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
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
and other proteins in the
(sections of DNA
that code for information) regulate the
production of different proteins. For a
to be made,
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
) 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.
The central core, structure, or
focal point of an object.
The nucleus of a living
is a roughly
spherical unit at the centre of the cell. It
), 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,
TYPICAL HUMAN CELL
the nucleus has
nucleolus, a smaller
dense region with no membrane that is
concerned with protein manufacture.
A nerve nucleus is a group of
(nerve cells) within the brain and spinal
cord that work together to perform a
The nucleus of an atom, composed
of protons and neutrons, accounts for
almost the total mass of the atom but
only a tiny proportion of its volume.
is produced through
changes in atomic nuclei.
Loss of sensation in part of
the body caused by interference with the
passage of impulses along sensory
. Numbness may be the result of
a disorder of or damage to the
or its blood supply.
can cause loss of sen-
sation in any part of the body through
damage to nerve pathways in the central
nervous system (CNS). In a
the peripheral nerves (nerves outside
the CNS) are damaged. In a
sure on, or reduced blood supply to,
nerve pathways in the brain often causes
loss of feeling on one side of the body.
direct action on the nerves. Numbness
may also be a feature of psychological
disorders, such as
anxiety panic attack,
or a hysterical
Treatment of numbness depends on
the underlying cause.