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Based on the glossary in 3E originally compiled by Joachim W. Engels

Acentric: a chromosome lacking a centromere.

Acetylation: covalent modification of a molecule by the addition of an acetyl, CH3CO, group; usually onto a hydroxyl or an amino group.

Adenosine Deaminase Acting on RNA (ADAR): A family of enzymes that catalyse the deamination of adenosine to inosine within double-stranded RNA.

Agarose: a polysaccharide isolated from seaweed used as a matrix in gel electrophoresis.

Alkylation: Reaction of a nucleotide (usually the base) with an alkyl electrophile (commonly a methyl).

Allele: one of two or more alternate forms of a gene: an individual inherits two alleles for each gene, one from each parent.

Allogeneic cell therapy: an approach in which a patient receives cells from another individual, often after those cells have been subjected to gene editing.

Allosteric control: the ability of an interaction at one site of a protein to influence (positively or negatively) the activity at another site.

Alternative splicing: a regulatory mechanism whereby variation in the incorporation of exons into messenger RNA (mRNA) results in production of multiple related proteins.

Alu family: a set of short (approximately 300 bp) related sequences dispersed throughout the human genome and cleaved once by the restriction enzyme AluI.

Aminoacylation: the process of charging a transfer RNA with an amino acid.

Amplification: the production of extra copies of a chromosomal sequence found either as intra- or extra-chromosomal DNA: with respect to plasmids, it refers to the increase in the number of plasmid copies per cell induced by certain treatments of transformed cells.

Anaphase: the phase of mitosis during which the chromosomes are segregated to the opposite poles of the mitotic spindle.

Anneal (re-anneal): the (re)establishment of base-pairing between complementary strands of double-stranded DNA (dsDNA) or of single-stranded DNA (ssDNA) with an RNA strand.

Anomerization: the interconversion of stereoisomers of a sugar that differ only in stereochemistry at the carbonyl carbon in its cyclic (furanose or pyranose) form.

Antibody: a protein produced by the immune system that specifically recognizes and binds to an antigen.

Anticodon: a triplet of nucleotides in a constant position in the structure of transfer RNA (tRNA) complementary to the triplet codon(s) in mRNA to which the tRNA binds.

Antigen: a molecule that is recognized by the immune system as foreign and stimulates antibody production.

Antisense: the strand of DNA that is complementary to the mRNA (also non-coding strand): also used to define a synthetic oligonucleotide (analogue) that is complementary to mRNA, often serving to inhibit its expression.

Apoptosis: programmed death of a cell within a multi-cellular organism, which follows an ordered process.

APOBEC: A family of enzymes that catalyse the deamination of cytosine to uracil within ssDNA or RNA, with substrate specificity depending on the specific apolipoprotein B mRNA-editing enzyme, catalytic polypeptide (APOBEC) enzyme.

Aptamer: DNA or RNA molecules selected from random pools based on their ability to bind other small or large molecules.

Archaea: one of the three domains of life, defined by cells that lack nuclei but possess replication, transcription and translation machinery more similar to eukaryotes than to bacteria.

Array: a spatial arrangement of e.g. oligonucleotides or peptides, which can be at high density (≥10 000 individual sequences).

Artificial miRNA: an expressed or synthetic small RNA designed to mimic the structural and sequence features of a microRNA.

ATPase: an enzyme that hydrolyses adenosine triphosphate (ATP), often converting potential energy derived from its P–O–P bonds into mechanical energy to drive movement within large molecular machines.

Autologous cell therapy: an approach in which a patient's own stem cells are isolated and their genome edited before reintroducing the cells into the patient.

Autoradiography: the detection of radioactively-labelled molecules present, for example, in a gel or on a filter by exposing an X-ray film to it.

Auxotrophy: the inability of microorganisms to live on minimal medium without supplemented (auxiliary) nutrients.

Back mutation: a mutation that reverses the effect of a mutation that had inactivated a gene.

Bacteria: one of the three domains of life, defined by cells that lack nuclei but excluding archaea.

Bacteriophage: a virus that infects bacteria; often abbreviated as phage.

Base: a heterocycle [adenine (A), guanine (G), cytosine (C), 5-methylcytosine (m5C) and thymidine (T) or uracil (U) are naturally occurring] bonded to a furanose sugar at carbon-1 in a nucleic acid strand.

Base-pair (bp): a pair of heterocycles in nucleotides that are hydrogen-bonded to each other; typically A with T(U) or C with G in duplex nucleic acids.

BER (base-excision repair): DNA repair by excision of a damaged base by a glycosylase enzyme, followed by excision of the sugar phosphate and gap repair.

Bioinformatics: the use of mathematical and computational tools to analyse biological data. Usually applied to large, systematic datasets, such as genome sequence or expression data.

Blotting: transfer of DNA, RNA or protein from a gel to nitrocellulose or other membrane.

Branch migration: movement of a Holliday junction (q.v.) along a pair of homologous chromosomes.

Budding yeast: a sub-phylum of yeast distinguished by producing progenitor cells by budding: often used for Saccharomyces cerevisiae.

Cap: the structure 7mG5′ppp5′Np added to the 5′-end of eukaryotic mRNA; introduced after transcriptional initiation.

Carcinogen: A substance or agent that causes cancer usually by reaction with DNA.

Cas: CRISPR-associated; refers to RNA-guided nucleases associated with prokaryotic defence systems. The paradigmatic example that has been repurposed for therapeutic applications is Cas9.

Catenane: a molecule in which two or more separate rings are interlocked, thus holding the structure intact without covalent bonding between the rings: a DNA catenane is a topoisomer of its components, i.e. it is a distinct topological structure that can be acted on by a topoisomerase.

cDNA: a complementary DNA, typically referring to a DNA synthesized in vitro by reverse transcription from an RNA.

CDK (Cyclin-dependent kinase): a family of protein kinases that require a cyclin regulatory subunit; so-called because the abundance of the founding member of the cyclin protein family oscillated in phase with the cell cycle: often involved in cell-cycle control.

Centromere: the region of a chromosome required for segregation during mitosis: the site of kinetochore assembly and thus spindle fiber attachment.

Chain termination sequencing: see Sanger–Coulson sequencing.

Chaperone: an RNA chaperone is a protein that binds transiently and non-specifically to single-stranded RNA (ssRNA) in order to resolve kinetically trapped, misfolded conformers.

ChIP-seq: Chromatin immuno-precipitation analysed by high-throughput sequencing; a technique in which DNA sequences bound by a protein of interest are identified.

