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The major distinguishing feature of B-DNA is that it has approximately 10 bases for

one turn of the double helix; furthermore a distinctive major and minor groove may

be identified (Fig. 5.5). In certain circumstances where repeated DNA sequences or

motifs are found the DNA may adopt a left-handed helical structure termed Z-DNA.

Thymine

CH 3

CC

OHN

H

CC

N

C

H

H C N

NC

C O

C

HN

CN

H

C N

C

C

Adenine

Cytosine Guanine

H

HNH

CC

O

CC

HHC N

N C

OHN

H

CN

NC

NH

C

N

constant distance between C–1

of deoxyriboses (1.1 nm)

Fig. 5.4Base-pairing in DNA. C in a circle represents carbon at the 1’ position of deoxyribose.

Table 5.1The various forms of DNA

DNA form % humidity Helix direction Base/turn helix Helix diameter (A)

B 92% RH 10 19

A 75% RH 11 23

C 66% RH 9.3 19

Z (Pu-Py)n LH 12 18

Notes:RH, right-handed helix; LH, left-handed helix; Pu, Purine; Py, Pyrimidine. Different forms of

DNA may be obtained by subjecting DNA fibres to different relative humidities. The B form is the

most common form of DNA whilst the A and C forms have been derived under laboratory conditions.

The Z form may be produced with a DNA sequence made up from alternating purine and pyrimidine

nucleotides.

142 Molecular biology, bioinformatics and basic techniques