Fundamentals of Medicinal Chemistry

P O

−O

O

N −O

N

N N

N

H H

O

H

N

O

N

Uracil

residue

Ribose

residue

Ribose

residue Adenine

residue

(b)

Anticodon

A

C

5' Terminal C

Anticodon loop

Variable loop

U

3' Terminal, amino

acid attached here

Stem

Hydrogen

bonds (thick

lines)

(a)

Figure 1.33 (a) The hydrogen bonding between uracil and adenine. (b) The two dimensional

cloverleaf representation of the structure of transfer RNA (tRNA) showing the hairpin loops in

the structure

C

GT

A

G

A

T

C

C

G

C

A

CT

C

G

C

G

A

G

C

AT

T

A

T

A C

G

C

GA

G

TA

C

C

G

T

A

C

G

C

C U

T

G U

G

T

AC

G G

G

A

G

A

ACT C

A

A

T

G

C

C

G

T T

AA

T

A

C

TAA

C G T A G C G C T

C

C

T

T

CAA

U

3' end. The new

RNA strand grows

in this direction

3' End

5' End

5' End

3' End

5' End of the

new RNA

strand

DNA double

helix strands

DNA parent strand

Figure 1.34 A schematic representation of a transcription process. Reproduced from G Thomas,

Chemistry for Pharmacists and the Life Sciences, 1996, by permission of Prentice-Hall, a Pearson

Education Company

bonding is catalysed by enzymes known as RNA polymerases. Since only

complementary base pairs can hydrogen bond, the order of bases in the new

RNA strand is determined by the sequence of bases in the parent DNA strand.

In this way DNA controls the genetic information being transcribed into the

RNA molecule. This information is in the form of a series of exons and

introns complementary to those found in the parent gene. The strands of

DNA contain start and stop signals, which control the size of the RNA molecule

32 BIOLOGICAL MOLECULES