Fundamentals of Medicinal Chemistry

AGGCAAT

GC

TA

G

A

CGA

TCC TTG G G

G

AT

CT

GC

TA

GGAA C CC

T CCGTTAC G A T C G C T

A G G A A CCC

T A G C G AT C C TTGGG

3' end of parent strand

5' end of daughter strand Leading strand

Lagging strand Okazaki fragments

5' end of
parent strand

3' end of
parent strand

5' end of parent strand

3' end of Okazaki fragments

Parent strand

Figure 1.31 A schematic representation of the replication of DNA. The arrows show the directions of the growth of the leading and lagging strands. Hydrogen bonding can only occur between either A and T or C and G. This means that the daughter strand is, in theory, an exact replica of the parent strand. Reproduced by permission of Prentice Hall fromChemistry for Pharmacists and the Life Sciencesby G Thomas

3 ’end of the parent strand can continue smoothly as the DNA helix continues to

unwind. This strand is known as the leading strand. However, this smooth

growth is not possible for the daughter strand that started from the 5’of the

parent strand. This strand, known as thelagging strand, is formed in a series of

sections, each of which is still grows in the 5’to 3’direction. These sections,

which are known as Okazaki fragments after their discoverer, are joined

together by the enzyme DNA ligase to form the second daughter strand.

Replication, which starts at the end of a DNA helix, continues until the entire

structure has been duplicated. The same result is obtained when replication

starts at the centre of a DNA helix. In this case unwinding continues in both

directions until the complete molecule is duplicated. This latter situation is more

common.

1.6.3 Genes and the human genome project

Each species has its own internal and external characteristics, which are deter-

mined by the information stored and supplied by the DNA in the nuclei of its

cells. The information is carried in the form of a code based on the consecutive

sequences of bases found in sections of the DNA structure. This code controls

the production of the peptides and proteins required by the body. The sequence

30 BIOLOGICAL MOLECULES