380
Cell Nucleus Biogenesis, Structure and Function
chromosomes. Somatic human cells are
diploid and a vast majority have the
same genetic information, which is held
within the 46 chromosomes – 22 pairs
of autosomes called chromosomes 1–22
and 1 pair of sex chromosomes, XX in
females or XY in males. This DNA is
folded as chromatin to form chromosomes
and it is this DNA–protein complex
that serves as the genetic substrate that
supports the major functions performed
by DNA. The major functions are as
follows:
1. DNA synthesis – called
DNA replica-
tion
– ensures that the same genetic
complement is passed from the fer-
tilized
egg
to
all
subsequent
cell
descendants.
2. RNA synthesis – called
RNA transcrip-
tion
– ensures that genes are expressed
in the appropriate cells at the required
times throughout the development of
an organism. Different patterns of gene
expression in different types of cells
and tissues are determined by patterns
of differentiation.
3. DNA repair ensures that the integrity
of genetic information is maintained
so that the same DNA sequence is
found in all cells of the body. De-
fects in DNA repair can lead to a
variety of diseases through sporadic
mutations, and are particularly im-
portant during the development of
cancer.
Different aspects of chromatin function
and nuclear structure are fundamental
to the development of multicellular or-
ganisms. The nucleus itself plays a vital
role in this development by ensuring that
functions performed on chromatin are
isolated from major cytoplasmic activi-
ties such as protein synthesis and energy
metabolism. This separation inevitably de-
mands that a critical step of the gene
expression process involves the transfer
of information from each gene to the
cytoplasm where it can be decoded to
generate the corresponding protein. The
genetic intermediary involved is called
mes-
senger RNA
(mRNA) and the process of
protein synthesis is called
protein trans-
lation
. The fact that eukaryotic cells are
divided into two major compartments by
the nuclear membrane has a number of
implications. One disadvantage is the need
to develop complex systems to regulate
compartmentalization and control trafFc
between the two compartments. The ma-
jor advantage is that separating protein
synthesis from the compartment in which
DNA function is performed allows qual-
ity controls to be installed to ensure that
protein synthesis is only performed on in-
tact and authentic mRNAs. In bacteria,
transcription, translation, protein folding,
and incorporation of the nascent folding
polypeptide into the functional sites are
coupled, so that in principle the nascent
protein might still be attached – through
the ribosome, mRNA and RNA poly-
merase – to its gene. In eukaryotes, much
more complex gene structures, and in par-
ticular, the fact that most genes contain
intervening sequences that do not hold
protein coding information, demands that
the events of mRNA and protein synthesis
are uncoupled.
2
Nuclear Function
The basic molecular mechanisms that
dictate most aspects of nuclear function
are
known
in
detail
and
it
is
only
appropriate to cover the fundamental
principles here.
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