Molecular Biology

A

Abundance of an mRNA is the average number of molecules per cell.
Abundant mRNAs consist of a small number of individual species, each present in a large number of copies per cell.
Acceptor splicing site is the junction between the right end of an intron and the left end of an exon.
Acentric fragment of a chromosome (generated by breakage) lacks a centromere and is lost at cell division.
Allele is one of several alternate forms of a gene occupying a given locus on the chromosome.
Allelic exclusion describes the expression in any particular lymphocyte of only one of the alleles coding for the expressed immunoglobulin.
Allosteric control refers to the ability of an interaction at one site of a protein to influence the activity of another site.
Amber codon is the nucleotide triplet UAG, one of three nonsense codons that cause termination of protein synthe
Amber mutation describes any change in DNA that creates an amber codon at a site previously occupied by a codon representing an amino acid in some protein.
Amber suppressors are mutant genes that code for tRNAs whose anticodons have been altered so that they can respond to UAG codons as well as or instead of to their previous codons.
Aminoacyl-tRNA synthetases are enzymes responsible for covalently linking amino acids to the 2′- or 3’-0H position of tRNA.
Amplification refers to the production of additional copies of a chromosomal sequence, found as either intrachromo somal or extrachromosomal DNA.
Anticoding strand of duplex DNA is used as template to direct the synthesis of RNA and is complementary to it.
Anticodon is a triplet of nucleotides in a constant position in the structure of tRNA that is complementary to the codon(s) in mRNA to which the tRNA responds.
Antigen is any molecule whose entry into an organism provokes synthesis of an antibody (immunoglobulin).
AP endonucleases make incisions in DNA on the 5′ side of either apurinic or apyrimidinic sites.
Apoinducer is protein that binds to DNA to switch on transcription by RNA polymerase.
att sites are the loci on a phage and the bacterial chromosome whose recombination integrates the phage into, or excises it from, the bacterial chromosome.
Attenuation is the regulation of termination of transcription involved in controlling expression of some bacterial operons.
Autogenous control describes the action of a gene product that either inhibits (negative autogenous control) or activates (positive autogenous control) expression of the gene coding for it.
Autoradiography detects radioactively labeled molecules by their effect in creating an image on photographic film.
Autosomes are all chromosomes except the sex chromosomes; a diploid cell has two copies of each autosome.
B

Backcross involves crossing an unknown genotype to a recessive homozygote so that the phenotypes of the progeny correspond directly to the chromosomes carried by the parent of unknown genotype.
Back mutation reverses the effect of a mutation that had inactivated a gene; thus it restores wild phenotype.
Bacteriophages are viruses that infect bacteria; often abbreviated as phages.
Balbiani ring is an extremely large puff at a band of a polytene chromosome.
Base pair (bp) is a partnership of A with T or of C with G in a DNA double helix; other pairs are possible in RNA under some circumstances.
Bidirectional replication is accomplished when two replication forks move away from the same origin in opposite directions.
Bivalent is the structure containing all four chromatids (two representing each homologue) at the start of meiosis.
Blunt-end ligation is a reaction that joins two DNA duplex molecules directly at their ends.
Branch migration describes the ability of a DNA strand partially paired with its complement in a duplex to extend its pairing by displacing the resident strand with which it is homologous.

 

