Bacteria
One of two prokaryotic domains, the other being Archaea.
Bacteriophage
A virus that infects bacteria; also called a phage.
Bacterium
Member of the prokaryotic domain Bacteria.
Chromatin
The complex of DNA and proteins that makes up a eukaryotic
chromosome. When the cell is not dividing, chromatin exists in
its dispersed form, as a mass of very long, thin fibers that are not
visible with a light microscope.
Deoxyribose
The sugar component of DNA nucleotides, having one fewer
hydroxyl group than ribose, the sugar component of RNA
nucleotides.
DNA ligase
A linking enzyme essential for DNA replication; catalyzes the
covalent bonding of the 3' end of one DNA fragment (such as an
Okazaki fragment) to the 5' end of another DNA fragment (such
as a growing DNA chain).
DNA polymerase
An enzyme that catalyzes the elongation of new DNA (for
example, at a replication fork) by the addition of nucleotides to
the 3' end of an existing chain. There are several different DNA
polymerases; DNA polymerase III and DNA polymerase I play
major roles in DNA replication in prokaryotes.
Double helix
The form of native DNA, referring to its two adjacent antiparallel
polynucleotide strands wound around an imaginary axis into a
spiral shape.
Euchromatin
The less condensed form of eukaryotic chromatin that is
available for transcription.
Helicase
An enzyme that untwists the double helix of DNA at the
replication forks, separating the two strands and making them
available as template strands.
Heterochromatin
Eukaryotic chromatin that remains highly compacted during
interphase and is generally not transcribed.
Histone
A small protein with a high proportion of positively charged
amino acids that binds to the negatively charged DNA and plays
a key role in chromatin structure.
Lagging strand
A discontinuously synthesized DNA strand that elongates by
means of Okazaki fragments, each synthesized in a 5'3' direction
away from the replication fork.
Leading strand
The new complementary DNA strand synthesized continuously
along the template strand toward the replication fork in the
mandatory 5'?3' direction.
Mismatch repair
The cellular process that uses specific enzymes to remove and
replace incorrectly paired nucleotides.
Nuclease
An enzyme that cuts DNA or RNA, either removing one or a few
bases or hydrolyzing the DNA or RNA completely into its
component nucleotides.
Nucleoid
A dense region of DNA in a prokaryotic cell.
Nucleoid region
A dense region of DNA in a prokaryotic cell.
Nucleosome
The basic, bead-like unit of DNA packing in eukaryotes,
consisting of a segment of DNA wound around a protein core
composed of two copies of each of four types of histone.
Nucleotide excision repair
A repair system that removes and then correctly replaces a
damaged segment of DNA using the undamaged strand as a
guide.
Okazaki fragment
A short segment of DNA synthesized away from the replication
fork on a template strand during DNA replication, many of which
are joined together to make up the lagging strand of newly
synthesized DNA.
Origin of Replication
Site where the replication of a DNA molecule begins, consisting
of a specific sequence of nucleotides.
Phage
A virus that infects bacteria; also called a bacteriophage.
Primase
An enzyme that joins RNA nucleotides to make the primer using
the parental DNA strand as a template.
Primer
A short stretch of RNA with a free 3' end, bound by
complementary base pairing to the template strand, that is
elongated with DNA nucleotides during DNA replication.
Pyrimidine
One of two types of nitrogenous bases found in nucleotides,
characterized by a six-membered ring. Cytosine (C), thymine (T),
and uracil (U) are pyrimidines.
Radioactive isotope
An isotope (an atomic form of a chemical element) that is
unstable; the nucleus decays spontaneously, giving off detectable
particles and energy.
Repetitive DNA
Nucleotide sequences, usually noncoding, that are present in
many copies in a eukaryotic genome. The repeated units may be
short and arranged tandemly (in series) or long and dispersed in
the genome.
Replication fork
A Y-shaped region on a replicating DNA molecule where the
parental strands are being unwound and new strands are being
synthesized.
Semiconservative model
Type of DNA replication in which the replicated double helix
consists of one old strand, derived from the old molecule, and
one newly made strand.
Single-strand DNA-binding protein
A protein that binds to the unpaired DNA strands during DNA
replication, stabilizing them and holding them apart while they
serve as templates for the synthesis of complementary strands of
DNA.
Telomerase
An enzyme that catalyzes the lengthening of telomeres in
eukaryotic germ cells.
Telomere
The tandemly repetitive DNA at the end of a eukaryotic
chromosome's DNA molecule that protects the organism's genes
from being eroded during successive rounds of replication. See
also repetitive DNA.
Template strand
The DNA strand that provides the pattern, or template, for
ordering, by complementary base pairing, the sequence of
nucleotides in an RNA transcript.
Topoisomerase
A protein that breaks, swivels, and rejoins DNA strands. During
DNA replication, topoisomerase helps to relieve strain in the
double helix ahead of the replication fork.
Transformation
(1) The conversion of a normal animal cell to a cancerous cell. (2)
A change in genotype and phenotype due to the assimilation of
external DNA by a cell. When the external DNA is from a
member of a different species, transformation results in
horizontal gene transfer.
X-ray crystallography
A technique used to study the three-dimensional structure of
molecules. It depends on the diffraction of an X-ray beam by the
individual atoms of a crystallized molecule.
antiparallel
Referring to the arrangement of the sugar-phosphate backbones
in a DNA double helix (they run in opposite 5' ---> 3' directions.)
virus
An infectious particle incapable of replicating outside of a cell,
consisting of an RNA or DNA genome surrounded by a protein
coat (capsid) and, for some viruses, a membranous envelope.