|Content of Nucleic Acid Synthesis
Metaphase chromosomes in cells undergoing mitosis are
visible under the light microscope. Their formation requires
some 104- to 105-fold condensation of uninterrupted
linear duplex DNA which has a 2-nm diameter.
Such compaction is accomplished in a highly complex
and stepwise fashion. Because DNA is a polyelectrolyte
with two negative charges per nucleotide, charge neutralization
and shielding is required before the polymer can
be folded in an ordered, condensed structure. In addition
to metal ions and polyamines, the major source of the
positive charge in chromatin is the family of highly basic
small proteins, called histones, which are rich in the basic
amino acid residues lysine and arginine needed to neutralize
the charge of the phosphate backbone of DNA. The
prokaryotes also have basic proteins (such as HU protein in E. coli) which induce DNA condensation. However, chromatin compaction in eukaryotes is carried out in stages.
The simplest folded unit of DNA is the 10-nm nucleosome,
consisting of a core histone octamer containing two
molecules each of histone H2A, H2B, H3, and H4 around
which nearly two turns of the DNA is wrapped. The nucleosome
cores are connected by a stretch of linear DNA
(linker) of variable length which is covered by histone H1
or H5. The polymeric chain nucleosomes are then folded in
a 30-nm fiber whose structure is stabilized by the interaction
among histones and a number of other proteins collectively
called nonhistone chromosomal proteins (NHC),
including high mobility group (HMG), which are not particularly
basic. Eventually, the fibers are condensed into
highly compacted metaphase chromosomes. The nature
of the interactions present in interphase and metaphase
chromosomes is not clear.
However, the implications of this compaction are profound.
It is absolutely essential to condense the mammalian
genome, which in an extended linear form more
than 1 m long, to a volume which can be accommodated
in the nuclear volume of 10–30 femtoliters. At the same
time, the genes will be buried in condensed chromatin, and
yet their specific sequences need to be exposed for various
processes of information transfer. Thus, for both transcription
and replication, the chromatin has to be decondensed.
This was evident in early in vitro studies which showed
that both these processes are severely inhibited when DNA
is complexed with histones.