C

C banding is a technique for generating stained regions around centromeres.
C genes code for the constant regions of immunoglobulin protein chains.
C value is the total amount of DNA in a haploid genome.
CAAT box is part of a conserved sequence at about 75 bp upstream from the startpoint of eucaryotic transcription units; it may be involved in binding RNA polymerase II.
CAP (CIRP) is a positive regulator protein activated by cyclic AMP, needed for RNA polymerase to initiate transcription of certain (catabolite-sensitive) operons of E. coli.
Capsid is the external protein coat of a virus particle.
Catabolite repression describes the decreased expression of many bacterial operons that results from addition of glucose; caused by decrease in intracellular level of cyclic AMP that inactivates CAP regulator.
Cell cycle is the period from one division to the next.
Centromere is a constricted region of a chromosome that includes the site of attachment to the mitotic or meiotic spindle.
Chemical complexity is the amount of a DNA component measured by chemical assay.
Chiasma (p/. chiasmata) is a site at which two homologous chromosomes appear to have exchanged material during meiosis.
Chromatids are the copies of a chromosome produced by replication.
Chromatin is the complex of DNA and protein in the nucleus of the interphase cell, originally recognized by its reaction with stains specific for DNA.
Chromocenter is an aggregate of heterochromatin from different chromosomes.
Chromosome is a discrete unit of the genome carrying many genes, consisting of proteins and a very long molecule of DNA, visible as a morphological entity only during the act of cell division.
Chromosome walking describes the sequential isolation of clones carrying overlapping sequences of DNA to span large regions of the chromosome (often in order to reach a particular locus):
Class switching is a change in the expression of the C region of an immunoglobulin chain during lymphocyte differentiation.
Clone describes a large number of cells or molecules identical with a single ancestral cell or molecule.
Closed reading frame contains termination codons that prevent its translation into protein.
Coconversion is the simultaneous correction of two sites during gene conversion.
Codon is a triplet of nucleotides that represents an amino acid or a termination signal.
Cointegrate structure is produced by fusion of two replicons, one originally possessing a transposon, the other lacking it; the cointegrate has copies of the transposon present at both junctions of the replicons, oriented as direct repeats.
Cold-sensitive mutant is defective at low temperature but functional at normal temperature.
Colony hybridization is a technique for using in situ hybridization to identify bacteria carrying chimeric vectors whose inserted DNA is homologous with particular sequences.
Compatibility group of plasmids contains members unable to coexist in same bacterial cell.
Complementation refers to the ability of independent (non allelic) genes to provide diffusible products that produce wild phenotype when two mutants are tested in trans configuration in a heterozygote.
In vitro complementation assay consists of identifying a component of a wild-type cell that can confer activity on an extract prepared from some mutant cell; identifies the component rendered inactive by the mutation.
Complementation group is a series of mutations unable to complement when tested in pairwise combinations in trans; defines a genetic unit (the cistron) that might better be called a noncomplementation group.
Complexity is the total length of different sequences of DNA present in a given preparation.
Composite transposons have a central region flanked on each side by insertion sequences, either or both of which may enable the entire element to transpose.
Concatemer of DNA consists of a series of unit genomes repeated in tandem.
Concatenated circles of DNA are interlocked like rings on a chain.
Conditional lethal mutations kill a cell or virus under certain (nonpermissive) conditions, but allow it to survive under other (permissive) conditions.
Consensus sequence is an idealized sequence in which each position represents the base most often found when many actual sequences are compared.
Conservative recombination involves breakage and reunion of preexisting strands of DNA without any synthesis of new stretches of DNA.
Core DNA is the 146 bp of DNA contained on a core particle.
Core particle is a digestion product of the nueleosome that retains the histone octamer and has 146 bp of DNA; its structure appears similar to that of the nucleosome itself.
Corepressor is a small molecule that triggers repression of transcription by binding to a regulator protein.
Cosmids are plasmids into which phage lambda cos sites have been inserted; as a result, the plasmid DNA can be packaged in vitro in the phage coat.
Crossing-over describes the reciprocal exchange of material between chromosomes that occurs during meiosis and is responsible for genetic recombination.
Crossover fixation refers to a possible consequence of unequal crossing-over that allows a mutation in one member of a tandem cluster to spread through the whole cluster (or to be eliminated).
Cruciform is the structure produced at inverted repeats of DNA if the repeated sequence pairs with its complement on the same strand (instead of with its regular partner in the other strand of the duplex).
Cytoplasmic protein synthesis is the translation of mRNAs representing nuclear genes; it occurs via ribosomes attached to the cytoskeleton.
Cytoskeleton is a network of fibers in the cytoplasm of the eucaryotic cell.

 

D

D loop is a region within mitochondrial DNA in which a short stretch of RNA is paired with one strand of DNA, displacing the original partner DNA strand in this region. The same term is used also to describe the displacement of a region of one strand of duplex DNA by a single-stranded invader in the reaction catalyzed by RecA protein.
Degeneracy in the genetic code refers to the lack of an effect of many changes in the third base of the codon on the amino acid that is represented.
Deletions constitute the removal of a sequence of DNA, the regions on either side being joined together.
Denaturation of DNA or RNA describes its conversion from the double-stranded to the single-stranded state; separation of the strands is most often accomplished by heating.
Denaturation of protein describes its conversion from the physiological conformation to some other (inactive) conformation.
Dicentric chromosome is the product of fusing two chromosome fragments, each of which has a centromere. It is unstable and may be broken when the two centromeres are pulled to opposite poles in mitosis.
Discontinuous replication refers to the synthesis of DNA in short (Okazaki) fragments that are later joined into a continuous strand.
Disjunction describes the movement of members of a chromosome pair to opposite poles during cell division. At mitosis and the second meiotic division, disjunction applies to sister chromatids; at first meiotic division it applies to sister chromatid pairs.
Divergence is the percent difference in nucleotide sequence between two related DNA sequences or in amino acid sequences between two proteins.
Down promoter mutations decrease the frequency of initiation of transcription.
Downstream identifies sequences proceeding farther in the direction of expression for example, the coding region is downstream from the initiation codon.