Chromatin: the protein–DNA complex into which the genome is assembled in the nucleus, consisting of nucleosomes and other chromatin-binding proteins.

Chromosome: a discrete segment of the genome defined by a single DNA molecule.

cis-Acting: the ability of a DNA or RNA sequence to influence only the molecule of which it forms a part, usually implying that it functions as a binding site for a regulatory protein.

Cistron: the genetic unit defined by the cistrans test; equivalent to gene in comprising a unit of DNA encoding a protein.

Clamp loader: an ATPase complex that loads the replicative processivity clamp onto the primer-template junction at the beginning of DNA replication.

Class switching: a recombination event that switches the type of antibody expressed. For instance, immunoglobulin M (IgM), the default antibody type, can be switched to immunoglobulin G (IgG), which is common in blood.

CLIPseq: (Crosslinking and immunoprecipitation sequencing) a method for finding which RNA species interact with a particular RNA-binding protein (or an RNA). Uses crosslinking between RNA and protein, then antibody immunoprecipitation of the protein.

Clone: a large number of cells or molecules genetically identical with a single ancestral cell or molecule.

Codon: a triplet of nucleotides that corresponds to an amino acid or a termination signal in mRNA.

Codon usage: the frequency with which a given codon is used to encode a particular amino acid in a particular species. For instance, in Escherichia coli, 68% of glutamate codons are GAA and 32% are GAG.

Competent: a culture of bacteria or yeast cells treated in such a way that its ability to take up DNA molecules without transduction or conjugation has been enhanced.

Complementation: the ability of an allele to provide a function lacking in another allele. Also called a cistrans test.

Conjugation: directional transfer of DNA between two bacteria.

Consensus sequence: an idealized sequence in which each position represents the base most often found when many actual sequences are compared.

Constitutive heterochromatin: heterochromatin that remains as heterochromatin regardless of the developmental fate of a cell.

Contig: a contiguous stretch of sequence in a genome assembly, uninterrupted by gaps or ambiguous sequence.

Copy number: the average number of copies of a genomic element in particular genes, repeats or plasmids.

Cotransformation: introduction of two or more genes carried on separate DNA molecules into a cell.

CpG island: a region of high CG dinucleotide density, generally associated with transcriptional regulatory regions.

CRISPR: clustered regularly interspaced short palindromic repeats, a family of DNA sequences found in the genomes of prokaryotic organisms such as bacteria and archaea. These repeats code for guide RNAs which are taken up by a Cas enzyme and used to detect and destroy DNA during subsequent infections, biology which has been applied to gene editing technology.

Cross-linking: introduction of covalent intramolecular or intermolecular bonds between groups that normally are not covalently linked. Used to detect close proximity of parts of (macro)molecules.

Crossover: a recombination event that results in a segment of a chromosome being exchanged with the equivalent segment of a homologous or sibling chromosome.

Cruciform: a DNA structure where normally double-stranded DNA is rearranged to form internal base-pairs in each strand such that the final structure resembles a cross.

Cytoplasm: the soluble contents of a cell; in eukaryotes, the soluble content outside the nucleus and other membrane-bound organelles.

Deletion: removal of a sequence of DNA, with the regions on either side being joined together.

Denaturation (of protein): conversion from the native conformation into some other (inactive) disordered conformation.

Deoxyribonucleotide (deoxynucleotide): a unit of DNA comprising a base, a ribofuranose sugar and a phosphate where the 2′-position of the sugar has no hydroxyl group.

Dianophore: the ensemble of molecular features determining pharmacokinetic parameters, such as tissue distribution and stability of a drug.

Dicentric: a chromosome with two centromeres.

Dicer: a cellular endoribonuclease that cleaves double-stranded RNA (dsRNA) and pre-microRNA into approximately 22-nucleotide double-stranded RNA fragments to give small interfering RNA and microRNA, respectively.

Direct repeats: identical (or closely related) sequences present in two or more copies in the same orientation on the same DNA (or RNA) molecule; they need not be necessarily adjacent.

DNA fingerprinting: generation of a pattern of bands, by Southern blotting and hybridization with a multi-locus probe, which is highly individual-specific.

DNase: an enzyme that degrades DNA molecules to nucleotides by cleaving P–O bonds.

DNAzyme: a short catalytic single-stranded DNA molecule.

Domain (of a protein): a discrete continuous part of the amino acid sequence that can be equated with a particular function or a particular substructure of the tertiary structure.

Domain (taxonomic): one of the three highest order taxonomic classifications: Archaea, Bacteria and Eukaryota.

Dominant (allele): determines the phenotype displayed in a heterozygote with another (recessive) allele.

Donor DNA: a fragment of DNA with partial or total homology to DNA containing a double-strand break, and which serves as a template for homology-directed repair of the broken DNA.

Double-stranded break (DSB): DNA damage that breaks the phosphodiester backbone of both strands of a duplex in sufficiently close proximity (usually within 3 or 4 bp) that the molecule can break in two.

Downstream: sequences that proceed further in the direction of expression, for example the coding region is downstream from the initiation codon and towards the 3′-end (see Upstream).

Drosha: a double-stranded RNA-specific cellular endoribonuclease involved in the initial step of microRNA biogenesis. It cleaves the 3′- and 5′-strands of a stem–loop in primary micro RNAs (pri-miRNAs; processing centre 11 bp from the dsRNA–ssRNA junction) to release hairpin-shaped pre-miRNAs.

Duplex: double-stranded nucleic acid consisting of two complementary strands of DNA, RNA or a hybrid of the two.

Electropherogram: the graphical output of electrophoresis devices in short tandem repeat (STR) and sequencing analysis, showing fluorescence intensity as a function of molecular weight. The peak at a particular wavelength (colour) corresponds to a specifically labelled molecule of a particular size.

End labelling: the addition of a radioactively labelled group to one end (5′ or 3′) of a DNA or RNA strand.

Endonuclease: an enzyme that cleaves an internal phosphodiester P–O bond within an oligonucleotide or polynucleotide. Such enzymes may be specific for RNA or for ssDNA or dsDNA.

Endosymbiont: a cell that lives co-operatively within another cell.

Enhancer: a DNA sequence that increases the utilization of a promoter in cis. It functions in either orientation and upstream or downstream, relative to the promoter.

Epigenetic: a change to the genome that does not affect DNA sequence but can be inherited. Commonly this involves base-modifications in DNA.

Episome: an extrachromosomal DNA fragment.

Epitope: any part of a molecule that acts as an antigenic determinant. A macromolecule can have many different epitopes each stimulating the production of a different specific antibody.

