Organic Chemistry

Section 27.7 The Nucleic Acids 045

O

O

CH 2

O−

−O

P

O

O

O−

P

O

O−

O

P

5 ′-end

5 ′

3 ′

3 ′-end

a phosphoester bond

a phosphoester bond

the 3′-OH group attacks

the -phosphorus of the

next nucleotide to be

incorporated into the chain

base

O

base

OH

O

OOP −

O

O

O−

−O

P

O

O

O−

P

O

O−

O

P

base

OH

O

O

O−

−O P

O

O

O−

P

O

O−

O

P

base

OH

O

O

O

O

>Figure 27.3

Addition of nucleotides to a

growing strand of DNA.

Biosynthesis occurs in the

5 ¿¡ 3 ¿direction.

O

O

O

O

O

O

O

O

O

O

O

T A

5 ′

3 ′

3 ′

5 ′

O

C

OH

5 ′

5 ′C

3 ′

OH

3 ′

P

P

T A

P

P

T A

P

P

G C

P

P

G C

P

P

C G

3.4A

Figure 27.4

Complementary base pairing in

DNA. Adenine (a purine) always

pairs with thymine (a pyrimidine);

guanine (a purine) always pairs

with cytosine (a pyrimidine).

(^2) Watson was having difficulty understanding the base pairing in DNA because he thought the bases
existed in the enol form (see Problem 10). When Jerry Donohue, an American crystallographer, in-
formed him that the bases more likely existed in the keto form, Chargaff’s data could easily be ex-
plained by hydrogen bonding between adenine and thymine and between guanine and cytosine.
one strand there is a C in the other strand (Figure 27.4). Thus, if you know the sequence
of bases in one strand, you can figure out the sequence of bases in the other strand.
What causes adenine to pair with thymine rather than with cytosine (the other
pyrimidine)? The base pairing is dictated by hydrogen bonding. Learning that the
bases exist in the keto form (Section 19.2) allowed Watson to explain the pairing.^2
Adenine forms two hydrogen bonds with thymine but would form only one hydrogen
bond with cytosine. Guanine forms three hydrogen bonds with cytosine but would
form only one hydrogen bond with thymine (Figure 27.5). The N¬H  Nand
thymine adenine
sugar
sugar
cytosine guanine
sugar
sugar
O
H
N
H
N H N
N
N
H N
H
O
N
H N N
CH 3 N
N
H
H
N
N
O
O
N
N

Figure 27.5
Base pairing in DNA: Adenine and
thymine form two hydrogen bonds;
cytosine and guanine form three
hydrogen bonds.
[G][C]
[A][T]