 

E

Early development refers to the period of a phage infection before the start of DNA replication.
Elongation factors are proteins that associate with ribosomes cyclically, for addition of each amino acid to the polypepticle chain.
End-product inhibition describes the ability of a product of a metabolic pathway to inhibit the activity of an enzyme that catalyzes an early step in the pathway.
Endonucleases cleave bonds within a nucleic acid chain; they may be specific for RNA or for single-stranded or double-stranded DNA.
Epigenetic changes influence the phenotype without altering the genotype.
Episome is a plasmid able to integrate into bacteria] DNA.
Evolutionary clock is defined by the rate at which mutations accumulate in a given gene.
Excision of phage or episome or other sequence describes its release from the host chromosome as an autonomous DNA molecule.
Exon is any segment of an interrupted gene that is represented in the mature RNA product.
Extranuclear genes reside in organelles such as mitochondria and chloroplasts outside the nucleus.

 

F

Facultative heterochromatin describes the inert state of sequences that also exist in active copies-for example, one mammalian X chromosome in females.
Fast component of a reassociation reaction is the first to renature and contains highly repetitive DNA.
Figure eight describes two circles of DNA linked together by a recombination event that has not yet been completed.
Footprinting is a technique for identifying the site on DNA bound by some protein by virtue of the protection of bonds in this region against attack by nucleases.

 

G

G banding is a technique that generates a striated pattern in metaphase chromosomes that distinguishes the members of a haploid set.
G1 is the period of the eucaryotic cell cycle between the last mitosis and the start of DNA replication.
G2 is the period of the eucaryotic cell cycle between the end of DNA replication and the start of the next mitosis.
Gamete is either type of reproductive cell-sperm or egg-with haploid chromosome content.
Gene (cistron) is the segment of DNA that is involved in producing a polypepticle chain; it includes regions preceding and following the coding region (leader and trailer) as well as intervening sequences (introns) between individual coding segments (exons).
Genotype is the genetic constitution of an organism.
Gyrase is a type II topoisomerase of E. coli with the ability to introduce negative supercoils into DNA.

 

H

Hairpin describes a double-helical region formed by base pairing between adjacent (inverted) complementary sequences in a single strand of RNA or DNA.
Haploid set of chromosomes contains one copy of each autosome and one sex chromosome; the haploid number n is characteristic of gametes.
Heterogametic sex has the chromosome constitution 2A +XY.
Histones are conserved DNA binding proteins of eucaryotes that form the nucleosome, the basic subunit of chromatin.
Homogametic sex has the chromosome constitution 2A + XX
Homologues are chromosomes carrying the same genetic loci; a diploid cell has two copies of each homologue, one derived from each parent.
Homomultimeric protein consists of identical subunits.
Homozygote is an individual with the same allele at corresponding loci on the homologous chromosomes.
Hotspot is a site at which the frequency of mutation (or recombination) is very much increased.
Housekeeping (constitutive) genes are those (theoretically) expressed in all cells because they provide basic functions needed for sustenance of all cell types.
Hybridization is the pairing of complementary RNA and DNA strands to give an RNA-DNA hybrid.
Hybridoma is the cell line produced by fusing a myeloma with a lymphocyte; it continues indefinitely to express the immunoglobulins of both parents.
Hyperchromicity is the increase in optical density that occurs when DNA is denatured.

