interspersed stretches of high and low density chromatin, which are responsible for the beautiful banding
pattern seen in these chromosomes. The much cruder bands seen in mammalian chromosome spreads are
caused by a similar phenomenon (Figure 6.12a).
6.4.3 Prokaryotic Chromosomes
Most prokaryote chromosomes contain circular double-stranded DNA but some of them are linear, like
those of eukaryotes. Prokaryotic DNA is associated with DNA gyrase, DNA topoisomerase^25 (Section 2.3.5)
and packaging proteins into a nucleoprotein complex, which is analogous to eukaryotic chromatin but the
details of which are dissimilar.
6.4.4 Plasmid and Plastid Chromosomes
Both prokaryotes and eukaryotes contain extra DNA besides that belonging to their regular chromosomes.
Bacteria contain a wide variety of plasmids, which are smaller double-stranded DNA. Most, but not all are
circular. In E. colithese plasmids are not absolutely essential for the life of the host but they carry genes
that can be useful, particularly those conferring resistance to antibiotics. In other prokaryotes, plasmids
can be more important. For example, the spirochaete Borrelia burgdorferi, the causative agent of Lyme
disease, carries many linear and circular plasmids. Long term culture of this prokaryote results in loss of
some of these plasmids and concomitant loss of infectivity.
Eukaryotes contain plastids, the most prominent of which constitute the genomes of the mitochondriaof
virtually all eukaryotes and the chloroplastsof plants and algae. These organelles and their associated
genomes are the descendants of ancient prokaryotes that either invaded or were engulfed by the ancestors of
their present day hosts. Mitochondrial genomes are typically circular and carry genes required for the gener-
ation of ATP from respiration, together with some genes needed for their translation. Strangely, the human
mitochondrialgenome is smaller than that of S. cerevisiae(17 kb compared to 75 kb). Chloroplast genomes are
generally 100–200 kb long and contain genes for the light-harvesting complex, which drives photosynthesis.
6.4.5 Eukaryotic Chromosome Structural Features
Eukaryotic chromosomes need to be replicated faithfully, with no loss of DNA from their ends. After repli-
cation they need to separate (segregate) into the daughter cells. The preservation of the ends of chromo-
somes depends on structures called telomeresand segregation requires centromeres.
218 Chapter 6
Figure 6.12 Chromosome banding. (a) Three human metaphase chromosomes. (b) Drosophila polytene chromosomes