Chapter 7 Nucleic Acids: The Molecular Basis of Life • MHR 229Genetic Material in Prokaryotes
Most prokaryotes have a single, double-stranded
DNA molecule. One of the characteristic features
of prokaryotes is that they have no nucleus.
Therefore, there is no nuclear membrane to keep
the DNA strand contained in a particular location
within the cell. Instead, the prokaryote uses an
arrangement that helps to pack genetic material
tightly into a specific region known as the nucleoid
or nuclear zone of the cell. To accomplish this, the
ends of the DNA molecule bind together to form a
closed belt. As shown in the photomicrograph and
simplified illustration in Figure 7.17, this belt is
then further twisted in on itself like a necklace that
is coiled into a series of small loops. Such a
structure is said to be supercoiled. The loops of the
supercoiled DNA are held in place by proteins.
In addition to the relatively large, supercoiled
DNA molecule found in the nucleoid, prokaryotes
often have one or more small, circular double-
stranded DNA molecules floating free in the
cytoplasm of the cell. These additional DNA
molecules are called plasmids. Although plasmids
are not physically part of the nucleoid DNA, these
small strands of genetic material can contribute to
cell metabolism and to the hereditary mechanism.
For example, the genes that confer resistance to
antibiotics may be found on plasmids rather than
within the nucleoid DNA. Plasmids can be copied
and transmitted between cells, or they can be
incorporated into the cell’s nucleoid DNA and
reproduced during cell division. As a result, the
hereditary information contained on a plasmid can
spread within bacterial colonies.
Plasmids have proven to be a valuable tool in
genetic engineering techniques. You will learn
more about the use of plasmids in DNA sequencing
processes and in other applications later in this unit.Genetic Material in Eukaryotes
Supercoiling is an effective way of arranging DNA
within a prokaryote. However, even the simplest
eukaryotic cell has over 10 times the DNA of a
bacterial cell, and mammalian cells may have
many thousands of times the quantity of genetic
material found in a prokaryote. Each human cell
nucleus contains about 2 m of DNA, or six billion
base pairs. By way of comparison, this is roughly
the equivalent of trying to pack 400 km of wet
spaghetti into a bathtub — yet these DNA fibres
never become entangled. A highly structured
arrangement of proteins and DNA helps to pack
and organize this material within the cell.
The nuclei of plant and animal cells contain
double-stranded DNA. This DNA is organized
into a number of separate chromosomes. Each
chromosome contains one linear double-stranded
DNA molecule together with different types of
the protein histone. Overall, the composition
of a chromosome is about 60 percent protein,
35 percent DNA, and five percent RNA. These
components are organized into the long fibres that
Miescher called “nuclein,” and which are now
known as chromatin.
Figure 7.18 on the following page shows the
arrangement of genetic material in a eukaryotic
cell. The DNA molecule wraps tightly around
groups of histone molecules in a regular patternFigure 7.17The supercoiled molecule of bacterial DNA shown at left is coiled
into a series of small loops, as shown in the simplified illustration at right.