence is a ribosomal binding site in bacterial mRNA that is crucial for the initiation of translation. It is a short, conserved sequence located upstream of the start codon, facilitating the alignment of the ribosome with the mRNA. In archaea, similar sequences exist, but eukaryotic mRNAs do not have Shine-Dalgarno sequences; instead, they utilize a 5' cap structure for ribosome binding.

Transcription and Translation Differences

Transcription and translation processes differ significantly between prokaryotes and eukaryotes. In prokaryotes, transcription and translation occur simultaneously in the cytoplasm, while in eukaryotes, transcription occurs in the nucleus, and mRNA must be processed (including splicing and capping) before it is translated in the cytoplasm. This separation allows for more complex regulation in eukaryotic cells.