Page 376 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Naturally Occurring Polymers—Animals 339

NH 2
N
N
N N
O

HO P O
O
OH

OH
Adenylic acid, adenosine-3 -phosphate (10.8)

The backbone of nucleic acids is connected through the 3′ and 5′ sites on the sugar with the base
attached at the 1′ site. Because the sugar molecule is not symmetrical, each unit can be connected
differently but there is order (also called sense or directionality) in the sequence of this connection
so that phosphodiester linkage between units is between the 3′ carbon of one unit and the 5′ carbon
of the next unit. Thus, nucleic acids consist of units connected so that the repeat unit is a 3′–5′ (by
agreement we consider the start to occur at the 3′ and end at the 5′ though we could just as easily
describe this repeat as being 5′-3′) linkage. Thus, the two ends are not identical—one contains an
unreacted 3′-hydroxyl and the other an unreacted 5′-hydroxyl.
A shorthand is used to describe sequences. Following is a trimer containing in order the bases
cytosine, adenine, and thymine.

NH 2

N
OH
N O
HO P O
O
O
NH 2
N
O N
OH
(10.9)
N
P O N
O O
O
H C
3
O NH
OH
N O
P O
O O

OH

This sequence is described as

p-5′-C-3′-p-5′-A-3′-p-5′-T-3′ or pCpApT or usually as simply CAT.

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