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Encyclopedia of Physical Science and Technology en010k-502 July 16, 2001 16:56

858 Nucleic Acid Synthesis

TABLE I Genomic DNA Characterized in Biology a
Organism Structure Total size (bp) Number of genes Sequence
Bacteriophage Linear, circular 5 ∼ 200 × 10 3 10∼100 Completed for many species
Virus Up to 2 × 10 5 10∼100 Completed for many species
Bacteria E. coli Circular 4.6 × 10 6 ∼4300 Completed
Eukaryote
yeast (S. cerevisiae) Linear 1.4 × 10 7 ∼6000 Completed
Drosophila Linear 1.4 × 10 8 1.4 × 10 4 Partially completed
4
Human Linear 3 × 10 9 4 × 10 to 1 × 10 5 Partially completed
a
As of Feb 2001 the data are to be renewed continuously and are available at the website http://ncbi.nlm.
nih.org/entrez.
complex organization of eukaryotic genomes is a distinc- which postulates that a three-nucleotide sequence in DNA,
tive feature which separates them from the prokaryotes. 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
D. Information Storage, Processing,
a cell needs not only to be preserved and passed on to the
and Transfer
progenycellsduringreplication,anessentialcharacteristic
The central dogma of molecular biology is that informa- and requirement of all living organisms, but also has to
tion is transferred from DNA to RNA to proteins. The be processed and transferred via proteins to the ultimate
proteins (which include the enzymes and structural com- cellular activities, including the metabolism.
ponents of cells) are directly responsible for most cellular Elucidation of the double-helical structure of DNA
activities and functions. The information needed for all lends itself to an elegant but simple mechanism of perpet-
functions of all organisms is stored in the genomic DNA uation of the DNA information during duplication, called
sequence, which contains discrete units defined as genes. semi-conservative replication. In this model (Fig. 2), the
Each gene encodes a protein whose function and activity two strands of DNA separate, and each then acts as the
are determined by its primary sequence. The discovery of template for synthesis of a new daughter strand based on
colinearity of the DNA nucleotide sequence and the amino base pair complementarity and strand polarity. Thus, the
acid sequence of the encoded polypeptide in prokaryotes two strands of the DNA double helix, though not identical
and their viruses led to the discovery of the genetic code 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 poly-

merase. (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.