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Nucleic Acid Synthesis 855

template. It is responsible for propagation of retro- termediate. The enzyme acts as a swivel for rotating
viruses via synthesis of a proviral DNA intermediate. DNA strands around each other.
Ribonucleotides Monomeric units of RNA, namely, Trans-acting factors Proteins that bind to specificDNA
adenylic (AMP), guanylic (GMP), cytidylic (CMP), sequences (cis elements) in genes and regulate tran-
and uridylic (UMP); the ribose sugar moiety of each scription positively or negatively.
contains a 2 -OH. Transcribed strand The 3 → 5 DNA strand utilized by

Ribosome Protein synthesis factory consisting of two dif- RNA polymerase as its transcriptional template.
ferently sized subunits of ribonucleoprotein complexes Transcriptional activator Trans-acting proteins which
with several active centers. It travels along mRNA and enhance transcription and, thus, the level of specific
reads triplet codons for individual amino acids which proteins.
are brought in by transfer RNAs via base pairing with Transcription unit Discrete segment of DNA, corres-
cognate anticodon sequences in these RNAs. Protein ponding to one or more genes, which is utilized as
synthesis occurs on the ribosome to which the growing a template by RNA polymerase. In prokaryotes, the
polypeptide chain remains attached. transcription unit is called an operon.
RNA Ribonucleic acid: linear copolymers usually of four Translation Synthesis of a protein, directed by mRNA
ribonucleotides. Three major types of RNA are syn- molecules on ribosome.
thesized in the cell: ribosomal RNA (rRNA), the ma-
jor component of ribosomes; transfer RNA (tRNA),
the adaptor for protein synthesis; and messenger RNA NUCLEIC ACIDS are involved in the most fundamen-
(mRNA), which is required for information transfer. tal processes of life. Their maintenance and production
Other small RNAs with specialized functions are also are essential in all living organisms. The hallmark of the
synthesized in small amounts in both prokaryotic and biosphere is diversity of biological processes, even among
eukaryotic cells. members of the same genera, e.g., bacteria. Each organism
RTPCR Reverse transcript polymerase chain reaction. may have some unique features in regard to nucleic acid
Modification of the PCR method to amplify RNA, composition, structure, and metabolism. Thus, studies on
which involves generation of a complementary DNA nucleic acid synthesis constitute a huge topic of research
molecule from RNA (by reverse transcriptase) which on which thousands of research articles are published each
is then used in PCR. year. Therefore, it is impossible to cover all aspects of nu-
Telomerase A special eukaryotic DNA polymerase that cleic acid synthesis in this short article. Our goal is to
adds a repeat sequence to chromosome termini without present a broad overview of the key and general features
a template. of structure, synthesis, and processing of the various types
Telomere Terminal region of a linear chromosome, con- of nucleic acids. We have limited our discussion mostly to
taining partial single-stranded DNA and repeat se- bacteria, specifically Escherichia coli, and to mammals,
quences of short oligonucleotides. Its loss could cause mostly humans and mice. Most of our current knowledge
chromosome fusion and rearrangement. has been derived from the studies of those organisms.
Template-independent poly(A) polymerase A temp- We have also provided appropriate references, which
late-independent RNA polymerase which catalyzes are mostly recent reviews. The readers should be able to
formation of AMP containing homopolymers up to peruse these for in-depth knowledge of the topics which

several hundred monomers at the 3 termini of nascent are covered only superficially here. Finally, we have in-
RNA molecules. The poly(A) tail promotes transport cluded a glossary at the beginning of this article which
of mRNA from the nucleus, enhances its stability, and lists common acronyms and short descriptions of key pro-
is necessary for translation. cesses and phenomena.
Terminator Specific sequence found at the end of genes
for termination of transcription due to release of RNA I. STRUCTURE AND FUNCTION
and RNA polymerase.
Topoisomerase Enzymes which alter topologically con- OF NUCLEIC ACIDS
strained DNA, including circular DNA, by changing
A. Basic Chemical Structure
the linking number. Topoisomerase I changes the link-
ing number one at a time and does not require an exter- The basic information for all activities in living systems,
nal energy source. Topoisomerase II changes the link- at least on our planet, is stored ultimately in nucleic acids,
ing number two at a time and generally requires ATP. namely, deoxyribonucleic (DNA) and ribonucleic (RNA)
The linking number is changed by transient breakage acids. Except for certain viruses, DNA is the universal
and rejoining, with an enzyme-DNA covalent bond in- genetic material (Fig. 1). The chemical structures of basic