 

I

Idling reaction is the production of pppGpp and ppGpp by ribosomes when an uncharged tRNA is present in the A site triggers the stringent response.
Immunity refers to the ability of a prophage to prevent an other phage of the same type from infecting a cell or to the ability of a plasmid to prevent another of the same type from becoming established in a cell. Different mechanisms are involved in the two types of immunity.
In situ hybridization is performed by denaturing the DNA of cells squashed on a microscope slide so that reaction is possible with an added single-stranded RNA or DNA; the added preparation is radioactively labeled and its hybridization is followed by autoradiography.
Incompatibility is the inability of certain bacterial plasmid to coexist in the same cell.
Induced mutations result from the addition of a mutagen.
Inducer is a small molecule that triggers gene transcription by binding to a regulator protein.
Induction refers to the ability of bacteria (or yeast) to synthesize certain enzymes only when their substrates are present; applied to gene expression, refers to switching on transcription as a result of interaction of the inducer with the regulator protein.
Integration of viral or another DNA sequence is its insertion into a host genome as a region covalently linked on either side to the host sequences.
Interbands are the relatively dispersed regions of polytene chromosomes that lie between the bands.
Intermediate component(s) of a reassociation reaction are those reacting between the fast (satellite DNA) and slow (nonrepetitive DNA) components; contain moderately repetitive DNA.
Interphase is the period between mitotic cell divisions; divided into G1, S, and G2.
Inverted repeats comprise two copies of the same sequence of DNA repeated in opposite orientation on the same molecule. Adjacent inverted repeats constitute a palindrome.
Inverted terminal repeats are the short related or identical sequences present in reverse orientation at the ends of some transposons.
Isotype is a group of closely related immunoglobulin chains.

 

K

kb is an abbreviation for 1000 base pairs of DNA or 1000 bases of RNA.
Kinetic complexity is the complexity of a DNA component measured by the kinetics of DNA reassociation.
Kinetochore is the structural feature of the chromosome to which microtubules of the mitotic spindle attach.

 

L

Late period of phage development is the part of infection following the start of DNA replication.
Leader is the nontranslated sequence at the 5′ end of mRNA that precedes the initiation codon.
Lethal locus is any gene in which a lethal mutation can be obtained.
Library is a set of cloned fragments together representing the entire genome.
Ligation is the formation of a phosphodiester bond to link two adjacent bases separated by a nick in one strand of a double helix of DNA
Linkage describes the tendency of genes to be inherited together as a result of their location on the same chromosome; measured by percent recombination between loci.
Linkage group includes all loci that can be connected (directly or indirectly) by linkage relationships; equivalent to a chromosome.
Linking number is the number of times the two strands of a closed DNA duplex cross over each other.
Liquid (solution) hybridization is a reaction between complementary nucleic acid strands performed in solution.
Locus is the position of a chromosome at which the gene for a particular trait resides; locus may be occupied by any one of the alleles for the gene.
Loop is a single-stranded region at the end of a hairpin in RNA (or single-stranded DNA); corresponds to the sequence between inverted repeats in duplex DNA.
Luxury genes are those coding for specialized functions synthesized (usually) in large amounts in particular cell types.
Lysis describes the death of bacteria at the end of a phage infective cycle when they burst open to release the progeny of an infecting phage.
Lysogenic Immunity is the ability of a prophage to prevent another phage genome of the same type from becoming established in the bacterium.
Lysogenic repressor is the protein responsible for preventing a prophage from reentering the lytic cycle.
Lysogeny describes the ability of a phage to survive in a bacterium as a stable prophage component of the bacterial genome.
Lytic Infection of bacteria by a phage ends in destruction of bacteria and release of progeny phage.

 

M

Main band of genomic DNA consists of a broad peak on a density gradient, excluding any visible satellite DNAs that form separate bands.
Map distance is measured as cM (centiMorgans) = percent recombination (sometimes subject to adjustments).
Marker (DNA) is a fragment of known size used to calibrate an electrophoretic gel.
Marker (genetic) is any allele of interest in an experiment.
Meiosis occurs by two successive divisions (meiosis I and II) that reduce the starting number of 4n chromosomes to 1n in each of four product cells. Products may mature to germ cells (sperm or eggs).
Melting of DNA means its denaturation.
Micrococcal nuclease is an endonuclease that cleaves DNA; in chromatin, DNA is cleaved preferentially between nucleosomes.
Mitosis is the division of a eucaryotic somatic cell.
Modification of DNA or RNA includes all changes made to the nucleoticles after their initial incorporation into the polynucleoticle chain.
Modified bases are all those except the usual four from which T, C, A, G) or RNA (U, C, A, G) are synthesized; they result from postsynthetic changes in the nucleic acid.
Multicopy plasmids are present in bacteria at amounts greater than one per chromosome.
Multiforked chromosome (in bacterium) has more than one replication fork, because a second initiation has occurred before the first cycle of replication has been completed.
Mutagens increase the rate of mutation by causing changes in DNA.
Mutation describes any change in the sequence of genomic DNA.
Myeloma is a tumor cell line derived from a lymphocyte; usually produces a single type of immunoglobulin.

