Dictionary of Chemistry [6th Ed.]

synthesis of a new one complemen-

tary to itself. Replication is initiated,

controlled, and stopped by means of

polymerase enzymes.

Döbereiner’s triads A set of triads

of chemically similar elements noted

by Johann Döbereiner (1780–1849) in

1817. Even with the inaccurate
      atomic mass data of the day it was
      observed that when each triad was
      arranged in order of increasing
      atomic mass, then the mass of the
      central member was approximately
      the average of the values for the
      other two. The chemical and physical
      properties were similarly related. The

triads are now recognized as consecu-

tive members of the groups of the

periodic table. Examples are: lithium,

sodium, and potassium; calcium,

strontium, and barium; and chlorine,

bromine, and iodine.

A

• Döbereiner’s original paper

dodecanoic acid (lauric acid)A

white crystalline *fatty acid,

CH 3 (CH 2 ) 10 COOH; r.d. 0.8; m.p. 44°C;

b.p. 225°C. Glycerides of the acid are

present in natural fats and oils (e.g.

coconut and palm-kernel oil).

dodeceneA straight-chain alkene,

Döbereiner’s triads 184

d

hydrogen bond

thymine

(T)

adenine

(A)

O–

O–

P

OOCH

(^2) O
OH
P
O–
OO
CH^2 O
O
P
O–
OO
H CH (^2) O
O
P
O–
O
CH^2
H
O
O
OH H
AT
TA
GC
CG
TA
GC
AT
AT
CG
base
sugar–
phosphate
backbone
hydrogen
bond
3.4 nm
Double helical structure of DNA
Detail of molecular structure of sugar–phosphate
backbone. Each deoxyribose unit is attached to a
phosphate group and a base, forming a nucleotide
CH (^3) O
N
N
HO
NH O N
NHN N
N
OHN
N
H
HN
H
NH
N
N
N
cytosine
(C)
guanine
(G)
sugar–
phosphate
backbone
The four bases of DNA, showing the
hydrogen bonding between base pairs
nucleotide
deoxyribose
bases
DNA