A

mRNA

Incoming ribosomal subunits

Growing polypeptide chain

Direction of translation

Drawing to be rendered from actual micrograph being used in part (b)

415.25 An mRNA molecule may be transcribed simultaneously by several ribosomes. (a) Four ribosomes are translating a eukaryotic mRNA molecule, moving from the 5' end to the 3' end. (b) In this electron micrograph of a polyribosome from the silkworm, the dark staining spheres are ribosomes, and the long, thin filament connecting the ribosomes is mRNA. The 5' end of the mRNA is toward the upper left-hand corner of the micrograph. The twisted filament coming out of each ribosome is a polypeptide chain. The polypeptide chains become longer as the ribosomes move toward the 3' end of the mRNA. (Part b, O. L. Miller, Jr., and Barbara A. Hamaklo).

Direction of transcription

RNA polymerase DNA

mRNA 5'

RNA polymerase DNA

mRNA 5'

Direction of translation -►

Direction of translation -►

415.26 In prokaryotic cells, transcription and translation take place simultaneously. While mRNA is being transcribed from the DNA template at mRNA's 3' end, translation is taking place simultaneously at mRNA's 5' end.

tion of eukaryotic nuclear genes, as will be discussed near the end of this chapter.

Other fundamental differences lie in the process of initiation. In bacterial cells, the small subunit of the ribo-some attaches directly to the region surrounding the start codon through hydrogen bonding between the Shine-Dalgarno consensus sequence in the 5' untranslated region of the mRNA and a sequence at the 3' end of the 16S rRNA. In contrast, the small subunit of a eukaryotic ribosome first binds to proteins attached to the 5' cap on mRNA and then migrates down the mRNA, scanning the sequence until it encounters the first AUG initiation codon. (A few eukaryotic mRNAs have internal ribosome-binding sites that utilize a specialized initiation mechanism similar to that seen in bacterial cells.) Additionally, more initiation factors take part in eukaryotic initiation than in bacterial initiation.

Elongation and termination are similar in bacterial and eukaryotic cells, although different elongation and termination factors are used. In both types of organisms, mRNAs are translated multiple times and are simultaneously attached to several ribosomes, forming polyri-bosomes.

What about translation in archaea, which are prokaryotic in structure (see Chapter 2) but are similar to eukaryotes in other genetic processes such as transcription? Much less is known about the process of translation in archaea, but available evidence suggests that they possess a mixture of eubacterial and eukaryotic features. Because archaea lack nuclear membranes, transcription and translation take place simultaneously, just as they do in eubacterial cells. As mentioned earlier, archaea utilize unformylated methionine as the initiator amino acid, a characteristic of eukaryotic translation. Findings from recent studies of DNA sequences that code for initiation and elongation factors in archaea suggest that some of them are similar to those found in eubacteria, whereas others are similar to those found in eukaryotes. Finally, some of the antibiotics that inhibit translation in eubacte-ria have no effect on translation in archaea.

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