 

N

Negative complementation occurs when interallelic com-plementation allows a mutant subunit to suppress the activity of a wild-type subunit in a multimeric protein.
Negative regulators function by switching off transcription or translation.
Nick in duplex DNA is the absence of a phosphodiester bond between two adjacent nucleoticles on one strand.
Nick translation describes the ability of E. coli DNA polymerase I to use a nick as a starting point from which one strand of a duplex DNA can be degraded and replaced by resynthesis of new material; is used to introduce radioactively labeled nucleoticles into DNA in vitro.
Nonpermissive conditions do not allow conditional lethal mutants to survive.
Northern blotting is a technique for transferring RNA from an agarose gel to a nitrocellulose filter on which it can be hybridized to a complementary DNA.
Nuclear matrix is a network of fibers surrounding and penetrating the nucleus.
Nucleolar organizer is the region of a chromosome carrying genes coding for rRNA.
Nucleoid is the compact body that contains the genome in a bacterium.
Nucleolus is a discrete region of the nucleus created by the transcription of rRNA genes.

 

O
Ochre codon is the triplet UAA, one of three nonsense codons that cause termination of protein synthesis.
Ochre mutation is any change in DNA that creates a UAA codon at a site previously occupied by another coclon.
Ochre suppressor is a gene coding for a mutant tRNA able to respond to the UAA codon to allow continuation of protein synthesis.
Open reading frame contains a series of triplets coding for amino acids without any termination codons; sequence is (potentially) translatable into protein.
Operator is the site on DNA at which a repressor protein binds to prevent transcription from initiating at the adjacent promoter.
Operon is a complete unit of bacterial gene expression and regulation, including structural genes, regulator gene(s), and control elements in DNA recognized by regulator gene product(s).

 

P
Packing ratio is the ratio of the length of DNA to the unit length of the fiber containing it.
Palindrome describes a sequence of DNA that is the same when one strand is read left to right or the other is read righ to left; consists of adjacent inverted repeats.
Papovaviruses are a class of animal viruses with small genomes, including SV40 and polyoma.
Periodicity of DNA is the number of base pairs per turn o the double helix.
Phenotype is the appearance or other characteristics of organism, resulting from the interaction of its genetic constitution with the environment.
Plasmid is an autonomous self-replicating extrachromo somal circular DNA.
Playback experiment describes the retrieval of DNA that has hybridized with RNA to check that it is nonrepetitive by a further reassociation reaction.
Point mutations are substitutions of single base pairs.
Polarity refers to the effect of a mutation in one gene in influencing the expression (at transcription or translation) of subsequent genes in the same transcription unit.
Polyadenylation is the addition of a sequence of polyadenylic acid to the 3′ end of a eucaryotic RNA after its transcription.
Polymorphism refers to the simultaneous occurrence in the population of genomes showing allelic variations
Polyprotein is a gene product that is cleaved into several independent proteins.
Position effect refers to a change in the expression of a gene brought about by its translocation to a new site in the genome; for example, a previously active gene may become inactive if placed near heterochromatin.
Positive regulator proteins are required for the activation of a transcription unit.
Positive supercolling describes the coiling of the double helix in space in the same direction as the winding of the two strands of the double helix itself.
Postmeiotic segregation describes the segregation of two strands of a duplex DNA that bear different information (created by heteroduplex formation during meiosis) when a subsequent replication allows the strands to separate.
Primary transcript is the original unmodified RNA product corresponding to a transcription unit.
Primer is a short sequence (often of RNA) that is paired with one strand of DNA and provides a free 3′-OH end at which a DNA polymerase starts synthesis of a deoxyribonucleotide chain.
Primosome describes the complex of proteins involved in the priming action that initiates synthesis of each Okazaki fragment during discontinuous DNA replication.
Procaryotic organisms (bacteria) lack nuclei.
Processive enzymes continue to act on a particular substrate, that is, do not dissociate between repetitions of the catalytic event.
Promoter is a region of DNA involved in binding of RNA polymerase to initiate transcription.
Prophage is a phage genome covalently integrated as a linear part of the bacterial chromosome.
Provirus is a duplex DNA sequence in the eucaryotic chromosome corresponding to the genome of an RNA retrovirus.
Pulse-chase experiments are performed by incubating cells very briefly with a radioactively labeled precursor (of some pathway or macromolecule); then the fate of the label is followed during a subsequent incubation with a nonlabeled precursor.

