MadSci Network: Molecular Biology
Query:

Re: How does DNA unravel?

Date: Tue Apr 13 16:35:47 1999
Posted By: Raymond Cheong, Undergraduate, Chemical Engineering, University of Maryland
Area of science: Molecular Biology
ID: 916101426.Mb
Message:

Hello Vic,

Your question was "How does DNA unravel and reravel?" As you probably know, DNA contains genetic information, so knowing how to access this information through DNA winding and unwinding is a very important question in biology and biochemistry.

A DNA double helix is made of two strands of DNA that are "wound" together. These strands have complementary base pairs (A,G,T,C), with A matching a T and G matching a C. Base pairs interact through hydrogen bonds, which are not full covalent bonds, but are strong intermolecular forces. Base pairs T and A are held together by two hydrogen bonds and G/C are held together by three hydrogen bonds. You can see that in a long strand of DNA, there is much hydrogen bonding, which helps hold the DNA in a wound helix.

Because of the numerous interactions, double-stranded DNA is very stable. For this reason, DNA raveling, or more technically, "annealing" or "hybridization", is spontaneous. So, if two complementary single strands of DNA are put in the same solution, double-stranded DNA spontaneously forms.

Temperature is one factor that can cause DNA to unravel. A technical term for this is "denaturation" or colloquially, "DNA melting" (i.e. separates, not really melting). "Under approximately physiological conditions, [DNA melting temperature] usually lies in a range of 85-95 C" (p.58). This is almost the boiling point of pure water! Human body temperature is approximately 37 C, so clearly DNA tends to stay double-stranded in the cell. Another factor that affects DNA melting is salt or ion concentration.

Now, here's the catch: if DNA is usually double-stranded in the cell, how can functions like DNA replication and gene transcription (first step in making proteins) occur? The cell has enzymes and proteins that can interact with DNA and unwind portions of the helix. Exactly how these proteins work is the subject of much current research.

There are other ways to exploit the fact that DNA "ravels" spontaneously. Techniques that you may have heard of, such as PCR and so-called "DNA fingerprinting", rely on this fact. It has even been used for "DNA computing".

I hope this gives you a taste of the beauty and power of Nature's DNA structure. Stay curious and if you have more questions, don't hesitate to ask!

Your MAD Scientist,
Raymond Cheong

Reference: Lewin, Benjamin. Genes, Third Edition. New York: John Wiley & Sons, 1987.


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