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Size of a DNA molecule?

  1. Dec 6, 2007 #1
    "Size" of a DNA molecule?

    Sorry for such a beginner's question in an advanced profile forum:
    In basic school we were taught that a molecule of a substance is "the smallest structure which still retains all the properties of that substance. If you divide it any further, (at least some of) its properties are gone".

    Based on this very simplistic interpretation, is it possible (or meaningful) to define "the minimum size of a DNA molecule"?

    My guess is "no", and if that's right, where is the point where the above simplistic definition of "a molecule" breaks down?, and what should be the proper definition of "a molecule" for complex substances with very variable sizes such as DNA?

  2. jcsd
  3. Dec 6, 2007 #2


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    Well, DNA is composed of base pairs, so the smallest molecule that could reasonably be called "DNA" would be a single base pair.

    - Warren
  4. Dec 7, 2007 #3
    Not exactly related, by fascinating to read:

    The thing about DNA is, just changing around one of the base pairs will change the properties (I surely don't want to have a genetic error and have 2.5 eyes!). It is veyr hard to define, as it IS a large molecule.
  5. Dec 7, 2007 #4

    jim mcnamara

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    Anyway, proteins and nucleic acids are in fact completely different molecules when one component in the chain of amino acids or bases changes. I don't see that as violating the definition of a molecule. Also you may not have learned this yet, but some organic molecules molecules have stereo-isomers. Example: L-glucose and R-glucose are different molecules but have the same number and order of component atoms. They are assembled 3-dimensionally either as left or right handed.
  6. Dec 7, 2007 #5
    As they have said, DNA is made of pairs of nucleotide bases. Each nucleotide is composed of a deoxyribose sugar, a nitrogenous base, and a single phosphate. They differ by having one of 4 different nitrogenous bases, adenine, guanine, cytosine, or thymine.

    If by asking what is the smallest "molecule", do you mean what is the shortest strand that still can code for something?
  7. Dec 7, 2007 #6


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    I think the confusion may be the somewhat "sloppy" use of the word molecule in biology compared with the way it is defined within your chemistry texts. Often in biology, we are referring to a class of molecules, not a specific one. All molecules with certain common structures are contained within the class.

    If it's the other possibility that Chilodonella raised, then you require at least 3 base pairs to encode an amino acid, but actually need more than that in order to have an appropriate start site for transcription to RNA/translation to amino acids/peptides/proteins, so there would be a lower limit to be functional, if function is important in your definition.
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