N-terminus (amino-terminus) - The end of a polypeptide chain that contians a free amino group that is not connected to any other amino acid.

Nitrogenous base - A nitrogen containing molecule having the chemical properties of a base.

Non-Disjunction - The failure of paired chromosomes or sister chromatids to separate and go to different cells during meiosis.

Nucleic acid - A large molecule composed of nucleotide subunits.

Nucleolus - Large sphere shaped organelle visible in interphase eukaryotic cells with the use of a light microscope.

Nucleosome - These are the basic building blocks of chromatin structure. Nucleosomes consist of a core of proteins called histones around which the DNA is wound.

Nucleotide - A subunit of DNA or RNA consisting of a nitrogenous base (adenine, guanine, thymine or cytosine in DNA; adenine, guanine, uracil or cytosine in RNA) , a phosphate molecule, and a sugar molecule. Thousands of nucleotides may be linked to form a DNA or RNA molecule.

Nucleus - The cellular organelle in eukaryotes that contains the genetic material.