In addition to the familiar duplex DNA, certain G-rich DNA sequences can fold into a variety of four-stranded structures, which are called G-quadruplexes.

富含鸟嘌呤碱基的DNA序列能够通过鸟嘌呤环的互联作用形成四链螺旋结构，这种结构被称为G-四链体。

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Cambridge University scientists say they have seen four-stranded DNA at work in human cells for the first time.

BBC: 'Quadruple helix' DNA seen in human cells

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This revealed the four-stranded DNA arose most frequently during the so-called "s-phase" when a cell copies its DNA just prior to dividing.

BBC: 'Quadruple helix' DNA seen in human cells