Nalidixic acid
An antibiotic that inhibits DNA gyrase.

Negative dominance
A gene product that inhibits the function of another gene product.

Negative regulation
A mechanism of genetic regulation in which a protein or RNA molecule inhibits gene expression, e.g. by preventing the transcription of a gene or translation of the mRNA. A negative regulator that inhibits transcription of a gene is commonly called a repressor.

Repair enzymes that are able to excise certain incorrectly paired or damaged nucleotides.

A single-strand break, involving the absence of one or more nucleotides, in a double-stranded DNA molecule. A break in the phosphodiester bond between adjacent nucleotides in one strand of a DNA duplex.

Nick translation
A method which uses DNA polymerase I to first produce a nick in a DNA duplex, then degrade stretch of single-stranded DNA using its 5'-exonuclease while synthesizing a new strand in its place. By including a labeled dNTP in the reaction, the newly synthesized DNA will be labeled.

Non-composite transposon
A transposible element that is NOT flanked by IS elements.

Nucleic acid molecules that do not share similar nucleotide sequences.

Non-permissive conditions
Growth conditions not allowing a conditionally lethal mutant to survive.

Nonsense codon
A codon which does not code for any amino acid, but signals a termination of translation, or punctuation. The three nonsense codons are UAG (amber), UAA (ochre), and UAG (opal).

Nonsense mutation
A mutation which replaces a codon for an amino acid with a codon for chain termination (UAG, UAA, or UGA).

Nonsense suppressors
A mutant tRNA that recognizes a nonsense (stop) codon and inserts an amino acid into the growing polypeptide chain. Nonsense suppressors are produced by base substitution mutations in the DNA corresponding to the anticodon of a tRNA that cause the anticodon to pair with one of the terminarion (or "nonsense") codons, UGA (Amber), UAA (Ochre), or UAG (Opal). Note that because of the wobble rules amber suppressing tRNAs can only read UAG codons, but ochre-suppressing tRNAs can read both UAA and UAG codons. Two possible events can occur when a ribosome encounters a nonsense codon in a strain with a nonsense suppressor: termination of peptide elongation can occur if the appropriate release factors associate with the ribosome, or an amino acid can be inserted into the growing peptide chain if the suppressor tRNA associates with the ribosome. The efficiency of suppression depends upon how well the suppressor tRNA is charged with the appropriate amino acid, the concentration of the suppressor tRNA in the cell, and the "context" of the nonsense codon in the mRNA -- especially the base on the 3' side of the codon. Although suppressor tRNAs are often inefficient, the amount of protein produced is often sufficient to repair the mutant phenotype.

Northern transfer
A technique for transferring bands of RNA from an agarose gel to a nitrocellulose or similar membrane.

The end of a polypeptide chain that has a free amino acid (-NH2) group.

Nucleotide triphosphate. Used for the synthesis of RNA or as an energy source.

An enzyme which cleaves phosphate-deoxyribose bonds within (endonuclease) or at the end (exonuclease) of a nucleotide sequence. Nucleases usually recognize a specific substrate, such as single- or double-stranded DNA or RNA.

Nucleic acid hybridization
Formation of a double-stranded molecule by base pairing between complementary or homologous polynucleotides.

The condensed organization of a prokaryote chromosome inside the cell.

Nucleoside analog
See analog.

A nucleoside with a phosphate group attached.

Completely absent. For example, a null mutation completely disrupts a gene.