Encyclopedia of Biology

contact (dipolar) shifts, and considerable increases in
the nuclear spin relaxation rates. From this type of
measurement, structural information can be obtained
about the paramagnetic site.

nuclear pores Openings in the membrane of a cell’s
nuclear envelope that allow the exchange of materials
between the nucleus and the cytoplasm.

nucleation The process by which nuclei are formed;
defined as the smallest solid-phase aggregate of atoms,
molecules, or ions that is formed during a precipitation
and that is capable of spontaneous growth.

nucleic acid probe (DNA probe) A single strand
of DNA that is labeled or tagged with a fluorescent or
radioactive substance and binds specifically to a com-
plementary DNA sequence. It is used to detect its
incorporation through hybridization with another
DNA sample. Nuclear acid probes can provide rapid
identification of certain species like mycobacterium.

nucleic acids Macromolecules composed of
SEQUENCEs of NUCLEOTIDESthat perform several func-
tions in living cells—e.g., the storage of genetic infor-
mation and its transfer from one generation to the next
(DNA), and the EXPRESSIONof this information in pro-
tein synthesis (mRNA, tRNA)—and can act as func-
tional components of subcellular units such as
RIBOSOMEs (rRNA). RNA contains D-ribose, while
DNA contains 2-deoxy-D-ribose as the sugar compo-
nent. Currently, synthetic nucleic acids can be made
consisting of hundreds of nucleotides.
See alsoGENETIC CODE;OLIGONUCLEOTIDE.

nucleobases SeeNUCLEOSIDES.

nucleoid The irregularly shaped, aggregate mass of
DNA that makes up the chromosome in a prokaryotic
cell; not bound by a membrane. Found in the nucleoid
region of the cell.

nucleolus A somewhat round structure in the nucle-

us that forms at the nuclear organizer, a specific chro-

mosomal region, consisting of ribosomal RNA (rRNA)

and protein. It disappears during nuclear division in

late prophase, is completely absent during meta and

anaphase, and then reappears during telephase. The

nucleus controls the synthesis of proteins in the cyto-

plasm through messenger RNA (mRNA). Messenger

nucleolus 249

base pairs

sugar-

phosphate

backbone

sugar-

phosphate

backbone

Deoxyribonucleic acid (DNA)

P P P P P P P

P

P

P

P

P

A

T

A

A

T

GC

CG

GC

T

S

S

S

S

S

S

S S S S S S

hydrogen

bonds

base pair

nucleotide

One of the structural components, or building blocks, of DNA and

RNA, a nucleotide consists of a base (one of four chemicals: ade-

nine, thymine, guanine, and cytosine) plus a molecule of sugar

and one of phosphoric acid.(Courtesy Darryl Leja, NHGRI,

National Institutes of Health)