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chapter 23

Structure and Properties of DNA

The

nucleoside

unit

Adenine

N'

I

NHj

I

C

6

deoxyadenosine

diphosphate (ADP or dADP)

- Adenosine triphosphate or -

deoxyadenosine

triphosphate (ATP or dATP)

Ribose (X = OH)

or

Deoxyribose (X = H)

FIGURE 23-1

Structure of the nucleotide in DNA and RNA. The base adenine is used as an example. See Table 27-1 for a complete

nomenclature of nucleosides and nucleotides.

A base linked to a sugar is called a

nucleoside;

a

base linked to a sugar linked to a phosphate is called a

nucleotide

or a nucleoside phosphate. The nucleotides in

nucleic acids are joined to one another by a second phos-

phodiester bond that joins the 5'-phosphate of one nu-

cleotide to the 3'-OH group of the adjacent nucleotide.

Such a doubly esterified phosphate is called a

phosphodi-

ester group

(Figure 23-2).

Base Pairing and Base Composition

The molar content of the four bases in DNA (called the

base composition) always satisfies the equalities [A] =

[T] and [G] = [C] where [ ] denotes molar concentration.

These equalities arise because of the base pairing rules in

FIGURE 23-2

Structure of a phosphodiester group. The vertical arrows show the bonds in

the phosphodiester group that are free to rotate. The horizontal arrows

show the N-glycosidic bond about which the base can rotate freely. A

polynucleotide consists of many nucleotides linked together by

phosphodiester groups.