Euchromatin: a region of the genome that is enriched in transcriptionally active chromatin and is more accessible to soluble factors than heterochromatin.

Eukaryote: one of the three domains of life, defined by cells that contain a nucleus.

Excision-repair: a repair system that removes a single-stranded sequence of DNA containing damaged or mis-paired bases and replaces it in the duplex by synthesis of a sequence complementary to the intact strand.

Exon: a segment of a spliced gene that is retained in the mature RNA.

Exon skipping: The use of a splice-switching oligonucleotide to induce exclusion of an exon leading to generation of a productive mRNA despite a disease-causing mutation within the pre-mRNA.

Exonuclease: an enzyme that cleaves nucleotides one at a time from the end of a polynucleotide chain. Such enzymes may be specific for either the 5′- or 3′-end of DNA or RNA.

Experimental noise: variation in the result of an experimental assay independent of the quantity being measured and attributable to factors that are beyond the control of the experimentalist.

Expression vector: a cloning vector from which a foreign gene inserted into the vector can be expressed in a heterologous host.

Ex vivo: any biological process that occurs outside a living cell or organism (see in vitro).

Facultative heterochromatin: chromatin that can switch between heterochromatin and euchromatin, dependent on the developmental fate of a cell, to regulate gene expression.

FDA (United States Food and Drug Administration): Federal Agency responsible for protecting and promoting public health through control and supervision of pharmaceutical drugs, vaccines, biopharmaceuticals etc.

Flap endonuclease: an enzyme that cleaves a single-stranded flap displaced from a DNA duplex by a helicase or polymerase.

Footprinting: a technique for identification of the site of DNA bound by some protein by virtue of the protection of bonds in this region against chemical modification or attack by nucleases.

Forensic genetics: the application of genetics for the resolution of disputes at law.

Frameshifting: the process on a ribosome when the reading frame of the messenger RNA is moved by one nucleotide in either the forward or reverse direction.

FRET: fluorescence resonance energy transfer is radiationless transmission of energy from a donor molecule (a chromophore or dye energy absorber) to an acceptor molecule (that emits radiation).

Fruit fly: an insect of the family Drosophilidae. Often used to refer to the specific species Drosophila melanogaster.

Fusion gene: a recombinant gene constructed from parts of two different genes.

Fusion protein: a protein expressed by a fusion gene containing parts of the coding sequence of two different genes.

G1 phase: the phase of the cell cycle after mitosis and before DNA replication.

Gapmer: an antisense oligonucleotide where the central section is DNA or a DNA-like oligonucleotide containing modifications such as phosphorothioate that permit recruitment of RNase H upon binding a complementary RNA strand. The 5′- and 3′-flanking regions typically contain RNA-like chemical modifications that bind target RNA with high affinity.

Gel electrophoresis: electrophoresis carried out in a gel matrix (usually agarose or polyacrylamide) that allows separation of molecules of similar electric charge density on the basis of difference in molecular weight.

Gene: the basic unit of genetic inheritance. Usually a DNA sequence expressed as an RNA or protein molecule, including the transcribed region and associated regulatory sequences.

Gene expression: the combined processes of transcription and translation of a gene.

Gene family: a group of genes of similar sequence and usually similar function derived by gene duplication and diversification.

Gene therapy: the treatment of disease by introducing a healthy gene to replace a disease-causing variant or introducing a new or modified gene that serves a therapeutic purpose.

Genetic code: the complete set of codons specifying the various amino acids, including the nonsense codons. The code is usually written in the form in which it occurs in mRNA (can be different in mitochondria).

Genetic fingerprint: a DNA sequence polymorphism, or collection thereof, that can precisely identify an individual.

Genetic recombination: exchange of polymorphic DNA sequence between homologous chromosomes, often in the context of sexual reproduction.

Genome: the entire genetic material of a cell. In eukaryotes, the genomes of separate organelles (e.g. nucleus, mitochondria, chloroplast, etc.) are considered independently.

Genomic island: a group of functionally related genes that can be inherited by horizontal gene transfer between bacteria, usually conferring a phenotypic advantage.

Germline: The cells in an organism that can contribute to the next generation.

Glycosylic bond (alternatively, glycosidic bond): the bond between carbon-1′ of an aldose sugar and N or O in the nucleoside heterocyclic base.

G-tetrad: a structure that involves four oligonucleotide strands in which there is participation by one guanine base in each strand.

Gyre: a spiral turn in DNA that gives rise to superhelical DNA.

Hairpin: double-stranded region formed by base-pairing of adjacent complementary sequences in the same DNA or RNA strand.

Haploid: an organism containing one copy of its genome.

Hapten: a small molecule that acts as an antigen when it is conjugated to a large (carrier) molecule.

Helicase: an enzyme that unwinds double-stranded nucleic acids.

Hemi-methylated: a palindromic, cytosine-containing DNA sequence (e.g. CpG) in which the cytosines on only one strand are methylated: the product of replicating fully-methylated dsDNA.

Heterochromatin: a region of the genome that is depleted in transcriptionally active chromatin and is less accessible to soluble factors than euchromatin.

Heteroduplex (hybrid) DNA: DNA that is generated by base-pairing between partly non-complementary single strands derived from the different parental duplex molecules; occurs during genetic recombination.

Hi-C: a specific chromosome conformation capture approach in which all pairwise DNA interactions are identified by high-throughput sequencing.

High-throughput sequencing: a class of sequencing technologies that produce many (105 to 1010) short sequence reads. Also referred to as next-generation sequencing (NGS).

Histone: a protein octamer containing two of each of histone H2A, H2B, H3 and H4.

Holliday junction: a four-stranded DNA structure that occurs during homologous recombination between two chromosomes.

Holoenzyme: a complete enzyme including all its subunits. Often used in reference to RNA and DNA polymerases.

Homologous chromosome: one of a number of equivalent chromosomes in a diploid or polyploid genome. In a sexual diploid, one homologous chromosome is maternal and the other is paternal.

Homologous recombination: recombination between identical or highly similar sequences on homologous or sibling chromosomes or an exogenous piece of homologous DNA; the earliest-studied pathway for homology-directed repair.

Homologue: one of two genes that are similar in sequence because they evolved from a common ancestor.

Homology: the relationship between two sequences that are similar because they evolved from a common ancestor.

Homology-directed repair: one of a number of mechanisms for DNA repair after a double-strand break, depending on the presence of a partially homologous DNA template (donor DNA).

Horizontal gene transfer: the transfer of DNA from one organism to another outside of the usual transfer of a genome from a parent to its progeny.

