1 mm long DNA molecule is so “packed” that it can easily be
accomodated by the much smaller recipient cell (2—3 x 1—1.5 μm).
DNA-molecule is structured in domains, some of them transcribing, others
— non-transcribing. The replication starts at a defined spot called origin
(oriC) and stops at another called terminus (terC), being attached to the
cytoplasmic membrane. The very site where at the given moment the act
of replication is taking place (the so-called replication fork) is also
attached to the cytoplasmic membrane. The mode of replication of the
circular bacterial chromosome is shown in Figure 2–18. The causes for
the uneven distribution of the genes in the bacterial chromosome, the
presence of plasmids, mobile elements and transpozons and their role in
the bacterial genome are treated in detail in Prozorov’s review (1989).
image
Figure 2–18. Autoradiography of a circular DNA-molecule of E. coli K-12, labeled with ³H-thymidine for two generations. Two circular structures are seen bound by a common segment (From Cairns, 1963).
In the cells of eukaryotic organisms, most of which are multicellular, the
chromosomes are located in the nucleus separated from the cytoplasm by a
well-defined nuclear membrane. In them nature has created a greater variety