  In this paper, we review the literature on statistical long-range correlation
in DNA sequences. We examine the current evidence for these correlations, and
conclude that a mixture of many length scales (including some relatively long
ones) in DNA sequences is responsible for the observed 1/f-like spectral
component. We note the complexity of the correlation structure in DNA
sequences. The observed complexity often makes it hard, or impossible, to
decompose the sequence into a few statistically stationary regions. We suggest
that, based on the complexity of DNA sequences, a fruitful approach to
understand long-range correlation is to model duplication, and other
rearrangement processes, in DNA sequences. One model, called
``expansion-modification system", contains only point duplication and point
mutation. Though simplistic, this model is able to generate sequences with 1/f
spectra. We emphasize the importance of DNA duplication in its contribution to
the observed long-range correlation in DNA sequences.
