The genes of eukaryotes are distributed among a
number of linear chromosomes that vary in size and
number. Eukaryotic chromosomes are condensed by
packing the DNA to different degrees (Figure
3-1). Nucleosomes consist of DNA wound twice
around an octet of proteins called histones (two each
b, H3, and H4). Approximately 200 base
pairs (bp) of the DNA are wound around the spherical
bodies formed by the histones, and about 50 bp of DNA connect the nucleosomes. Further compaction may be accomplished
by histone H1 binding, which induces the nucleosomes to associate
into a ring of six nucleosomes and the rings to associate into a cylinder
called a solenoid. Phosphorylation of histone H1 results in the
dissociation of the solenoid into an extended nucleosome form. The solenoid is the form in which most of the cell’s DNA exists during interphase.
However, further packing can occur by certain proteins binding
the solenoid and stimulating it to loop back and forth from a central core
of proteins called a scaffold. Dephosphorylation of topoisomerase II
and other proteins causes dissociation of the scaffold and results in the
decondensation of the chromosomes to the solenoid form. In some eukaryotes,
18 loops of the solenoid form a disklike structure and the chromosome
condenses as hundreds of disks stack together. This is the form
that is predominant during nuclear division.
|Figure 3-1 Eukaryotic chromosome packaging: (a) extended
nucleosome form; (b) solenoid form; and (c) looped solenoid form.
Heterochromatin is highly condensed DNA that remains in the solenoid
form throughout the cell cyle except during DNA replication,
when it decondenses. Most of the genes associated with heterochromatin
are not expressed because of the DNA ’s condensed state. In contrast, euchromatin
is decondensed DNA that exists in the solenoid form or in an
extended nucleosome form.
A centromere is a highly constricted region of a mitotic or meiotic
chromosome where the spindle fibers attach. Complex sequences of
DNA constitute centromeres. If the centromere is in the middle of the
chromosome, the chromosome is said to be metacentric. If the centromere
is near the tip, it is called telocentric. The short and long arms
of the chromosome with respect to the centromere are designated as p and
q, respectively. Special staining techniques reveal that each chromosome has a specific pattern of dark and light regions called bands. Homologous
chromosomes have the same banding pattern.
Protein complexes associated with the centromeric regions are called
kinetochores. Kinetochores bind microtubules of the spindle bundle and
function to distribute chromosomes as cells proliferate.
Propagation and maintenance of any piece of DNA requires the presence
of one or more origin of replication sites (OriR) and special ends
called telomeres. Origin of replication sites are special sequences where
DNA replication initiates. Telomeres protect the ends of linear chromosomes
from cellular enzymes that degrade nucleic acids from their ends.
Yeasts have 4 chromosomes; haploid human cells
Euchromatin in the nucleosome form
can be expressed; in the solenoid
form it cannot.
Bacterial genomes range from 106
to 3 x 107
whereas a diploid human cell has 5.6 x 109
bp among its 46 chromosomes. However, 90–95% of
the human genome is not expressed as protein.