N

Native Gel:

An electrophoresis gel run under conditions which do not denature proteins (i.e., in the absence of SDS, urea, 2-mercaptoethanol, etc.).

Nested PCR:

A very sensitive method for amplfication of DNA, which takes part of the product of a single PCR reaction (after 30-35 cycles), and subjects it to a new round of PCR using a different set of PCR primers which are nested within the region flanked by the original primer pair

Negative Supercoling:

It comprises the twisting of a duplex of DNA in space in the opposite sense to the turns of the strands in the double helix.

Neutral Substitutions:

Substitutions in a protein are those changes of amino acids that do not effect the activity.

Nick:

In duplex DNA, this refers to the absence of a phosphodiester bond between two adjacent nucleotides on one strand.

Nick Translation:

A method for incorporating radioactive isotopes (typically 32P) into a piece of DNA. The DNA is randomly nicked by DNase I, and then starting from those nicks DNA polymerase I digests and then replaces a stretch of DNA. Radiolabeled precursor nucleotide triphosphates can thus be incorporated.

Nonconservative Substitution:

A mutation which results in the substitution of one amino acid within a polypeptide chain with an amino acid belonging to a different polarity/charge group.

Nontranslated RNA (NTR):

The segments located at the 5' and 3' ends of a mRNA molecule which do not encode any part of the polyprotein; may contain important translational control elements.

Nonsense Codons:

It is one of the three triplets(UAG,UAA,UGA) that causes the termination of protein synthesis.

Normalization:

A control gene that is expressed at a constant level is used to normalize the gene expression results for variable template amount or template quality.

Northern Blot:

A technique for analyzing mixtures of RNA, whereby the presence and rough size of one particular type of RNA can be ascertained.

Nonsense Suppressor:

It is a gene coding for a mutant tRNA able to respond to one or more of the termination codons.

Nontranscribed spacer:

It is the region between transcription units in a tandem gene cluster.

NT:

Abbreviation for nucleotide; i.e. the monomeric unit from which DNA or RNA are built.

Nuclease:

An enzyme which degrades nucleic acids. A nuclease can be DNA-specific (a DNase), RNA-specific (RNase) or non-specific. It may act only on single stranded nucleic acids, or only on double-stranded nucleic acids, or it may be non-specific with respect to strandedness.

Nucleic Acid Target (also called “target template”):

DNA or RNA sequence that is going to be amplified.

Nucleoytic Reactions:

It involves the hydrolysis of a phosphodiester bond in a nucleic acid.