Chromosomes, prokaryotic WORLD OF MICROBIOLOGY AND IMMUNOLOGY122•
carry only half the full complementof chromosomes. This
reduction in the number of chromosomes within sex cells is
accomplished during two rounds of cell division, called meio-
sis I and meiosis II. Prior to meiosis I, the chromosomes repli-
cate and chromosome pairs are distributed to daughter cells.
During meiosis II, however, these daughter cells divide with-
out a prior replication of chromosomes. Mistakes can occur
during either meiosis I and meiosis II. Chromosome pairs can
be separated during meiosis I, for instance, or fail to separate
during meiosis II.
Meiosis produces four daughter cells, each with half of
the normal number of chromosomes. These sex cells are called
haploid cells (meaning half the number). Non-sex cells in
humans are called diploid (meaning double the number) since
they contain the full number of normal chromosomes.
Most alterations in chromosome number occur during
meiosis. When an egg or sperm that has undergone faulty
meiosis and has an abnormal number of chromosomes unites
with a normal egg or sperm during conception, the zygote
formed will have an abnormal number of chromosomes. If the
zygote survives and develops into a fetus, the chromosomal
abnormality is transmitted to all of its cells. The child that is
born will have symptoms related to the presence of an extra
chromosome or absence of a chromosome.See alsoCell cycle (eukaryotic), genetic regulation of; Cell
cycle (prokaryotic), genetic regulation of; Chromosomes,
prokaryotic; DNA (Deoxyribonucleic acid); Enzymes;
Genetic regulation of eukaryotic cells; Genetic regulation of
prokaryotic cells; Molecular biology and molecular geneticsCHROMOSOMES, HUMAN•seeCHROMOSOMES,
EUKARYOTICCChromosomes, prokaryoticHROMOSOMES, PROKARYOTIC
The genetic material of microorganisms, be they prokaryotic
or eukaryotic, is arranged in an organized fashion. The
arrangement in both cases is referred to as a chromosome.
The chromosomesof prokaryotic microorganisms are
different from that of eukaryotic microorganisms, such as
yeast, in terms of the organization and arrangement of the
genetic material. Prokaryotic DNAtends to be more closely
packed together, in terms of the stretches that actually code for
something, than is the DNA of eukaryotic cells. Also, the
shape of the chromosome differs between many prokaryotes
and eukaryotes. For example, the deoxyribonucleic acidof
yeast (a eukaryotic microorganism) is arranged in a number of
linear arms, which are known as chromosomes. In contrast,
bacteria(the prototypical prokaryotic microorganism) lack
chromosomes. Rather, in many bacteria the DNA is arranged
in a circle.
The chromosomal material of virusesis can adopt dif-
ferent structures. Viral nucleic acid, whether DNA or ribonu-
cleic acid(RNA) tends to adopt the circular arrangement whenpackaged inside the virus particle. Different types of virus can
have different arrangements of the nucleic acid. However,
viral DNA can behave differently inside the host, where it
might remain autonomous or integrating into the host’s
nucleic acid. The changing behavior of the viral chromosome
makes it more suitable to a separate discussion.
The circular arrangement of DNA was the first form dis-
covered in bacteria. Indeed, for many years after this discov-
ery the idea of any other arrangement of bacterial DNA was
not seriously entertained. In bacteria, the circular bacterial
chromosome consists of the double helix of DNA. Thus, the
two strands of DNA are intertwined while at the same time
being oriented in a circle. The circular arrangement of the
DNA allows for the replication of the genetic material.
Typically, the copying of both strands of DNA begins at a cer-
tain point, which is called the origin of replication. From this
point, the replication of one strand of DNA proceeds in one
direction, while the replication of the other strand proceeds in
the opposite direction. Each newly made strand also helically
coils around the template strand. The effect is to generate two
new circles, each consisting of the intertwined double helix.
The circular arrangement of the so-called chromosomal
DNA is mimicked by plasmids. Plasmids exist in the cyto-
plasmand are not part of the chromosome. The DNA of plas-
mids tends to be coiled extremely tightly, much more so than
the chromosomal DNA. This feature of plasmid DNA is often
described as supercoiling. Depending of the type of plasmid,
replication may involve integration into the bacterial chromo-
some or can be independent. Those that replicate independ-
ently are considered to be minichromosomes.
Plasmids allow the genes they harbor to be transferred
from bacterium to bacterium quickly. Often, such genes
encode proteins that are involved in resistance to antibacterial
agents or other compounds that are a threat to bacterial sur-
vival, or proteins that aid the bacteria in establishing an infec-
tion (such as a toxin).
The circular arrangement of bacterial DNA was first
demonstrated by electron microscopy of Escherichia coliand
Bacillus subtilusbacteria in which the DNA had been deli-
cately released from the bacteria. The microscopic images
clearly established the circular nature of the released DNA. In
the aftermath of these experiments, the assumption was that
the bacterial chromosome consisted of one large circle of
DNA. However, since these experiments, some bacteria have
been found to have a number of circular pieces of DNA, and
even to have linear chromosomes and sometimes even linear
plasmids. Examples of bacteria with more than one circular
piece of DNA include Brucellaspecies, Deinococcus radiodu-
rans, Leptospira interrogans, Paracoccus denitrificans,
Rhodobacter sphaerodes, and Vibriospecies. Examples of
bacteria with linear forms of chromosomal DNA are
Agrobacterium tumefaciens, Streptomyces species, and
Borreliaspecies.
The linear arrangement of the bacterial chromosome
was not discovered until the late 1970s, and was not defini-
tively proven until the advent of the technique of pulsed field
gel electrophoresisa decade later. The first bacterium shown
to possess a linear chromosome was Borrelia burgdorferi.womi_C 5/6/03 2:04 PM Page 122