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I know that ssRNA molecule can fold over itself (e.g. in t-RNA). Can DNA do the same? Is there any example of this in nature?

Why is this phenomenon more common in RNA than in DNA?

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Are you talking about single stranded DNA? –  terdon Jul 28 '13 at 15:25
    
You may also be interested in DNA supercoiling. –  terdon Jul 28 '13 at 15:39
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up vote 6 down vote accepted

DNA can adopt secondary structures like RNA, the main difference is that DNA is usually present as double-stranded DNA while RNA is in most cases present as single-stranded RNA. Double-stranded DNA won't easily adopt any other conformation than the well-known double helix as this one is more stable than possible structures each single strand could adopt on its own.

One example that occurs in nature are G-Quadruplexes which for example occur in telomeres.

There are also artificially created DNA enzymes (also called DNAzymes or deoxyribozymes) that adopt tertiary structures like ribozymes do. But there are no known DNA enzymes that occur in nature as far as I know.

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Could you add a link for the DNA (deoxy)-ribozymes? Sounds cooks and I had not heard of them. –  terdon Jul 28 '13 at 15:38
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@terdon I've added a Wikipedia link. The first one was created by Ron Breaker (who's more famous for having found most of the riboswitches we know so far), if you're looking for some newer papers on this I'd look for Scott Silverman, he has written a few reviews on the topic. –  Mad Scientist Jul 28 '13 at 15:42
    
Good answer; might benefit from mentioning ssDNA viruses as the most common natural occurrence of ssDNA, as well as other dsDNA conformations (A, B, Z) –  Armatus Jul 28 '13 at 16:00
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