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854 Nucleic Acid Synthesis

mutagenicbecauseofmisreplicationatthedamagesite. ment to another. Mutations could also be subtle, in-
Replication errors are also corrected by DNA repair. cluding changes of a single base (known as point
Repair involves removal of the DNA damage site in mutation), which include loss or addition of a single
duplex DNA, followed by resynthesis of the damaged base.

strand using the unaffected complementary strand as Nontranscribed strand The complementary strand (5 -

the template. 3 ) of DNA with the same sequence as the RNA tran-
Enhancer elements DNA sequences which activate the scribed from the other (transcribed or template) strand.
expression of genes in an orientation- and position- Nucleosome Smallest repeat unit of chromatin nucleo-
independent fashion. protein, containing 145 bp of DNA wrapped around a
Episome Small extrachromosomal and sometimes self- histone octamer core (2 subunits each of histone H2A,
replicating DNA molecules, including infecting viral H2B, H3, and H4) along with linker DNA of variable
DNA, founded in both prokaryotes and eukaryotes. length. Mild treatment of chromatin with DNase di-
Error-bypassDNApolymerases Anewclassofrecently gests the linker and generates nucleosome fragments
discovered DNA polymerases in both prokaryotes and of different repeat lengths (“ladder”).
eukaryotes which are more tolerant of improper base Okazaki fragments Nascent DNA fragments generated

pairing and may function in maintaining genomic by discontinuous synthesis of the lagging (5 → 3 )

continuity when damaged DNA bases have not been strand in all organisms.
repaired. Operator A small, specific, and often palindromic DNA

Function The intrinsic 3 exonuclease activity of replica- sequence or its repeats cognate to regulated bacterial
tiveDNApolymeraseorpolymerasecomplexesneeded genes. A repressor (or activator) binds the operators to
to excise incorrect deoxynucleotides inserted at the ter- prevent (or activate) transcription.
minus of a growing DNA chain. Ori (origin) Origin of replication in the genome. These
Gene Basic functional unit in the genome which is tran- are unique sequences which bind the replication initi-
scribed to produce messenger RNA, which in turn is ation complex as a prerequisite for primer synthesis.
translated into protein. (Some genes, e.g., those for ri- PCR Polymerase chain reaction.
bosomal and transfer RNAs, are only transcribed and Plasmid Extrachromosomal DNA molecule, usually
not translated.) much smaller than the cell genome. Plasmids are au-
Genome Complete genetic information stored in the nu- tonomously replicated in the cell, utilizing the cellular
cleotide sequence (usually DNA) of an organism, or- replication machinery.
ganelle, or episome. Pol DNA or RNA polymerase.
HMG proteins High mobility group (based on gel elec- Primase Enzyme (sometimes with other accessory pro-
trophoresis) proteins which are associated with chro- teins) which is a component of the DNA replication
matin; a subset of nonhistone chromosomal (NHC) machinery and is needed for synthesis of an oligori-
proteins. bonucleotide primer.
Lagging strand Nascent DNA strand synthesized dis- Promoter Specific DNA sequence usually found at the

continuously by replication of the 5 → 3 template beginning of a gene, which binds the transcriptional
strand. machinery as a prerequisite to transcription initiation
Leading strand Nascent DNA synthesized by continu- from the gene.

ous replication of the 3 → 5 template strand. Replicon Unit of DNA replication in the genome, con-
Mitochondrial genome Multiple copies of the circular taining one ori site. Small genomes of bacteria, plas-
DNA duplex molecule in eukaryotic mitochondria. Be- mids, and viruses have single replicons, while larger
lieved to be a vestigial prokaryotic genome, it is repli- eukaryotic genomes have hundreds or thousands of
cated by a special DNA polymerase (Pol γ ) which, replicons which could be simultaneously or sequen-
along with other proteins required for mitochondrial tially fired for synthesis of different segments of the
DNA replication, is encoded by the nuclear genome. genome. This is necessary to reduce the overall repli-
3
Mutation Change in the genome sequence via the pro- cation time of a genome which is 10 times larger than
cess of mutagenesis, which can occur either spon- a bacterial genome.
taneously due to endogenous reactions or after ex- Repressor Proteins which bind to specific operators and
posure to external mutagens, including radiation and thus negatively regulate gene expression by inhibiting
chemicals. Mutations include large-scale sequence al- transcription.
terations, including deletion or insertion of thousands Reverse transcriptase (RT) Specialized DNA poly-
of DNA base pairs and genomic rearrangement which merase encoded by retroviruses, including the AIDS
could involve translocation of one chromosomal seg- virus (HIV), which utilizes both RNA and DNA