Page 236 - Review of Medical Microbiology and Immunology ( PDFDrive )

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PART III BASIC VIROLOGY

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mebooksfree.com mebooksfree.com mebooksfree.com mitochondria, and ribosomes. Prokaryotic cells, such as mebooksfree.com
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The other infectious agents described in this book, namely,
bacteria, fungi, protozoa, and worms, are either single cells
do not have mitochondria but do have ribosomes; there-
or composed of many cells. Cells are capable of indepen-
dent replication, can synthesize their own energy and pro-
fore, they can synthesize their own proteins.
teins, and can be seen in the light microscope. In contrast, bacteria, are not divided into nucleus and cytoplasm and
(2) Viruses must reproduce (replicate) within cells,
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mebooksfree.com mebooksfree.com mebooksfree.com obligate intracellular parasites are chlamydiae and rickett- mebooksfree.com
viruses are not cells; they are not capable of independent
because they cannot generate energy or synthesize proteins.
replication, can synthesize neither their own energy nor
Because they can reproduce only within cells, viruses are
their own proteins, and are too small to be seen in the light
obligate intracellular parasites. (The only bacteria that are
microscope.
Viruses are characterized by the following features:
siae. They cannot synthesize sufficient energy to replicate
independently.)
(1) Viruses are particles composed of an internal core
(3) Viruses replicate in a manner different from that of
containing either DNA or RNA (but not both) covered by a
cells (i.e., viruses do not undergo binary fission or mitosis).
protective protein coat. Some viruses have an outer lipo-
One virus can replicate to produce hundreds of progeny
protein membrane, called an envelope, external to the coat.
Viruses do not have a nucleus, cytoplasm, mitochondria, or
daughter cells.
ribosomes. Cells, both prokaryotic and eukaryotic, have
Table III–1 compares some of the attributes of viruses
both DNA and RNA. Eukaryotic cells, such as fungal, pro- viruses, whereas one cell divides to produce only two
tozoal, and human cells, have a nucleus, cytoplasm,
and cells.
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TABLE III–1 Comparison of Viruses and Cells
Viruses
Cells
Property
DNA and RNA
DNA or RNA but not both
Type of nucleic acid
Few
Many
Proteins
Cell membrane present in all cells
Envelope present in some viruses
Lipoprotein membrane
Ribosomes
Absent
Present in eukaryotic cells but not in prokaryotic cells
Mitochondria
Many
Enzymes Absent 1 Present
None or few
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Yes
Multiplication by binary fission or mitosis
No
1
Arenaviruses have a few nonfunctional ribosomes.
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