Unsustainable DNA molecule when unfolded to be in a line

[DaTario]

Main Question or Discussion Point

Hello,

Some years ago I have heard from the brazilian physicist, Constantino Tsalis, that the DNA molecule, if unfolded and made to lay in a line, will be close to one meter long. But he said that thermodynamically this configuration could be taken as an impossibility, for no such long and linear molecule could exist for long time. Is there any clear principle in thermodynamics that put the things in this terms?

Best Regards,

DaTario

 

[Bystander]

close to one meter long.

Numbers that get thrown around for DNA run on the order of magnitude of billions of base pairs. Dimension of a single unit at one nm isn't too bad (ten atoms). That works out to an unraveled length of the order of one to couple meters.
Base units (including the ribose chains) run 25-30 atoms each. 3 x10¹⁰ times three degrees of freedom per atom is a heat capacity of 10¹¹k_B. Bond energies for organic compounds are of the order of four to five hundred kJ/mol. That works out to a maximum chain length of a few hundred atoms before there is sufficient energy stored in two sections of the chain to pull bonds apart if the situation is such that bond strength is the only thing holding the chain together.

 

[DaTario]

Sorry, but are you considering that the weight of the chain is the responsible for the breaking?

best wishes,

DaTario

 

[Bystander]

that the weight of the chain is the responsible for the breaking?

Not the weight, but the thermal energy in some length of the chain is sufficient to break bonds. Or, the bond is of insufficient strength to play "crack the whip" with the lengths of the chain to either side.

 

[Nugatory]

If you try imagining the scale that we're talking about, a meter-long chain of nanometer-sized units... That's like a single strand of overcooked angel-hair spaghetti several hundreds of kilometers long.

 

[Bystander]

overcooked angel-hair spaghetti several hundreds of kilometers long.

And King Kong trying to use it for a jump rope. Perfect analogy.

 

[DaTario]

Ok, thank you a lot to both. The analogy was very nice.

Best Regards,

DaTario