Bma Illustrated Medical Dictionary

The ladder is twisted into a spiral shape
called a double helix.
Each DNA chain has a “backbone”
consisting of a string of sugar and phos-
phate chemical groups. Attached to
each sugar is a chemical called a base,
which can be any of 4 types (adenine,
thymine, guanine, and cytosine) and
forms half a rung of the DNA ladder. The
4 bases can occur in any sequence
along the chain. The sequence, which
may be many millions of individual
bases long, provides the code for the
activities of the cell (see genetic code).
RNA is like a single strand of DNA; the
main difference is that the base thymine
is replaced by another base, uracil.
DNA controls a cell’s activities by
specifying and regulating the synthesis
of enzymesand other proteins in the
cell. Different genes(sections of DNA
that code for information) regulate the
production of different proteins. For a
particular protein to be made, an
appropriate section of DNA acts as a
template for an RNA chain.This “mes-
senger” RNA then passes out of the
nucleus into the cell cytoplasm, where
it is decoded to form proteins (see
genetic code; protein synthesis).
When a cell undergoes mitotic (see
mitosis) division, identical copies of its
DNA must go to each of the 2 daughter
cells. The 2 DNA chains separate, and 2
more chains are formed, side by side
with the original chains. Because only
certain base pairings are possible, the
new double chains are identical to the
original DNA molecule. Each of a per-
son’s cells carries the same DNA replica
that was present in the fertilized ovum,
so the DNA message passes from one
generation of cells to the next.
nucleusThe central core, structure, or
focal point of an object.
The nucleus of a living cellis a roughly
spherical unit at the centre of the cell. It
contains the chromosomes(composed
mainly of nucleic acid), which are respon-
sible for directing the cell’s activities, and
is surrounded by a membrane. The mem-
brane has small pores through which
various substances can pass between the
nucleus and the cytoplasm, a thick fluid
that forms the bulk of the cell. Usually,

the nucleus has 1 nucleolus, a smaller

dense region with no membrane that is

concerned with protein manufacture.

A nerve nucleus is a group of neurons

(nerve cells) within the brain and spinal

cord that work together to perform a

particular function.

The nucleus of an atom, composed

of protons and neutrons, accounts for

almostthe total mass of the atom but

only a tiny proportion of its volume.

Nuclear energyis produced through

changes in atomic nuclei.

numbnessLoss of sensation in part of

the body caused by interference with the

passage of impulses along sensory

nerves. Numbness may be the result of

a disorder of or damage to the nervous

systemor its blood supply.

Multiple sclerosiscan cause loss of sen-

sation in any part of the body through

damage to nerve pathways in the central

nervous system (CNS). In a neuropathy,

the peripheral nerves (nerves outside

the CNS) are damaged. In a stroke, pres-

sure on, or reduced blood supply to,

nerve pathways in the brain often causes

loss of feeling on one side of the body.

Severe cold causes numbness by

direct action on the nerves. Numbness

may also be a feature of psychological

disorders, such as anxiety, panic attack,

or a hysterical conversion disorder.

Treatment of numbness depends on

the underlying cause.

NUCLEUS NUMBNESS

N

Nucleolus

Nucleus

Cytoplasm

Mitochondrion Cell membrane

TYPICAL HUMAN CELL

NUCLEUS