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Messenger RNA in bacteria often carries several coding sequences.

Bacterial Messenger RNA Can Code for Several Proteins

In bacteria, several genes may be transcribed to give a single messenger RNA. The term operon refers to clusters of genes that are co-transcribed. The result is that several proteins may be encoded by the same mRNA. As long as each open reading frame has its own Shine-Dalgarno sequence in front of it, the ribosome will bind and start translating. Open reading frames that are translated into proteins are sometimes known as cistrons; consequently, mRNA which carries several of these is called polycistronic mRNA (Fig. 8.20).

operon A cluster of prokaryotic genes that are transcribed together to give a single mRNA (i.e. polycistronic mRNA)

polycistronic mRNA mRNA carrying multiple coding sequences that may be translated to give several different protein molecules;only found in prokaryotic (bacterial) cells

FIGURE 8.18 Polysome

A single mRNA molecule is associated with several ribosomes. At initiation the two subunits assemble; during chain elongation several ribosomes are at different stages in reading the same mRNA message; at termination the ribosome complex disassembles.

FIGURE 8.18 Polysome

A single mRNA molecule is associated with several ribosomes. At initiation the two subunits assemble; during chain elongation several ribosomes are at different stages in reading the same mRNA message; at termination the ribosome complex disassembles.

FIGURE 8.19 False Color TEM of Polysome

This false-color transmission electron micrograph (TEM) shows a polysome from a human brain cell. Polysomes consist of several individual ribosomes connected by slender strands of messenger RNA. Magnification: x240,000. CNRI / Science Photo Library.

Eukaryotic mRNA molecules each code for a single protein.

Start

End codon codon

Cistron

Cistron

Cistron

Cistron

Protein

1 2 FIGURE 8.20 Polycistronic mRNA in Bacteria

Cistron

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