The central dogma of molecular biology is that information
is transferred from DNA to RNA to proteins. The
proteins (which include the enzymes and structural components
of cells) are directly responsible for most cellular
activities and functions. The information needed for all
functions of all organisms is stored in the genomic DNA
sequence, which contains discrete units defined as genes.
Each gene encodes a protein whose function and activity
are determined by its primary sequence. The discovery of
colinearity of theDNAnucleotide sequence and the amino
acid sequence of the encoded polypeptide in prokaryotes
and their viruses led to the discovery of the genetic code
which postulates that a three-nucleotide sequence in DNA,
called a codon
, is responsible for insertion of a specific
amino acid in the polypeptide chain during its synthesis.
Thus, the information content in the genomic DNA of
a cell needs not only to be preserved and passed on to the
progeny cells during replication, an essential characteristic
and requirement of all living organisms, but also has to
be processed and transferred via proteins to the ultimate
cellular activities, including the metabolism.
Elucidation of the double-helical structure of DNA
lends itself to an elegant but simple mechanism of perpetuation
of the DNA information during duplication, called
semi-conservative replication. In this model (Fig. 2), the
two strands of DNA separate, and each then acts as the
template for synthesis of a new daughter strand based on
base pair complementarity and strand polarity. Thus, the
two strands of the DNA double helix, though not identical
in sequence, are equivalent in information content.
|FIGURE 2 DNA polymerization reaction. (A) According to the base pairing rules, a deoxythymidinetriphosphate
(dTTP) is added at the 3´-OH end of the top strand through a transesterification reaction catalyzed by a DNA polymerase.
(B) Two units of DNA polymerase form a heterodimer complex to carry out replication in a semi-conservative
way. Because the reaction goes only in the 5´→3´ direction, one side (the leading strand) is synthesized continuously,
while the other (the lagging strand) consists of short DNA fragments (Okazaki fragment). DNA replication is initiated
by an RNA primer (waved line) which is synthesized by a primase. There are a number of accessory but essential
proteins besides the polymerase unit.
|FIGURE 3 An RNA polymerase unit
(filled circle), which consists
factors, opens DNA helix (shown as a
synthesizes RNA in the
The intermediate carrier in the transfer of information
from DNA to protein is the messenger RNA (mRNA),
which is copied (transcribed) from only one of the two
strands (Fig. 3), based on base pair complementarity (except
for the presence of U in RNA in the place of T;
Fig. 1C). In the synthesis of both DNA (replication) and
RNA (transcription), the polynucleotide chains are synthesized
by sequential addition of monomeric units (deoxyribonucleotide
for DNA and
ribonucleotides for RNA)
to the 3 ´ end of the growing chain (Fig. 3).
The mRNA is read out by ribosomes, the ribonucleoprotein
complex which functions as the factory for protein
synthesis. The codons are recognized as blocks because
they code for specific amino acids. Thus, the linear
polypeptide sequence is determined by the linear mRNA