Chemistry - A Molecular Science

Chapter 13 Organic Chemistry

O

H OH

H H

H

HCH^2 OP

O

O

O

O

HH

H

H

HCH^2 OP

O

O

O

O

HH

H

H H

O

sugar

(deoxyribose)

N-containing phosphate

Base

N-containing

Base

N-containing

Base

Figure 13.38 Three residues of a DNA molecule Each nucleotide contains the sugar deoxyribose,

a phosphate group, and one of four N-containing

bases. In the above, three deoxyribose units

are connected by two phosphate groups. The N-

containing base can be different in each.

The N-containing bases interact with one

another by hydrogen bonding, but the

interactions are between specific pairs of bases only. The bases that interact with one another are called

base pairs

. Interactions between base pairs is specific due to the

structures of the bases. As shown in Figure 13.39,

Adenine and

Thymine form a base pair

(A

• T

), as do

Cytosine and

G

uanine (

C-

G). It is the sequence of these bases along the

DNA backbone that forms the genetic code. Thus, who we are is spelled out using an alphabet containing only four

different chemical letters (

A,

T,

C and

G). The alphabet

may be small, but the words are not: the human

cell contains about three billion base pairs!

The biological importance of relatively weak

interactions, such

as hydrogen bonds and

dipole-dipole interactions, cannot be overemphasized

.

The secondary structure of DNA, shown in Figure 13.40, is known as the

double

helix

. The backbone of each strand (shown as ribbons in the figure) is composed of the
phosphate groups of the nucleotide residues.

The nitrogen containing base of each residue

is matched by its base-pair partner in the other strand. The strong hydrogen bonding between the base pairs maintains the double helix.

N

N

N

N

N

N

N

O O

H H

H N

N

N

N

O N

H H

H

G

N

N

N O

H

H

backboneA DN

DNA backbone

A

T C

Figure 13.39 Hydrogen bonds responsible for DNA double helix The two units in each base pair hydrogen bond specifically with one another. The genetic code is carried by the sequence of the base pairs along the DNA backbone. This base pair specificity allows DNA to reproduce strands that are identical to the original strands. A = adenine, T = thymine, G = guanine, and C = cytosine.

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Carolina

State

University