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betweenTmand (CþG) content arises because cytosine and guanine form three

hydrogen bonds when base-paired, whereas thymine and adenine form only two.

Because of the differential numbers of hydrogen bonds between A–T and C–G pairs

those sequences with a predominance of C–G pairs will require greater energy to

separate or denature them. The conditions required to separate a particular nucleotide

sequence are also dependent on environmental conditions such as salt concentration.

If melted DNA is cooled it is possible for the separated strands to reassociate,

a process known asrenaturation. However, a stable double-stranded molecule will

only be formed if the complementary strands collide in such a way that their bases

are paired precisely, and this is an unlikely event if the DNA is very long and complex

(i.e. if it contains a large number of different genes). Measurements of the rate of

renaturation can give information about the complexity of a DNA preparation.

Strands of RNA and DNA will associate with each other, if their sequences are

complementary, to give double-stranded, hybrid molecules. Similarly, strands of radio-

actively labelled RNA or DNA, when added to a denatured DNA preparation, will act

as probes for DNA molecules to which they are complementary. This hybridisation of

complementary strands of nucleic acids is very useful for isolating a specific fragment

of DNA from a complex mixture. It is also possible for small single-stranded fragments of

DNA (up to 40 bases in length) termedoligonucleotidesto hybridise to a denatured

sample of DNA. This type of hybridisation is termedannealingand again is dependent

on the base sequence of the oligonucleotide and the salt concentration of the sample.

5.3 Genes and genome complexity

5.3.1 Gene complexity

Each region of DNA which codes for a single RNA or protein is called a gene, and the

entire set of genes in a cell, organelle or virus forms its genome. Cells and organelles

1.3

Absorbance at 260 nm

40

Tm

1.2

1.1

1.0

Degree of denaturation (%)

50

60 80 100

0

Temperature (°C)

Fig. 5.7Melting curve of DNA.

145 5.3 Genes and genome complexity