2-20 Translation involves messenger RNA

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• Most messenger RNAs do not contain a great deal of secondary or tertiary structure.
• Messenger RNA is normally unstable with half-lives of less than three minutes.
• Prokaryotic messenger RNA has short stretches of adenine added to the 3' end of the message. This poly-A tail serves as a target for degradation.

Translation is the process of converting the instructions coded in the DNA into the proteins that actually carry out the work. The macromolecules that perform this task consist of mRNA, transfer RNA (tRNA) and the ribosome, which is made of ribosomal RNA (rRNA) and ribosomal proteins. In brief, this process consists of making an mRNA copy of a region in the DNA that gives directions for the synthesis of a protein or proteins. The mRNA is then bound by a ribosome that translates the mRNA into a amino acid sequence. The amino acids necessary for the protein are carried to the ribosome by tRNA that actually read the information in the mRNA and add the appropriate amino acid to the nascent protein chain. This is described in much greater detail in the chapter on the central dogma.

We will now examine the structure of the molecules involved in translation, starting with mRNA, where the primary structure is simple - merely unmodified A, G, C and U bases. In almost all prokaryotic mRNAs there is not a great deal of secondary and tertiary structure, since they are typically being translated by ribosomes and the translating ribosome certainly removes any structure as it moves along the mRNA. What structure there is tends to be in the regions that are not translated, notably the 5' and 3' ends of the mRNA. One of the roles of structure, especially at the 3' end, is to stabilize the mRNA. Now it happens that most prokaryotic mRNAs are not very stable in the cell because they are rapidly degraded by RNases. However, different types of RNA structures can impede the progress of nucleases, particularly the type that degrades from the 3' end of the mRNA (termed exonucleases, because they attack from the exterior ends). As a complication, however, there are some RNA structures that actually serve as a specific target for other types of RNases (termed endonucleases, because they cut within an RNA) and thus lead to destabilization of the mRNA. This is biologically important because a more stable mRNA is translated by more ribosomes and therefore leads to more protein product.

As a final structural feature, most if not all prokaryotic mRNAs have short stretches of adenosine residues added to the 3' end after transcription. The presence of these also tends to lead to mRNA degradation. This is a bit surprising because the presence of long adenosine stretches on the 3' ends of eukaryotic mRNAs actually tends to stabilize those mRNAs. This appears to be a case where evolution has taken a single feature, addition of adenosines to mRNAs, and changed its functional importance through evolution.

In eukaryotes, the fact that mRNA is transcribed in the nucleus and must be exported to the cytoplasm for translation changes some details. One of these is that eukaryotic mRNAs are rather more stable than in prokaryotes.

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