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Figure 2.3. Nucleotide residues in RNA.

2.5.2. Functional Regions

The physical length of the mRNA chain is always greater than the length of its coding sequence. The coding sequence includes only part of the total mRNA length. The first codon is preceded by a noncoding (untranslated) 5'-terminal sequence (5'-UTR) the length of which varies for different mRNAs. Furthermore, the terminal codon is never located at the 3'-end of an mRNA chain, but is always followed by a noncoding 3'-terminal sequence (3'-UTR). In addition, most eukaryotic mRNAs contain a long noncoding sequence of adenylic acid residues at their 3'-end. This poly(A) tract (tail) is added to mRNA after the end of transcription by a special enzyme, polyadenylate polymerase.

Identifying the factors that determine the starting point of the coding nucleotide sequence within an mRNA chain is an important problem. Each polypeptide is known to begin with a N-terminal methionine

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