Biology 12

226 MHR • Unit 3 Molecular Genetics

individual strand, how the two strands are bound

together in the double helix, and how the molecule

as a whole remains stable.

Figure 7.12 shows both the primary and secondary

structure of the DNA molecule. On each strand, the

nucleotides are joined to form a long chain. The

5 ′carbon of the pentose (five-carbon) sugar of one

nucleotide is connected to the 3 ′hydroxyl group

on the next, with the phosphate group serving as a

bridge between the two nucleotides. All the

phosphate bridges have the same orientation, so

each strand of DNA also has a specific orientation

or directionality, which is opposite in direction to

the other strand of the double helix. Thus each

DNA strand (and any severed fragment of a DNA

strand) has a 5 ′and a 3 ′end. By convention, the

sequence of nucleotides along a strand of DNA is

always read in the 5 ′to 3 ′direction.

As Figure 7.12 also shows, the nitrogenous

bases protrude at regular intervals from the sugar-

phosphate handrails into the interior of the DNA

molecule. One of the challenges facing Watson and

Crick was to determine how the bases could be

arranged in such a way that the distance between

the two handrails remained constant. They knew

that the four bases fell into two different categories.

Adenine and guanine are derived from the family

of nitrogenous compounds known as purines,

which have a double ring structure. Thymine and

cytosine are derived from pyrimidines, which have

a single ring structure. Watson and Crick hit upon

the idea that if a purine always bonded with a

pyrimidine, the base pairs would have a constant

total width of three rings.

It was not until after they had been experimenting

with models for some time that Watson and Crick

examined the molecular structure of the bases

themselves. When they did so, they discovered

that the structure of the bases allows only certain

complementary base pairings: Adenine (A) can

only form a stable bond with thymine (T), and

cytosine (C) with guanine (G). The complementary

bases are linked by hydrogen bonds, as shown in

Figure 7.13. These pairings provided for the

constant width of the molecule and, equally

importantly, also supported Chargaff’s rule. Thus,

wherever an A nucleotide appears on one DNA

strand, a T must appear opposite it on the other,

and wherever a C nucleotide appears on one

strand, the other strand will have a G nucleotide.

Figure 7.13Complementary base pairing in DNA. C-G pairs

are joined by three hydrogen bonds, while A-T pairs are

joined by two hydrogen bonds.

In all, three kinds of forces contribute to the

molecular stability of the DNA molecule. First,

the phosphate bridges link the sugar-phosphate

handrails. Second, hydrogen bonds between base

pairs keep the two strands together in the helix.

Finally, hydrophobic and hydrophilic reactions

cause the bases to remain on the inside of the

molecule and the handrails to face out into the

watery nucleus of the cell.

As illustrated in Figure 7.14, the two strands of

DNA that make up each double helix are not

identical but rather complementary to each other.

The strands are also antiparallel— that is, the

phosphate bridges run in opposite directions

in each strand. This means that the end of each

double-stranded DNA molecule contains the 5 ′end

of one strand and the 3 ′end of the other. These

two properties have important implications for

DNA replication and protein synthesis, as

discussed later in this chapter and in Chapter 8.

You will have a chance to test your

understanding of the composition of DNA and apply

Chargaff’s rule in the Thinking Lab that follows.

If you could arrange all the DNA strands contained in

your body end to end, their total length would stretch

2 × 1010 km. This is well over 100 times the distance

between Earth and the Sun.

BIO FACT

CH 3

N

NH

H

O

N

N

N

NH

N

O sugar

Adenine (A) Thymine (T)

sugar

N

OH

H

N

N

N

N

NH

N

sugar O

Cytosine (C) Guanine (G)

sugar

NH

H