Denaturation and Melting of DNA

FIGURE 10.46 Melting point curves of DNA and RNA (approx). The exact shapes and positions of the curves are dependent upon base compositions. 

Unwound chains can be separated by chromatographic or centrifuging techniques. Single-stranded DNA is more flexible than the usual double-stranded variety and hence denatured solutions exhibit lowered viscosities. They also show less optical rotation. 

Chemical denaturation of the double helix at neutral pH can be effected by addition of urea or formamide. These have the effect of disrupting the H-bonding systems which in turn reduces the inter-base hydrophobic forces which otherwise help to stabilise the double helix. These stabilising forces can also be curtailed by the action of alkali which shifts the bases into enol forms (10.70). 

Single strands of DNA will associate with single-stranded RNA, if their base sequences are complimentary, to give double-stranded hybrid molecules. This has analytical applications (Chapter 14.3). 

The average cell contains at least one kind of t-RNA for each of the 20 amino acids and many thousands of different m-RNA molecules to code for different proteins (see Chapter 11.6). 

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