Proteins and Nucleic Acids: Information Carriers
Information Storage: DNA and RNA
DNA is the most important of the nucleic acids
because it is the ultimate repository of all genetic information.
It is a long-chain polymer of D-deoxyribose (right), which differs
from D-ribose in having its -OH group at the 2` position replaced
by -H. The polymer connection is made by esterifying a phosphate
group with the 5` hydroxyl of one sugar molecule and the 3`hydroxyl
of the next. The resulting polymer chain has a "sense"
or a direction, with a 5` end and a 3`end as shown by the arrow
(right). Ribonucleic acid is derived from a similar
polymer, but uses D-ribose instead of D-deoxyribose.
In both DNA and RNA, the 1` carbon of each sugar ring is covalently
bonded to one of four purine or pyrimidine bases: A, C, G, or T
for DNA, and A, C, G, or U for RNA. (T differs from U only by an
extra methyl group on the six-membered ring.) Genetic information
is coded by the sequence of bases along a strand of DNA or RNA,
with three consecutive bases containing the code for one amino acid.
The three-base sequence for one amino acid is called a triplet codon.
With a choice of four different bases at each of three positions,
43 = 64 different codons are possible. Because only 20
amino acids are coded, there obviously must be redundancy within
the system, with the same amino acid represented by more than one
codon. This redundancy is inevitable, since a two-base codon scheme
would have permitted only 42 = 16 different amino acids
to be coded. Three of the 64 codons are used for "punctuation,"
to tell the polypeptide chain when to stop, and the other 61 represent
individual amino acids. Right: The backbone of DNA is a long polymer of alternating
phosphates and deoxyribose molecules esterified at the 3` and 5`
positions of the sugar.