Hybridization: pairing of two complementary nucleic acid strands to form a DNA, RNA or RNA–DNA duplex.

Hybridoma: a cell line produced by fusion of a myeloma cell with a lymphocyte that indefinitely expresses the immunoglobulins of both parents.

Hyperchromicity: increase in optical density that occurs when DNA is denatured.

Hypochromicity: decrease in optical density that occurs when bases become stacked in DNA.

Immunoglobulin: see Antibody.

Immunoprecipitation: the technique of precipitating a protein antigen from solution by an antibody that specifically binds to that particular protein. A process used to isolate and concentrate a selected protein from a sample containing thousands of different proteins.

i-Motif: an intercalated DNA structure formed by cytosine bases at low pH.

Incompatibility: the inability of certain bacterial plasmids to coexist in the same cell.

Indels: small insertions and deletions resulting from errors in DNA repair after a double-strand break.

Inducer: a small molecule that triggers gene transcription by binding to a regulator protein.

Initiation codon: AUG (sometimes GUG), three bases in mRNA that code for the first amino acid in a protein.

Initiation zone: a region of a mammalian genome, of the order of 50 kb in length, enriched for DNA replication initiation events.

In situ hybridization: detection of a nucleic acid in a cell or tissue by hybridization with a labelled probe.

Intasome: a protein–DNA complex between the phage lambda integrase (Int) and the phage lambda attachment site (attP).

Intercalation: the insertion of a molecule between two adjacent bases in double-stranded DNA.

Interphase: the phase of cell cycle outside of mitosis, in which chromosomes are decondensed. It consists of G1, S and G2 phases.

Intron: a segment of a spliced gene that is removed in the mature RNA.

In vitro: (“in glass”) any experimental (biological) process that occurs outside the living cell.

In vivo: any biological process that occurs within the living cell or organism.

IPTG: isopropyl β-D-thiogalactoside; an artificial inducer of the lac operon.

Isochore: a large region of a chromosome with nucleotide content significantly different from the genome average.

Isoform: one of a set of similar gene products. Isoforms can be produced by variation in RNA processing from a single gene or by expression from closely related members of a gene family.

kb: 1000 base-pairs of DNA or 1000 bases of RNA.

Kinase: an enzyme that catalyses the transfer of a phosphoryl group from ATP to an acceptor, usually a protein or a nucleotide.

Kinetochore: protein complex that assembles at a centromere and connects a chromosome to the mitotic spindle.

Klenow fragment: an N-terminal truncation of DNA polymerase I that retains polymerase activity, but lacks a 5ʹ→3ʹ exonuclease activity; can also be engineered to contain mutations to abolish the 3ʹ→5ʹ exonuclease proof-reading activity.

Lac operon: an inducible operon in E. coli that codes for three genes involved in the metabolism of lactose.

Lagging strand: strand of the replication fork replicated discontinuously in the opposite direction from the movement of the replisome.

Leader sequence: sequence at the 5ʹ-end of an mRNA not translated into protein. Contains encoded information that the ribosome and special proteins read to initiate synthesis of a polypeptide.

Leading strand: strand of the replication fork replicated continuously in the same direction as replisome movement.

Lentivirus: a type of retrovirus that has a long delay between infection and onset of symptoms (from Latin lentus, slow). HIV is the paradigmatic lentivirus.

Library: set of cloned fragments that together represent the entire genome.

Ligase: enzymes such as DNA ligase catalyse the formation of a phosphodiester bond at the site of a single-strand break in duplex DNA (ligation). RNA ligases covalently link two ssRNA molecules.

Linker (fragment): short synthetic duplex oligonucleotide containing a target site for a restriction enzyme. A linker may be added to the end of a DNA fragment prepared by cleavage with another enzyme during reconstruction of recombinant DNA.

Lipid nanoparticle (LNP): a formulation containing a nucleic acid and a combination of lipids, typically including an ionizable lipid and neutral lipids, and often functionalized with polyethylene glycol. Used to deliver nucleic acid therapeutics such as synthetic mRNAs and small interfering RNAs (siRNAs).

LNA: Locked nucleic acid, a nucleoside analogue where O2ʹ of the ribofuranose is attached to C4ʹ via a methylene bridge [also known as bridged nucleic acid (BNA)]: locks the furanose ring in an RNA-like 2ʹ-exo-conformation.

Long non-coding RNA (LncRNA): an RNA polymerase II transcript of greater that 200 nucleotides that is not translated into protein.

LTR (long-terminal repeat): a sequence directly repeated at both ends of a retroviral DNA.

Lysis: death of bacteria at the end of a phage infection cycle; they burst open releasing the infecting phage.

M13: an E. coli phage containing single-stranded circular DNA forming the basis for a series of cloning vectors.

Major groove: the wider of the two grooves in a B-form DNA duplex. In A-form DNA or a dsRNA duplex, the major groove is narrow and deep.

Mass spectrometry: analytical technique used to measure mass-to-charge ratio of ions. A mass spectrum is a plot of intensity as a function of mass-to-charge ratio, used to determine the mass of a nucleotide, oligonucleotide or a nucleotide fragment.

Match probability: the probability of two unrelated individuals sharing the same DNA profile.

Meiosis: a cell cycle specialized for sexual reproduction that produces haploid gametes from diploid cells. It involves two rounds of cell division, but only one round of DNA replication.

Meiotic recombination: programmed genetic recombination occurring during meiosis.

Melting temperature (Tm): temperature at which hyperchromicity is half-maximal.

Messenger RNA (mRNA): RNA that codes for a protein.

Metaphase: phase of mitosis during which condensed chromosomes align on the mitotic spindle.

Metaphase chromosome: a condensed chromosome prepared for mitotic segregation.

Microarray: tool used for the simultaneous detection of the expression of thousands of genes: a microscope slide printed with thousands of tiny spots in defined positions, each containing a known DNA sequence.

Microhomology: a short (2–10 nucleotides) stretch of sequence identity used to align broken DNA ends during MMEJ.

Microhomology-mediated end joining (MMEJ): a variation of non-homologous end joining, in which the broken ends are held together by microhomology.

MicroRNA (miRNA): a small, imperfectly-paired, non-coding dsRNA molecule of usually 21–23 nucleotides found in plants, animals and some viruses. miRNAs are formed by Drosha and Dicer enzyme processing from primary RNA transcripts to function in RNA silencing and post-transcriptional regulation of gene expression by targeting mRNA.

Minimal medium: a fully-defined chemical growth medium containing only inorganic sources of essential elements in addition to an organic carbon source.