 

Q

Quick-stop dna mutants of E. coli cease replication immediately when the temperature is increased to 42°C.

 

R

R loop is the structure formed when an RNA strand hybridizes with its complementary strand in a DNA duplex, thereby displacing the original strand of DNA in the form of a loop extending over the region of hybridization.
Reassociation of DNA describes the pairing of complementary single strands to form a double helix.
RecA is the product of the recA locus of E. coli; a protein with dual activities, acting as a protease and also able to exchange single strands of DNA molecules. The protease activity controls the SOS response.
Reciprocal recombination is the production of new genotypes with the reverse arrangements of alleles according to maternal and paternal origin.
Recombinant progeny have a different genotype from that of either parent.
Recombinant joint is the point at which two recombining molecules of duplex DNA are connected (the edge of the heteroduplex region).
Recombination nodules (nodes) are dense objects present on the synaptonermal complex; could be involved in crossing-over.
Renaturation is the reassociation of denatured complementary single strands of a DNA double helix.
Repeating unit in a tandem cluster is the length of the sequence that is repeated; appears circular on a restriction map.
Repetitive DNA behaves in a reassociation reaction as though many sequences are present in a component, allowing any pair of complementary sequences to reassociate.
Replication eye is a region in which DNA has been replicated within a longer, unreplicated region.
Replication fork is the point at which strands of parental duplex DNA are separated so that replication can proceed.
Repression is the ability of bacteria to prevent synthesis certain enzymes when their products are present.
Repressor protein binds to operator on DNA or to RNA prevent transcription or translation.
Restriction enzymes recognize specific short sequence of (usually) unmethylated DNA and cleave the duplex.
Restriction map is a linear array of sites on DNA cleave by various restriction enzymes.
Retroregulation describes the ability of a sequence dow stream to regulate translation of an mRNA.
Retrovirus is an RNA virus that propagates via conversic, into duplex DNA,
Reverse transcription is synthesis of DNA on a template of RNA; accomplished by reverse transcriptase enzyme
Rifamycins (including rifampicin) inhibit transcription bacteria.
RNA-driven hybridization reactions use an excess of RNA to react with all complementary sequences in a single stranded preparation of DNA.
Rolling circle is a mode of replication in which a replication fork proceeds around a circular template for an indefinite number of revolutions; the DNA strand newly synthesized each revolution displaces the strand synthesized in the previous revolution, giving a tail containing a linear series of sequences complementary to the circular template strand.

 

S

S phase is the restricted part of the eucaryotic cell cycle during which synthesis of DNA occurs.
S1 nuclease is an enzyme that specifically degrades unpaired (single-stranded) sequences of DNA.
Saltatory replication is a sudden lateral amplification to produce a large number of copies of some sequence.
Satellite DNA consists of many tandem repeats (identical or related) of a short basic repeating unit.
Scaffold of a chromosome is a proteinaceous structure in the shape of a sister chromatid pair, generated when chromosomes are depleted of histones.
Selection describes the use of particular conditions to allow survival only of cells with a particular phenotype.
Semiconservative replication is accomplished by separation of the strands of parental duplex, each then acting as a template for synthesis of a complementary strand.
Semidiscontinuous replication is mode in which one new strand is synthesized continuously while the other is synthesized discontinuously.
Sex chromosomes are those whose content are different in the two sexes; usually labeled X and Y (or W and Z), one sex has XX (or WW), the other sex has XY (or WZ).
Sex linkage is pattern of inheritance shown by genes carried on a sex chromosome (usually the X).
Sigma factor is the subunit of bacterial RNA polymerase needed for initiation; is the major influence on selection of binding sites (promoters).
Silent mutations do not change the product of a gene.
Silent sites in a gene describe those positions at which mutations do not alter the product.
Single-copy plasmids are maintained in bacteria at a ratio of one plasmid for every host chromosome.
Single-strand assimilation describes the ability of RecA protein to cause a single strand of DNA to displace its homologous strand in a duplex; that is, the single strand is assimilated into the duplex.
Single-strand exchange is a reaction in which one of the strands of a duplex of DNA leaves its former partner and instead pairs with the complementary strand in another molecule, displacing its homologue in the second duplex.
Somatic cells are all the cells of an organism except those of the germ line.
Southern blotting describes the procedure for transferring denatured DNA from an agarose gel to a nitrocellulose filter where it can be hybridized with a complementary nucleic acid.
Spheroplast is a bacterial or yeast cell whose wall has been largely or entirely removed.
Spindle describes the reorganized structure of a eucaryotic cell passing through division; the nucleus has been dissolved and chromosomes are attached to the spindle by microtubules.
Splicing describes the removal of introns and joining of exons in RNA; thus introns are spliced out, while exons are spliced together.
Spontaneous mutations are those that occur in the absence of any added reagent to increase the mutation rate.
Sporulation is the generation of a spore by a bacterium (by morphological conversion) or by a yeast (as the product of meiosis).
Startpoint (startsite) refers to the position on DNA corresponding to the first base incorporated into RNA.
Stem is the base-paired segment of a hairpin.
Stringent replication describes the limitation of single-copy plasmids to replication pari passu with the bacterial chromosome.
Structural gene codes for any RNA or protein product other than a regulator.
Suppression describes the occurrence of changes that eliminate the effects of a mutation without reversing the original change in DNA.
Suppressor (extragenic) is usually a gene coding a mutant tRNA that reads the mutated codon either in the sense of the original codon or to give an acceptable substitute for the original meaning.
Suppressor (intragenic) is a compensating mutation that restores the original reading frame after a frameshift.
Synapsis describes the association of the two pairs of sister chromatids representing homologous chromosomes that occurs at the start of meiosis; resulting structure is called a bivalent.

