We describe the structural implications of a periodic pattern found in human 
exons and introns by hidden Markov models. We show that exons (besides the 
reading frame) have a specific sequential structure in the form of a pattern 
with triplet consensus non-T(A/T)G, and a minimal periodicity of roughly ten 
nucleotides. The periodic pattern is also present in intron sequences, although 
the strength per nucleotide is weaker. Using two independent profile methods 
based on triplet bendability parameters from DNase I experiments and nucleosome 
positioning data, we show that the pattern in multiple alignments of internal 
exon and intron sequences corresponds to a periodic "in phase" bending potential 
towards the major groove of the DNA. The nucleosome positioning data show that 
the consensus triplets (and their complements) have a preference for locations 
on a bent double helix where the major groove faces inward and is compressed. 
The in-phase triplets are located adjacent to GCC/GGC triplets known to have the 
strongest bias in their positioning on the nuclesome. Analysis of mRNA sequences 
encoding proteins with known tertiary structure exclude the possibility that the 
pattern is a consequence of the previously well-known periodicity caused by the 
encoding of alpha-helices in proteins. Finally, we discuss the relation between 
the bending potential of coding and non-coding regions and its impact on the 
translational positioning of nucleosomes and the recognition of genes by the 
transcriptional machinery.