36
Chapter 2
their subunits, whereas RNA molecules are involved in decod-
ing this information into instructions for linking together a
specifi c sequence of amino acids to form a specifi c polypeptide
chain.
Both types of nucleic acids are polymers and are there-
fore composed of linear sequences of repeating subunits. Each
subunit, known as a
nucleotide,
has three components: a
phosphate group, a sugar, and a ring of carbon and nitrogen
atoms known as a base because it can accept hydrogen ions
(
Figure 2–22
). The phosphate group of one nucleotide is
linked to the sugar of the adjacent nucleotide to form a chain,
with the bases sticking out from the side of the phosphate–
sugar backbone (
Figure 2–23
).
DNA
The nucleotides in DNA contain the fi ve-carbon sugar
deoxy-
ribose
(hence the name “deoxyribonucleic acid”). Four dif-
ferent nucleotides are present in DNA, corresponding to the
four different bases that can be bound to deoxyribose. These
bases are divided into two classes: (1) the
purine
bases,
adenine
(A) and
guanine
(G), which have double rings of
nitrogen and carbon atoms, and (2) the
pyrimidine
bases,
cytosine
(C) and
thymine
(T), which have only a single ring
(see Figure 2–23).
A DNA molecule consists of not one but two chains of
nucleotides coiled around each other in the form of a double
helix (
Figure 2–24
). The two chains are held together by
hydrogen bonds between a purine base on one chain and a
pyrimidine base on the opposite chain. The ring structure of
each base lies in a fl at plane perpendicular to the phosphate–
sugar backbone, like steps on a spiral staircase. This base pair-
ing maintains a constant distance between the sugar–phosphate
backbones of the two chains as they coil around each other.
Specifi city is imposed on the base pairings by the
location of the hydrogen-bonding groups in the four bases
(
Figure 2–25
). Three hydrogen bonds form between the
purine guanine and the pyrimidine cytosine (G–C pairing),
while only two hydrogen bonds can form between the purine
adenine and the pyrimidine thymine (A–T pairing). As a
result, G is always paired with C, and A with T. It is this speci-
fi city that provides the mechanism for duplicating and trans-
ferring genetic information.
Table 2–6
Bonding Forces Between Atoms and Molecules
Bond
Strength
Characteristics
Examples
Hydrogen
Weak
Electrical attraction between polarized
bonds, usually hydrogen and oxygen
Attractions between peptide bonds, forming the
alpha helix structure of proteins, and between polar
amino acid side chains, contributing to protein
conformation; attractions between water molecules
Ionic
Strong
Electrical attraction between oppositely
charged ionized groups
Attractions between ionized groups in amino acid
side chains, contributing to protein conformation;
attractions between ions in a salt
Hydrophobic
interactions
Weak
Attraction between nonpolar molecules
and groups when very close to each other
Attractions between nonpolar amino acids in
proteins, contributing to protein conformation;
attractions between lipid molecules
Covalent
Very strong
Shared electrons between atoms
Nonpolar covalent bonds share electrons
equally, while in polar bonds the electrons
reside closer to one atom in the pair
Most bonds linking atoms together to form
molecules
α
2
α
1
β
1
β
2
Figure 2–21
Hemoglobin, a multimeric protein composed of two identical alpha
(
α
) chains or subunits and two identical beta (
β
) chains. (The
iron-containing heme groups attached to each globin chain are not
shown.)
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