Biology 12

G

G

G

G

G

A A A A A A A A

C

C

C

C

C

T

T T T T T T T

3.4 nm

0.34 nm

1 nm

sugar-phosphate “handrails”

phosphate bridge

OH

hydrogen bonds between

nitrogenous bases

end

5 ′ end

5 ′

5 ′

5 ′

5 ′

5 ′

5 ′

5 ′

3 ′

3 ′

3 ′

3 ′

3 ′

3 ′

3 ′

3 ′

3 ′

T

A

G

G

C

C

P

P

P

P P P P P P

P

A

T

Chapter 7 Nucleic Acids: The Molecular Basis of Life • MHR 225

Figure 7.11James Watson (left) and Francis Crick
in 1953 with their model of a DNA molecule.

DNA — The Double Helix

As you probably know, DNA is a thread-like

molecule made up of two long strands of nucleotides

bound together in the shape of a double helix. If

the helix were unwound, the molecule would look

something like a ladder, as shown in Figure 7.12.

The sugar-phosphate “handrails” form the two

sides, while the paired nitrogenous bases form the

rungs. The space between the rungs is 0.34 nm,

while the strand as a whole makes one complete

turn of the helix for every 10 base pairs, or 3.4 nm.

Thus, the structure accounts for the two different

recurring patterns indicated by Franklin’s X-ray

diffraction photograph. The following paragraphs

explain how the nucleotides are joined within each

Figure 7.12The DNA molecule is made up of two chains
of nucleotides wound around each other. The “handrails”
of the molecule are made up of alternating sugar and

phosphate groups, with the phosphate groups serving as

bridges between nucleotides. The nitrogenous bases

protrude at regular intervals into the interior of the molecule.

A DNA double helix B DNA structure