Physics and Radiobiology of Nuclear Medicine

Effects of Radiation

DNA Molecule

The nucleus of the cell is the part most sensitive to radiation and this sen-

sitivity has been attributed to the DNA molecule. To understand the effect

of radiation on the DNA molecule, a knowledge of its structure is essen-

tial. It has a double-helical structure consisting of two strands, which are

like the two rails of a ladder (Fig. 15.5A). The strands are composed of

sugars interlinked by phosphate bonds. The two strands are connected to

each other by rungs made of four bases: thymine (T), adenine (A), guanine

(G), and cytosine (C) (Fig. 15.5B). The bases are bonded to the sugar mole-

230 15. Radiation Biology

A

B

G C

A T

T A

T A

T A

A T

A T

C G

C G

C G

G C

G C

Sugar

Sugar

Sugar

Sugar

Base Base

Base Base

PO 4 PO 4

PO 4 PO 4

Fig. 15.5.A.Double-helical structure of DNA molecule composed of four bases:

adenine (A), guanine (G), thymine (T), and cytosine (C).B.Configuration of a DNA

molecule: strands are formed by sugar molecules bonded by phosphate groups. The

rungs of the ladderlike structure are formed by bases connected to each other by

the hydrogen band (dashed line) and to the sugar molecule on the strands on both

sides.