Minisatellites: loci made up of a number (approximately 10–1000) of tandemly repeated sequences, each typically 10–100 bp long, usually GC-rich and often hypervariable.

Minor groove: the narrower of the two grooves in a B-form DNA duplex. In A-form DNA or dsRNA, the minor groove is wide and shallow.

Mitosis: the phase of the cell cycle in which chromosomes are condensed and segregated during nuclear division. Usually coincides with cell division.

Mitotic spindle: a proteinaceous scaffold composed of microtubule fibers and associated proteins that attach to chromosomes during metaphase and segregate them during anaphase.

Modified base: results from enzymatic modification of one of the five standard bases (A, C, G, T or U) following chemical or enzymatic polymerization.

Monoclonal antibody: a specific immunoglobulin molecule (one protein sequence) produced by monoclonal cells derived from the fusion of a B lymphocyte with a myeloma cell. It is directed against a single epitope of the antigen used to raise the antibody.

Multicopy plasmid: a plasmid present in bacteria at amounts greater than one per chromosome.

Multiple displacement amplification: a method for whole-genome amplification using a highly processive polymerase from bacteriophage fp29 and random primers.

Mutagens: chemicals that increase the rate of mutation by causing changes in DNA composition (mutagenesis).

Mutation: any change in the sequence of genomic DNA.

Nascent strand: a newly synthesized strand produced by DNA replication.

Nick: a single-stranded P–O bond break in a duplex nucleic acid.

Nick translation: 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.

NMR: nuclear magnetic resonance spectroscopy (see Section 15.2).

Non-coding RNA: ncRNA, an RNA that does not code for a protein.

Non-homologous end joining: NHEJ, an error-prone double-strand break repair mechanism that functions by directly ligating the broken DNA ends.

Non-productive splicing: a pattern of splicing that generates an isoform of mRNA containing a frameshift or other mutation or which is rapidly degraded or otherwise not useful for protein expression. This can include splicing onto a poison exon (see poison exon).

Nonsense codon: a premature stop codon (UAG, UAA, UGA) that occurs within an open reading frame, and leads to truncated protein synthesis.

Northern blotting: transfer of RNA from an agarose or polyacrylamide gel to a membrane on which it can be made visible by hybridization to a complementary DNA probe.

Nuclease: an enzyme that cleaves P–O bonds in a nucleic acid chain.

Nucleobase: see Base.

Nucleoid: the region of prokaryotic cytoplasm containing genomic DNA.

Nucleolus: the region in the nucleus where rRNA synthesis takes place.

Nucleoside: a monomeric unit of a nucleic acid with a furanose sugar joined to a heterocyclic base; joined by phosphodiester bridges to form a nucleic acid.

Nucleosome: a histone octameric protein wrapped 1.7 times with 146 bp of DNA: the fundamental unit of eukaryotic chromatin.

Nucleotide: a nucleoside having a 5ʹ- or 3ʹ-phosphate: the monomeric building block of a nucleic acid.

Nucleus: the eukaryotic organelle containing the genome.

Okazaki fragment: a short DNA replication product synthesized on the lagging strand of the replication fork. Okazaki fragments are ligated to produce a continuous nascent strand.

Oligonucleotide: a short single-stranded nucleic acid formed of nucleotides (ribo-, 2ʹ-deoxyribo-, analogue or combinations) joined by phosphodiester linkages.

Oncogene: a gene that when incorrectly activated causes cancer.

Open reading frame (ORF): a sequence of triplet-nucleotide codons, starting with an initiation (start) codon, ending with a termination (stop) codon, that encodes a protein sequence.

Operator: the site on DNA where a bacterial repressor protein binds to prevent transcription from initiating at the adjacent promoter.

Operon: a unit of bacterial genes, including co-transcribed structural genes, regulatory genes and the control elements recognized by regulatory gene products.

ORC (Origin Recognition Complex): a multi-subunit ATPase that loads an MCM replicative helicase at origins.

Origin: a site in the genome where DNA replication is initiated.

Orthologue: a homologue in a different species that has diverged as a consequence of speciation; e.g. mouse β-globin is an orthologue of human β-globin.

Palindrome: a sequence of double-stranded DNA that is the same when one strand is read left to right or its complement is read right to left: consists of adjacent inverted repeats.

PAM density: the frequency of PAM (see under ‘Protospacer’ below) sites within the genome; determines the ease of finding a target site for a given Cas enzyme.

Paralogue: a homologue within a species that has diverged as a consequence of gene duplication; e.g. human α-globin is the paralogue of human β-globin.

Parental strand: the template strand of DNA upon which the nascent strand is synthesized.

Paternity/Maternity testing: the use of DNA profiles to ascertain if a particular individual is the biological parent of another person; uses similar markers to individual genetic fingerprinting.

Pattern Recognition Receptors (PRRs): protein components of the innate immune system which recognize molecules having structural elements frequently found in pathogens.

PCR (polymerase chain reaction): in vitro amplification of DNA based on thermal cycling in the presence of a pair of primers, a DNA template and a thermostable DNA polymerase.

PCR stutter: a PCR artifact in which an additional band is seen as well as a band of the expected size, typically one repeat unit smaller; results from slippage synthesis errors by the PCR polymerase.pegRNA (prime editing guide RNA): an engineered guide used in reverse-transcriptase based gene editing (prime editing).

Peptide nucleic acid (PNA): a nucleic acid analogue with a neutral pseudopeptide backbone of 2-aminoethylglycine repeat units and standard heterocyclic bases.

Permissive: a gene-regulatory environment that allows gene expression but does not cause it.

Personalized medicine: medical diagnosis and treatment based on individual characteristics of a patient, usually based on genetic information.

Phage (bacteriophage): a virus that infects bacteria.

Pharmacophore: the ensemble of molecular features determining target recognition and modulation by a drug.

Phenotype: the observable characteristics of an organism.

Phosphatase: an enzyme hydrolysing a phosphomonoester in nucleotides (or in proteins), to liberate inorganic phosphate.

Phosphodiester linkage: a negatively charged phosphate bridge between two nucleosides in DNA and RNA.

Phosphoramidite: a phosphorus(iii) group containing two oxygen substituents and one nitrogen substituent; on treatment with mild acid, the nitrogen substituent becomes an excellent leaving group. Nucleoside phosphoramidites are the monomers used in the most common method for oligonucleotide synthesis.

Phosphorodiamidate morpholino oligonucleotide (PMO): a nucleic acid analogue where a morpholino group replaces the sugar moiety and a neutral phosphoramidate linkage replaces the phosphodiester.

Phosphorodithioate (PS2): modification of a phosphodiester linkage where two sulfur atoms replace both of the non-bridging oxygens.

