Glossary of Biotechnology Terms
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- complementary DNA (cDNA): a DNA
sequence which was produced from mRNA by
reverse transcription. A cDNA
is so-called because its sequence is the
complement of the original mRNA sequence. However, when
double-stranded cDNA is synthesized,
it contains both the original sequence and its complement.
- cDNA library: a collection
of cDNA's, each of which has been inserted in
a DNA vector (e.g. a circular DNA plasmid) and replicated
in a bacterium such as E. coli. The bacteria maintain
a ready pool of the cDNA's and can be cultured to make copies of
the library for many experiments. A population of bacteria
containing a single inserted cDNA is called a clone. To
extract a cDNA from the library, the insert from the corresponding
clone can be amplified using PCR primed off the
vector sequences connected to the ends of the cDNA.
- complement: The complement of a
nucleic acid sequence replaces each base by its complementary base:
adenine (A) by thymidine (T), cytosine (C) by guanine (G), and vice
versa. In RNA, adenine is paired not with thymidine but with uracil
(U). By convention, DNA and RNA molecules have a consistent
orientation (5' to 3') which is used in writing their sequences. To
preserve this orientation, the complement of a sequence is written
backwards compared to the original. For example, an RNA sequence
ACGGUACU has the DNA complement AGTACCGT.
constitutively active: A constitutively active
gene is always transcribed, regardless of any regulatory
influences. Many, perhaps most genes are constitutively
transcribed at some (possibly low) level; however, the
level of transcription can be turned up or down by the
action of regulatory genes.
- double-stranded: consisting of
two bound strands, each of which is the complement
of the other. DNA is usually double-stranded, while mRNA is not.
- emission wavelength: the
wavelength of the light emitted by a fluorescent
reporter molecule when stimulated by a laser at its characteristic
- excitation wavelength:
the wavelength of the light required to stimulate emission by
a fluorescent reporter molecule.
- expressed sequence tag (EST): a sequence
from a transcribed mRNA for a possibly unknown
gene. Individual EST's are neither especially informative nor highly
accurate; however, they are easy to collect in large numbers. A
database of EST's can be helpful for picking protein-coding sequences
out of a long stretch of DNA, or for providing a larger context for
very short sequences, such as the small pieces of protein sequence
obtainable by mass spectrometry. EST's are compiled by the National Center for Biotechnology
Information in a database called dbEST.
- gene transcription:
the process by which a gene's DNA is read to produce
genetically heterogeneous: caused by more than
one genetic factor that leads to the same
phenotype. Many cancers, as
well as other genetic diseases like type-I diabetes,
are genetically heterogeneous. Note that this term
is used when different genetic defects can independently
cause disease. When multiple defective genes must combine to
cause a disease, we say that they act in epistasis.
the collection of all genes in a cell. In a multicellular
organism, every cell has a copy of the same genome, but
not all cells express the same genes.
to bind complementary pairs of DNA molecules. A DNA molecule has a
very strong preference for its sequence complement,
so just mixing complementary sequences is enough to
induce them to hybridize. Hybridization is temperature dependent, so
DNA's that hybridize strongly at low temperature can be temporarily
separated (denatured) by heating.
the process of cell replication by division.
- messenger RNA (mRNA):
the type of RNA which codes for protein, as opposed to
ribosomal RNA (rRNA) and transfer RNA (tRNA). mRNA
is translated to protein by a cell's
- oligonucleotide: a short
sequence (usually 2-50 bases) of DNA. Oligonucleotides of up to 30
bases are routinely synthesized from scratch for use as PCR primers or as probes for their sequence complements in a complex mixture of DNA.
- open reading frame (ORF):
a DNA sequence which is believed, on the basis of statistical evidence,
to code for protein, but for which no matching protein or mRNA is known.
ORFs can often be confirmed by matching their sequences to a database of
known genes or EST's.
- polymerase chain reaction (PCR):
a technique for making many copies of a specific DNA sequence.
The reaction is initiated using a pair of short primer
sequences which match the ends of the sequence to be copied.
Thereafter, each cycle of the reaction copies the sequence between
the primers. Primers can bind to the copies as well as the original
sequence, so the total number of copies increases exponentially with
the aggregate of all the observable characteristics of a
cell or organism. The phenotype results from the expression
of a subset of a cell's genome.
an enzyme which assembles nucleic acid residues into
DNA or RNA polymers. Polymerases work from the DNA
complement of the sequence
to be built. DNA polymerases copy DNA to DNA to replicate
the genome before mitosis, while
RNA polymerases copy DNA to RNA as the first step in
- probe: in a microarray experiment, the
solution of labeled DNA that is hybridized
with the array. For comparative transcription studies, two cDNA probes are prepared from the total mRNA of two different kinds of cells and labeled with
two different reporters.
- regulatory gene:
a gene whose expressed product controls the transcription of
other genes, either directly by DNA binding or indirectly
by activating a cellular protein.
- reporter: a molecule which can be bound
to cDNA and subsequently detected. Fluorescent reporters are used to
detect the presence of cDNA on the spots of a microarray.
- reverse transcription:
the copying of an RNA molecule back into its DNA complement. The
enzymes that perform this function are called reverse transcriptases.
Reverse transcription is is used naturally by retroviruses to insert
themselves into an organism's genome. Artifically-induced reverse
transcription is a useful technique for translating unstable
mRNA molecules into stable cDNA.
a complex of protein and rRNA in the cytoplasm which
can translate mRNA into protein.
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(jbuhler AT wustl DOT edu)
Last Update: 8/27/2002