The Structure of Messenger RNA

Messenger RNA functions as the template for protein synthesis; it carries genetic information from DNA to a ribosome and helps to assemble amino acids in their correct order. Each amino acid in a protein is specified by a set of three nucleotides in the mRNA, called a codon. Both prokaryotic and eukaryotic mRNAs contain three primary regions (IFigure 14.5). The 5' untranslated region (5' UTR; sometimes call the leader) is a sequence of nucleotides at the 5' end of the mRNA that does not code for the amino acid sequence of a protein. In bacterial mRNA, this region contains a consensus sequence called the Shine-Dalgarno sequence, which serves as the ribosome-binding site during translation; it is found approximately seven nucleotides upstream of the first codon translated into an amino acid (called the start codon). Eukaryotic mRNA has no equivalent consensus sequence in its 5' untranslated region. In eukaryotic cells, ribosomes bind to a modified 5' end of mRNA, as discussed later in the chapter.

The next section of mRNA is the protein-coding region, which comprises the codons that specify the amino acid sequence of the protein. The protein-coding region begins with a start codon and ends with a stop codon. The last region of mRNA is the 3' untranslated region (3' UTR), a sequence of nucleotides at the 3' end of mRNA that is not translated into protein. The 3' untranslated region affects the stability of mRNA and the translation of the mRNA protein-coding sequence.

Shine-Dalgarno sequence in prokaryotes only

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