Phosphorothioate (PS): modification of a phosphodiester linkage where one sulfur replaces one oxygen in a non-bridging position: a widely used modification for therapeutic oligonucleotides; other phosphorothioate analogues may have a bridging oxygen atom replaced by sulfur.

Phosphorylation: addition of a phosphate ester to an oxygen, nitrogen or sulfur in another molecule.

Phosphotriester linkage: the phosphate bridge between two nucleosides in DNA and RNA where one non-bridging oxygen atom is linked to an alkyl or aryl moiety.

Phylogeny: the evolutionary relationship between a group of related species, often depicted as an evolutionary tree.

Physical mapping: an approach to establish the physical location of a genetic marker on a chromosome.

piRNA [P-element induced wimpy testis protein (Piwi) interacting RNA]: the largest class of small non-coding RNA molecules expressed in animal cells that form RNA–protein complexes via interactions with piwi-subfamily Argonaute proteins; mostly involved in epigenetic and post-transcriptional silencing of transposable elements and other repeat-derived transcripts: also involved in regulation of genetic elements in germ line cells.

Plasmid: an autonomous self-replicating extra-chromosomal circular DNA.

Plastid: family of membrane-bound organelles unique to plant cells, only one type is found in each cell. All types derive from a proplastid, a common precursor organelle.

Platform potential: as applied to nucleic acid therapeutics, this is the principle that technologies, including mechanisms, chemical modification strategies and delivery vehicles, can be optimized for one target and then applied to other targets, enabling rapid drug discovery for rare diseases once a platform technology is established in a particular tissue or cell type.

Ploidy: the number of genome copies in a cell before DNA replication.

Poison exon: a naturally occurring alternative exon containing a premature termination codon.

Polyadenylation: post-transcriptional attachment of up to several hundred adenylate residues to the 3′-terminus of most eukaryotic mRNAs.

Polylinker: synthetic dsDNA oligonucleotide having a number of different restriction sites.

Polymerase: an enzyme that catalyses the assembly of ribonucleotides into RNA or of deoxynucleotides into DNA, usually in a template-directed manner.

Polymerase switching: replacement of a replicative polymerase by a repair polymerase to replicate damaged DNA followed by subsequent reinstatement of the replicative polymerase.

Polymorphic sequence: a sequence that differs between individual genomes or between individual copies of repeated genes.

Polynucleotide: a long single-stranded nucleic acid formed of nucleotides (ribo-, 2ʹ-deoxyribo-, analogue or combinations) joined by phosphodiester linkages.

Polyploid: a cell or organism with more than the normal number of copies of its genome.

Polytene: a chromosome that has undergone reduplication, resulting in as many as thousands of copies of the chromosome.

Pre-initiation complex: the complex of proteins established at a promoter and required to initiate transcription.

Primer: a short sequence (of DNA or RNA) paired with a single-stranded DNA to provide a free 3′-OH end at which a DNA polymerase starts the synthesis of a complementary DNA chain.

Probe: a labelled DNA or RNA molecule used to detect a complementary sequence by molecular hybridization.

Processivity clamp: a ring-shaped protein complex that encircles dsDNA just behind the site of DNA synthesis; increases polymerase processivity by topologically attaching the polymerase to a template.

Prokaryote: an organism whose cells lack a nucleus, such as bacteria and archaea.

Promoter: the region of the gene, usually directly upstream of the transcriptional start site, involved in recruiting the RNA polymerase to initiate transcription.

Protein A: a protein from Staphylococcus aureus that binds specifically to immunoglobulin G molecules; used in detection of proteins by immunological techniques.

Protein-coding gene: a gene transcribed into RNA that encodes a protein.

Proteinase K: a protease enzyme used to remove contaminating protein in preparations of nucleic acids.

Proteome: the entire collection of proteins expressed by a genome, cell, tissue or organism at a certain time under defined conditions.

Proteomics: the study of the proteome.

Proton sponge effect: a mechanism by which a cationic polymer is thought to contribute to endosomal escape. As the endosome matures and becomes acidic, the cationic polymer becomes protonated, leading to recruitment of anions and swelling and eventual bursting of the endosomal membrane.

Protoplast: a cell from which the cell wall has been removed but which retains an intact cell membrane.

Protospacer-adjacent motif (PAM): a two-to-four base-sequence adjacent to the target site of a CRISPR–Cas protein which enables self–non-self discrimination by prokaryotes.

Pseudogene: a protein-coding gene that has lost its ability to be expressed and/or its coding capacity; usually a gene in the process of being evolutionarily discarded.

Pseudoknot: an RNA secondary structure minimally composed of two helical segments connected by single-stranded regions or loops.

Purine: a heterocyclic base having fused five- and six-membered rings [A, G, hypoxanthine (H) or xanthine (X)] occurring in nucleotides and in DNA and RNA.

Pyrimidine: a six-membered ring heterocyclic base occurring in nucleotides and in DNA (C, 5mC and T) and RNA [C, dihydrouridine (D), pseudouridine (Ψ) and U].

Quadruplex: a four-stranded box-like structure with a central cavity created by parallel stacking of two or more G-tetrads.

RAN translation (repeat associated non-AUG translation): an irregular mode of mRNA translation that can occur in eukaryotic cells.

Recombinant DNA: a DNA molecule created by ligating pieces of DNA, normally not contiguous.

Recombinase: an enzyme that catalyses recombination between similar or identical sequences.

Recombination: the coordinated exchange of phosphodiester bonds between DNA duplexes resulting in a segment of one duplex being ligated to a segment of the other and vice versa.

Recombinational exchange: the exchange of DNA sequence between chromosomes by recombination.

Recombinational repair: repair of DNA damage by recombinationally replacing the damaged DNA with an undamaged copy from another chromosome.

Reference genome: the consensus genome assembly for a species.

Renaturation (of DNA or RNA): re-establishment of a DNA duplex or intrastrand hairpin structures in an RNA species after denaturation.

Replication-defective viruses: engineered vectors in which components of a viral genome that are required for viral replication are replaced with a therapeutic gene cargo and used for gene therapy.

Replication fork: a three-stranded structure formed during DNA replication consisting of the partially unwound parental duplex annealed to the nascent leading and lagging stands.

Replication origin: a site in the genome where DNA replication initiates.

Replicon: regulatory unit of an origin and proteins necessary for initiation of replication (origin specific).

Replisome: the large and dynamic protein complex that assembles at a replication fork to catalyse coupled leading- and lagging-strand replication.

