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Within reading frames all nucleotides either contribute to the specification of amino acids are serve as the termination point of the reading frame.
Individual codons, in other words, are arrayed directly against each other, with no intervening nucleotides. The sequence AUGTATCGAAAC thus is read [AUG] [TAT] [CGA] [AAC], with each nucleotide a portion of a codon and each codon specifying an amino acid. The exceptions to this "rule" are stop codons which instead serve to terminate the reading frame. See also lack of ambiguity in the genetic code and redundancy of the genetic code.
The oligopeptide specified by the above sequence, by the way, is Met-Try-Arg-Asn.
The lack of punctuation is reason why insertion and deletion mutations can give rise to frameshift mutations. Codons are read in three-nucleotide units rather than from (hypothetical) punctuations to punctuations within a base sequence.
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