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5’ Nucleic acids are examples of informational molecules—
Base P Direction of that is, large molecules that carry information in the
chain growth
sequence of nucleotides that make them up. The genetic
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information in DNA is contained in the sequence, or order,
Base P in which successive nucleotides occur along the molecule.
1 The bases attached to the sugar give each nucleotide its
Base P
chemical identity. Successive nucleotides along a DNA
strand can occur in any order, so a long molecule could
OH
OH
OH
OH
3’ 3’ contain any of an immense number of possible nucleo-
tide sequences. This is one reason why DNA is an efficient
Base
carrier of genetic information.
P P P
®
PREP FOR THE AP EXAM
OH
®
AP EXAM TIP
P P

You should know what is meant by a ′′ →→ ′′ 3 direction,
5

Pyrophosphate (PP ) i and understand that it applies to nucleic acids, such as
FIGURE 5.5 DNA synthesis dNA and RNA, not other organic molecules, such as
proteins and carbohydrates.
The synthesis of DNA occurs as an incoming nucleotide triphosphate
is added to an existing DNA chain. As the incoming nucleotide is
added, it is joined by the first phosphate group of its triphosphate. Two
phosphates, called pyrophosphate, are released in the reaction. New
nucleotides are added to the 3′-hydroxyl group, so chain growth is Concept Check
always in the 5′-to-3′ direction.
4. Identify the bond that links successive nucleotides
in DNA.
backbone. The other two are released as pyrophosphate, or 5. Describe how genetic information is stored in a
two phosphate groups attached to each other and abbrevi- molecule of DNA.

ated PP , as shown in the figure.
i

5.3 Cellular DNA takes the form Crick set out to build a molecular model of the structure

of a double helix of DNA. To do this, they combined three critical pieces
of information. The first consisted of results from X-ray
Up to now, we have discussed the building blocks of DNA crystallography of DNA carried out by Rosalind Franklin
and how these building blocks connect to one another and Maurice Wilkins, also at Cambridge University. X-ray
to form a long chain. In 1953, using key data and insights crystallography is a technique in which X-rays are passed
obtained by Rosalind Franklin, James Watson and Francis through crystals of a substance and the pattern that results
Crick of Cambridge University announced a description of on the X-ray film provides information about the structure
the three-dimensional structure of DNA. In the cell and even of the substance. Franklin’s results, in particular, suggested
in a laboratory solution, DNA consists of two chains wound that DNA had some sort of helical structure with a simple
around each other to form what is known as a double helix repeating structure all along its length.

because each strand takes the shape of a helix. This discovery The second piece of information consisted of results of
marked a turning point in modern molecular biology, as the experiments carried out by Erwin Chargaff, a biochemist at
structure of DNA revealed a great deal about its function. Let’s Columbia University. Chargaff had shown that DNA from
examine this shape in more detail and how it was determined. many organisms has a characteristic feature: the number of
molecules of the nucleotide adenine (A) always equals the
Double Helix number of molecules of thymine (T), and the number of
With the knowledge of the chemical makeup of the nucle- molecules of guanine (G) always equals the number of mol-
otides and their linkages in a DNA strand, Watson and ecules of cytosine (C).
MODULE 5 Nucleic Acids 81

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