Repression: inhibition of transcription (or translation) by binding a repressor protein to a specific site on DNA (or mRNA).

Restriction enzyme: an enzyme that recognizes short double-stranded DNA sequences and cleaves them at or near the target site.

Restriction fragment: a duplex DNA fragment produced by restriction enzyme digestion.

Retrotransposon: a transposon that moves by being transcribed into RNA and then using reverse transcription to create a DNA copy for reintegration.

Retrovirus: a virus containing an ssRNA genome that propagates via conversion into dsDNA by reverse transcription.

Reverse transcriptase: an RNA-dependent DNA polymerase that catalyses reverse transcription.

Reverse transcription: enzymatic synthesis of ssDNA on an RNA template.

Reversion: a change in DNA that either reverses the original alteration (true reversion) or compensates for it (second site reversion in the same gene).

Ribonucleotide: monomeric unit of RNA having a ribofuranose, heterocyclic base and phosphate: distinguished from a deoxyribonucleotide by its 2ʹ-OH group.

Ribosomal RNA: large non-coding RNAs that comprise the core of the ribosome.

Ribosome: a subcellular molecular machine composed of RNA and protein that synthesizes proteins by translating the genetic code of mRNAs.

Riboswitch: a part of an mRNA molecule that can directly bind a small target molecule, thereby affecting the activity of the RNA.

Ribozyme: an RNA molecule that can catalyse a chemical reaction; as in the amino-transferase activity of the ribosome or in the chain-cleavage (or ligation) of itself or of another RNA molecule.

RISC: an RNA-induced silencing multiprotein complex that incorporates one strand of a small interfering RNA (siRNA) or micro RNA (miRNA) as a template for recognizing complementary mRNA; its complex with a complementary RNA strand activates an RNase to cleave that RNA.

RNA editing: post-transcriptional modification of an RNA sequence.

RNA interference (RNAi): a process in which double-stranded small RNAs inhibit expression of complementary or partially complementary mRNAs by cleaving, blocking translation and/or promoting degradation of the target mRNA.

RNA processing: removal, addition and/or modification of nucleotides of an RNA transcript giving rise to a mature, functional RNA product.

RNase: an enzyme that degrades RNA molecules to nucleotides.

RNase H: a non-sequence-specific endonuclease that cleaves the RNA strand in an RNA–DNA duplex.

RNase P: a ribozyme that cleaves extraneous nucleotides from tRNA molecules.

RNA-seq: high-throughput sequencing of cDNA produced from an RNA sample.

Rossmann Fold: a tertiary fold in nucleotide binding proteins that uses a conserved β,α,β (or α,β) region to bind the ADP moiety of nucleotides and cofactors [ATP, flavin adenine dinucleotide (FAD), nicotinamide adenine dinucleotide (NAD+) and NAD phosphate (NADP+)]: Found in >20% of all known protein structures with hydrogen bonding between up to six parallel β-strands.

Roundworm: a member of the phylum Nematoda; often used to refer to the specific species Caenorhabditis elegans, a widely used experimental organism.

RRM (RNA-recognition motif): an RNA-interacting protein domain playing an important role in features of RNA metabolism; e.g. degradation, editing, export, splicing and regulation of translation.

S phase: the phase of the cell cycle after G1 and before G2 during which DNA is replicated.

Sanger–Coulson sequencing: DNA sequencing based on polymerase transcription of ssDNA in the presence of dideoxynucleotides: the same technique is also used for sequencing RNA.

Satellite repeat: a diverse collection of short tandem genomic DNA repeats, typically 10 bp to several hundred bp in length.

SDS gel electrophoresis: gel electrophoresis of proteins in polyacrylamide gels in the presence of sodium dodecyl sulfate (SDS) as detergent. SDS associates with the proteins to give them all a similar electric charge density and thereby allows separation on the basis of molecular weight differences.

Seed sequence: (of miRNA or siRNA) bases 2–8 from the 5ʹ-end of the guide strand, used to nucleate binding to target mRNA. [of CRISPR RNA (crRNA)] the section of the guide region used to nucleate binding to target DNA, typically the region proximal to the PAM site.

Segmental duplication: large, often tandem, duplication of chromosomal regions, typically 10 to 100 kb in length.

Selection: use of particular conditions to allow survival only of cells with a particular phenotype.

SELEX: Systematic evolution of ligands by exponential enrichment, a technique that allows simultaneous screening of highly diverse pools of different RNA or DNA molecules in order to obtain a particular feature.

Semi-conservative replication: a DNA replication strategy in which the product of replication is two DNA double helices, both of which have one parental stand and one nascent strand.

Sequencing: the process of determining the linear order of amino acids or nucleotides in a protein, DNA or RNA strand.

Sequencing gel: a very thin (0.1–1 mm) polyacrylamide gel used for determining the sequence of DNA or RNA.

Shine–Dalgarno sequence: part or all of the polypurine sequence AGGAGG located on bacterial mRNA just prior to an AUG initiation codon: is complementary to the sequence at the 3′-end of 16S rRNA and is involved in binding a ribosome to an mRNA.

Short tandem repeat (STR): a DNA sequence containing a variable number (≤50) of tandemly repeated short (2–6 bp) sequences, such as (GATA)n: forensic STRs are usually tetranucleotide repeats, which show little PCR stutter.

Shotgun sequencing: a DNA sequencing strategy in which copies of a template are broken into random short fragments, whence each nucleotide of the template is sequenced on many fragments. The template sequence is then reconstructed by overlapping the fragment sequences.

shRNA (short hairpin RNA): a self-complementary RNA of about 22 base-pairs that is used to silence gene expression selectively via RNA interference.

Shuttle vector: a vector able to replicate in different host organisms e.g. in E. coli and in mammalian cells.

Sibling chromosomes: the two identical chromosomes produced by DNA replication: also known as sister chromosomes.

Sigma factor: the bacterial RNA polymerase promoter-recognition subunit.

Signal hypothesis: a process by which proteins synthesized in the cytoplasm are exported either out of the cell or into one of its organelles; the signal peptide of the protein plays an important role in this process.

Signal transduction: the process of transferring information within or between cells, often across membranes.

Simple STRs: short tandem repeat loci composed of uninterrupted runs of a single repeat type.

Single-cell-seq: a high-throughput sequencing approach used on a single-cell sample.

Single-nucleotide polymorphism (SNP, pronounced ‘snip’): a polymorphic sequence that differs by only one nucleotide.

siRNA (small interfering RNA): double-stranded RNAs of about 22 nucleotides that mediate gene silencing and mRNA degradation via RNA interference.

