The interphase chromosomes of eukaryotic cells are complex molecular structures
composed primarily of a DNA core and a protein matrix complexed into a long
thread-like structure. This basic chromosome thread is then coiled through
several layers of organization and ultimately gives rise to a structure that can
be visualized with a light microscope.
Chemically, the interphase nucleus is composed of a substance known as
“chromatin”, which is further subdivided into euchromatin and heterochromatin.
The distinction between these subdivisions is based on quantitative distribution
of the basic chromosome fiber, with a higher concentration found in heterochromatin.
Heterochromatin, therefore, will stain more intensely than
euchromatin, since the fiber is packed tighter within a given volume.
Proteins extracted from chromatin have been classified as either basic or
acidic in nature. The basic proteins are referred to as “histones” and the acidic
as “nonhistone proteins”. Histones play an integral part in the structural
integrity of a eucaryotic chromosome. They are organized into specific complexes,
known as nucleosomes, and around which the DNA molecule is coiled. Acidic
proteins within the nucleus compose many of the DNA replication and RNA
transcription enzymes and regulatory molecules. They vary in size from small
peptides of a few amino acids to large duplicase and replicase enzymes
(respectively, DNA and RNA polymerases).
Transcription of DNA on the chromosome fiber results in the presence of
a host of RNA species found within the nucleus of the cell. When the RNA
is transcribed from the “nucleolar organizer” region of a genome and complexed
with ribosomal proteins, granules are formed, which collectively produce a
“nucleolus,” visible at the light microscope level of resolution. When transcribed
from other portions of the genome, the RNA is either in the form of pretransfer
RNA, or heterogeneous nuclear RNA (hnRNA). The precursor tRNA must be
methylated and altered before becoming functional within the ytoplasm, and
the hnRNA will also be significantly modified to form functional mRNA in
Thus, a chemical analysis of chromosomes will yield DNA, RNA, and both
acidic and basic proteins. It is possible to extract these compounds from an
interphase nucleus (i.e., from chromatin) or to physically isolate metaphase
chromosomes and then extract the components. For the former, the nuclear
envelope will be a contaminating factor, as will the nucleolus. For Isolated
chromosomes, many of the regulatory molecules may be lost, since the
chromosomes are essentially nonfunctional during this condensation period.