 

T

Tandem repeats are multiple copies of the same sequence lying in series.
TATA (Hogness) box is a conserved A-T-rich septamer found about 25 bp before the startpoint of each eucaryotic RNA II transcription unit; may be involved in positioning the enzyme for correct initiation.
Telomere is the natural end of a chromosome.
Temperature-sensitive mutation creates a gene product that is functional at low temperature but inactive at higher temperature (the reverse relationship is usually called coldsensitive).
Terminal redundancy describes the repetition of the same sequence at both ends of (for example) a phage genome.
Termination codon is one of three triplet sequences, UAG (amber), UAA (ochre), or UGA that cause termination of protein synthesis; they are also called nonsense coclons.
Terminator is a sequence of DNA, represented at the end of the transcript, that causes RNA polymerase to terminate transcription.
Topological isomers are molecules of DNA that are identical except for a difference in linking number.
Tracer is a radioactively labeled nucleic acid component included in a reassociation reaction in amounts too small to influence the progress of reaction.
Transcription is synthesis of RNA on a DNA template.
Transcription unit is the distance between sites of initiation and termination by RNA polymerase; may include more than one gene.
Transfection of eucaryotic cells is the acquisition of new genetic markers by incorporation of added DNA.
Transformation of bacteria describes the acquisition of new genetic markers by incorporation of added DNA.
Transition is a mutation in which one pyrimidine is substituted by the other or in which one purine is substituted for the other.
Translocation of the ribosome is movement one codon along mRNA after the addition of each amino acid to the polypeptide chain.
Transplantation antigen is protein coded by a major histocompatibility locus, present on all mammalian cells, involved in interactions between lymphocytes.
Transposition immunity refers to the ability of certain transposons to prevent others of the same type from transposing to the same DNA molecule.
Transposon is a DNA sequence able to replicate and insert one copy at a new location in the genome.
Transversion is a mutation in which a purine is replaced by a pyrimidine or vice versa.
True-breeding organisms are homozygous for the trait under consideration.
Twisting number of a DNA is the number of base pairs divided by the number of base pairs per turn of the double helix.

 

U

Unequal crossing-over describes a recombination event in which the two recombining sites lie at nonidentical locations in the two parental DNA molecules.
Unidirectional replication refers to the movement of a single replication fork from a given origin.

 

V

V gene is sequence coding for the major part of the variable (N-terminal) region of an immunoglobulin chain.
Cloning vector is any plasmid or phage into which a foreign DNA may be inserted to be cloned.
Virion is the external protein coat of a virus particle.
Virulent phage mutants are unable to establish lysogeny.

 

W

Writhing number is the number of times a duplex axis crosses over itself in space.

 

Z

Zero time-binding DNA enters the duplex form at the start of a reassociation reaction; results from intramolecular reassociation of inverted repeats.
Zygote is produced by fusion of two gametes-that is, it is a fertilized egg.