Site-directed mutagenesis: introduction of a specific mutation into a DNA molecule at a predetermined site.

snRNA (Small nuclear RNAs): a class of small RNA molecules involved as ribonucleoproteins (snRNPs) in splicing RNAs in the nucleus of a eukaryotic cell.

snoRNA (Small nucleolar RNAs): a class of small RNA molecules that primarily guide chemical modifications of other RNAs, mainly rRNAs, tRNAs and snRNAs.

Somatic cell: A cell that cannot contribute to the next generation.

Southern blotting: transfer of denatured DNA from an agarose gel onto a nitrocellulose filter and identification by hybridization with a complementary nucleic acid probe.

Spindle fibre: a microtubule or bundle of microtubules that makes up the mitotic spindle.

Spine of hydration: the network of water molecules existing in the major groove of a DNA molecule.

Spliceosome: a complex of several RNAs and proteins responsible for removing the non-coding introns from unprocessed mRNA.

Splicing: sequential removal of introns and the splicing together of exons in RNA.

Stem: the base-paired segment of a hairpin.

Start codon: the triplet sequence AUG, which codes for methionine and is the first codon in an open reading frame (also called the initiation codon).

Sticky ends: overhanging single-stranded regions on a segment of DNA produced by many kinds of restriction enzymes; their single-stranded nature allows them to base-pair with complementary sticky ends.

Stop codon: one of three triplet sequences, UAG, UAA or UGA, causing termination of protein synthesis; also called a termination codon (UAG is known as amber; UAA as ochre, UGA as opal).

Structural gene: gene coding for any RNA or protein product other than a regulator.

Stutter: see PCR stutter.

Subcloning: cloning of fragments of an already cloned DNA sequence.

Supercoil: over- or under-winding of dsDNA relative to the number of twists in a relaxed duplex.

Synonymous codon: one of several codons that encodes the same amino acid.

Synthesis-dependent strand annealing (SDSA): a form of homologous recombination in which one end of a ds-break invades a homologue, is extended, displaced from the template and annealed to the other end of the break – leaving the template homologue unaltered.

Tac-promotor: a chimeric bacterial promotor of high strength constructed from parts of the Trp and lac promotors of E. coli.

Tandem array: repeated sequences arranged in a head-to-tail orientation.

TATA box: a conserved A–T-rich heptamer found approximately 25 bp before the start-point of many eukaryotic RNA polymerase II transcription units; involved in positioning the enzyme for correct initiation.

Taxonomy: classification of organisms by shared characteristics.

Telomerase: a reverse transcriptase that elongates chromosome ends by adding short, repetitive sequences.

Telomere: the end of a linear chromosome terminating in a long stretch of short repetitive sequences at which a nucleoprotein complex assembles to maintain chromosome-end stability.

Template: portion of ssDNA or ssRNA used to direct the synthesis of a complementary strand.

Termination codon: see Stop codon.

Toll-like receptor: receptor molecules in vertebrates that are able to stimulate activation of the adaptive immune system, linking innate and acquired immune responses.

Topoisomerase: an enzyme that acts on the topology of DNA; required to untwist supercoiled DNA and untangle DNA strands that are topologically linked.

trans-Acting: the ability of a DNA or RNA sequence to exert its influence at a distance by production of a diffusible protein.

Transcript: the RNA transcribed from a gene.

Transcription: synthesis of RNA on a DNA template.

Transcription unit: the section of a gene transcribed into RNA.

Transcriptional start site: DNA site at which transcription is initiated.

Transcriptional stop site: DNA site corresponding to the last nucleotide in a transcript. Because the 3′-end of eukaryotic mRNA is formed by nuclease cleavage and polyadenylation, the transcriptional stop site does not correspond to the site of RNA pol II termination.

Transcription factor: a protein that recognizes a specific sequence, usually in the promoter sequence of a gene and which regulates the expression of that gene.

Transcriptome: the entire collection of RNA transcripts expressed in a particular gene or gene cluster.

Transduction: the transfer of a bacterial gene from one bacterium to another by a phage.

Transfection: the introduction of native, protein-free DNA or RNA into cells other than by viral infection.

Transfer RNA (tRNA): a folded RNA that transports an amino acid into the ribosome for protein synthesis.

Transformation: the acquisition into a cell of new genetic markers by incorporation of added DNA: in eukaryotic cells also refers to conversion into a state of unrestrained growth in culture resembling or identical to a tumorigenic condition.

Transgene: a gene transferred naturally or by any of a number of genetic engineering techniques from one organism to another.

Transition: a mutation in which a purine is replaced by another purine (e.g. G to A) or a pyrimidine by another pyrimidine (e.g. T to C).

Transition State (TS): highest energy point on the reaction coordinate from reactants to products; regarded as the complex of its constituents that has maximum affinity for the enzyme catalysing that interconversion.

Translation: the synthesis of protein encoded by an RNA.

Translocation: the movement of a segment of DNA, usually a large segment including a telomere, from one chromosome to another.

Transposable element: a heterogeneous class of genetic element (transposon) that can move from one location to another within the genome, often duplicating itself in the process.

Transposase: an enzyme that catalyses the movement of a transposable element.

Transversion: a mutation in which a purine is replaced by a pyrimidine or vice versa.

Triplet: a sequence of three nucleotides in DNA or RNA; see Codon.

Two-dimensional gel electrophoresis: a protein separation technique using separation by charge in the first direction on the plate and separation by mass in the second, perpendicular direction.

Untranslated region (UTR): an untranslated portion of an mRNA transcript, typically including regions upstream (5′-UTR) and downstream (3′-UTR) of the translated region (open reading frame).

Upstream: refers to the relative position of genetic code in DNA or RNA; upstream is toward the 5ʹ-end in RNA while in DNA upstream is towards the 5ʹ-end of the coding strand which is towards the 3ʹ-end of the template strand.

Watson–Crick rules: base-pairing rules that underlie gene structure and expression: in DNA, G pairs with C and A pairs with T; in RNA, A pairs with U.

Western blotting: transfer of proteins from a gel onto a nitrocellulose filter on which they can subsequently be imaged by use of antibodies.

Wild-type: the genotype or phenotype found in nature or in the standard laboratory stock for a given organism.

Wobble pairing: the ability of a tRNA to recognize more than one codon by non-Watson–Crick pairing for the third base of its anticodon, i.e. G with C or U; some wobble positions can pair with any of the four bases.

Xeno Nucleic Acid (XNA): synthetic nucleic acid composed of non-natural nucleotides, typically with a sugar backbone not based on ribose.

Deceased.

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