Sunday, 20 July 2008

Why is DNA double-stranded, but RNA single-stranded?

When we refer to DNA as being 'double-stranded', we mean that it consists of two strands of DNA bound together. These strands can be separated from each other either temporarily (e.g. when transcribing a gene into RNA) or permanently (e.g. when duplicating genetic material prior to mitosis), and each can act as a template for the synthesis of a complementary nucleic acid (either DNA or RNA), using the original strand as a template.
So why isn't RNA double-stranded?

Actually, a better question is to ask why DNA is double stranded in the first place. Many people assume that being double-stranded allows for it to be copied, but that explanation doesn't hold water. It could just as well be single-stranded - both RNA and DNA are only copied from a single strand of DNA anyway. In fact, it would save on the whole 'unzipping' process that DNA has to undergo just to present a single strand to RNA or DNA polymerase*.

No, a better answer comes from understanding that DNA is absolutely frantic in its efforts to keep itself from undergoing mutations. Mutations are generally bad things (Dawkins once famously said that there are many more ways of being dead than they are of being alive), and a zero-mutation rate is evolutionarily favoured. DNA, not RNA, is the ultimate genetic repository of information, and so you would expect it to be fiercely guarded. In this regard, being double-stranded helps in at least two ways.

Firstly, the 'information' part of DNA is the nitrogenous base, as opposed to the pentose sugar or the phosphate residues. In a single-stranded molecule, this important part would be exposed to the cellular environment, providing more opportunity for it to be mutated by the various chemicals there. In a double-stranded configuration, however, the two nitrogenous bases are locked within the complex, facing each other in the centre of the molecule. This organisation helps to safeguard them from local mutagens.

Secondly, having two complementary strands facing each other fundamentally means having two copies of the same thing placed right next to each other. This allows for proof-reading. George C. Williams summarised this beautifully in this pithy passage** (recall that adenine [A] on one strand should always bind to a thymine [T] on the complementary strand, and visa versa; likewise cytosine [C] always binds to guanine [G], and vice versa):

[Imagine a] gene containing the sequence CCAXT. The X cannot possibly be right because, whatever it is, it is not one of the expected DNA components. Fortunately, all we need to do here is to consult the complementary strand of the DNA. If it is GGTCA, we immediately know that the X should be replaced by a G.

In this way, mutations may be corrected or at least limited. (Actually, the proof-reading exercise and mechanism is not usually so simple as the above, but that doesn't matter for the present purposes.) And the proof is in the pudding. When you compare viruses that use DNA as the repository of their genetic material with those that use RNA, it can be seen that RNA has a higher mutation rate than the more robust and correctable DNA.

When used mainly as an intermediary between genes and proteins (rather than as an ultimate repository, like with the viruses above), RNA doesn't need this elaborate double-stranded structure. It is rather unlikely that the mRNA strand will mutate before it carries out its job anyway, but if it should, a perfect new one could always be synthesised from the DNA again. More importantly, the chemical inertness that double-strandedness grants to DNA would be a hindrance in the case of RNA, which relies on its ability to fold and contort somewhat (a little like a protein) to accomplish some of its tasks.

* the enzymes that make RNA or DNA from a single strand of 'parent' DNA, respectively
** The quote is from Williams' excellent book, "The Pony Fish's Glow" (1997)

32 comments:

  1. nice. helped me alot

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  2. Yes, it is a good reason for DNA to be double and RNA as single strand, i want to add a point, this DNA contains thymine instead of uracil(RNA), this might give the enzymes of replicatory and translation pathway to recognise its parent strand..

    please let me knoe the details of it,.


    Thanking you.
    Khalander

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  3. its really autheintic proof
    i wanna ask 1 thng tht why there s 2 H-bond b/w adinine n thymine while 3 H-bonds b/w guanine n cytocine
    THANKS, MAIRA

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  4. Hi Maira,

    To DNA, the number of hydrogen bonds don't matter; rather, it is the ability of the bases to form ANY hydrogen bonds that is at issue. T can only form hydrogen bonds with A, and C can only form hydrogen bonds with G. Any other form doesn't work, and pairing can't take place.

    As to exactly why there are three H-bonds in the G-C interaction but 2 H-bonds in the A-T interaction, the reason is quite complicated, and has to do with the three dimensional shapes of the various bases, as determined by their molecular makeup. In the A-T interaction, there simply isn't an opportunity for a third H-bond to form. Have a look at this picture to see why.

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  5. Can you please clarify - DNA sense anti-sense strand both are involved in DNA replication as per my understanding.

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  6. Thanks for the post!

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  7. Yeah. Perhaps a clarification has to be made. both the strands can be used as template for DNA replication, but only the 3'-5' (anti-sense) is used during transcription. the reason being RNA polymerase only reads from the 3'-5' DNA strand and synthesize RNA in the 5'-3' direction.

    do correct me if i am wrong. =)

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  8. why dna isnt in the form of ladder or not straight??

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  9. why does dna form a double helix what forces are responsible for the structure of dna??? plzz hurry ans..tomorrow is my presentation...

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  10. thank you so much! this helped me tremendously with my AP Biology homework.

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  11. why is DNA a double stranded (double helix) instead of single stranded?

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  12. cool helped me with some of my biology homework

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  13. Thank You.......was really helpful. I have a question in this regard. Does RNA exist as dsRNA in a cell normally? Is keeping the genome intact,the sole purpose of DNA being double stranded? Kindly explain

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  14. thanks for the post.. its help me a lot

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  15. Awesome post. Eukaryotic biochemistry is complex. It is no wonder natural selection took billions of years to reach this stage of life.

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  16. very helpful and good at describing the reason WHY!

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  17. yeah, understand now why dNA is doubled stranded. There is an explanation for everything.

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  18. Sudheesh.A.P1 April 2012 18:27

    still this doesnt give an explanation why RNA is single stranded?
    and why uridine is found in RNA and not in DNA??

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  19. Sudheesh.A.P1 April 2012 18:28

    can any one answer this??

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  20. Sudheesh, I think it does answer the first question. Read the final paragraph again.

    Re: the second question, have a look here.

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  21. for the reason number one,'safeguard'can also be acted if it is present as triple or more.....
    if anyone has answer....

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  22. Thank you for being clear.

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  23. can anyone tell me why DNA is designated as acid(deoxyribonucleic acid)

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  24. sir, I'm still confusd bout wat's da reason of single strandedness of RNA . HOW dis structure stable ? nd is dis a GENITIC MATERIAL or not ?

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  26. RNA do not exist as single stranded because:

    RNA in a cell is produced only by transcription and no other means. Remember that only one strand of DNA gets transcribed. So obviously only one RNA strand gets synthesized.

    one more possible explanation would be:
    The complementary strand of DNA is synthesized by DNA dependent DNA polymerase (during replication). And no such RNA dependent RNA polymerase exist in the nucleus to synthesize a complementary RNA strand.

    Also dsRNA is of no biological significance as,
    # It cannot pass the nuclear pore. (I hope)
    # It cannot be translated by the ribosomes (basic principle of antisense RNA therapy)


    However RNA can form intra-strand base pairing as in tRNA etc.

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  27. correct me if I were wrong..!!

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