Viruses in Microbiology
I.
Viruses: Structure, Function, and Uses
A. General
1. Definition: - virus -A small parasite consisting of nucleic acid (RNA or
DNA) enclosed in a protein coat that can replicate only a susceptible host
cell. Used widely in biological research.
2. Functions:
a. A virus is a small parasite that cannot reproduce by itself.
b. Upon infection of a cell, it can redirect the cell to producing more of
itself – the virus.
c. Most viruses have RNA or DNA as their genetic material – double or
single strand.
d. The entire section of the virus that can infect host cells is called a viron
that contains the genetic material and a protein covering.
e. Viruses can encode from 4-200 proteins.
f. All viruses are obligate intracellular parasites.
B. Structure of Viruses
1. Viruses range in size from 10 – 400 nm
2. A virus is made up generally of a head, collar, tube, helical sheath,
baseplate, and tails.
C. Viral caspids are regular arrays of one or a few types of proteins
1. The nucleic acid of a viron is enclosed within a protein coat, a caspid,
composed of multiple copies of one or a few proteins – each encoded by
one gene.
2. Large caspids can be created with very few genes.
3. A caspid and the enclosed nucleic acid are called a nucleocaspid.
4. Four major ways to arrange the multiple caspid protein subunits and the
viral genome into a nucleocaspid.
a. The simple structure is a protein helix or Helical with the RNA or
DNA protected within.
-
They are shaped like hollow cylinders that are rigid or flexible.
b. Icosahedral or quasi-spherical viruses – a solid object of 20 identical
faces with an equilateral triangle shape. This creates symmetry along
its surface. The vertices of the triangles are called clefts that interact
with the host cells for attachment.
c. Enveloped – These viruses have a outer membranous layer
surrounding the nucleocaspid that creates a roughly spherical shape.
d. Complex Viruses – These are a combination of the Icosahedral and
Helical.
5. In some viruses, the symmetrically arranged external membrane, which is
composed of a phosolipid layer and viral glyoproteins, covers the
nucleocpsid.
D. Most viral host ranges are narrow
1. Viruses can only effect a limited type of cell.
2. A bacteriophage or a phage is a virus that effects bacteria only.
3. Most viruses do not cross phyla and many only infect closely related
species.
E. Viruses can be cloned and counted in plague assays.
1. Plaque assay – a dilute sample of viral particles is cultured on a plate with
host cells on its surface. The number of lesions or plagues that developed
is counted.
2. This is a standard for plant and animal viruses.
F. Viral growth cycles are classified as lytic or lysogenic
1. There are two phases of both life cycles – the extracellular and the
intercellular.
a.
Extracellular – virus is metabolically inert but infectious.
b.
Intracellular – virus induces a host cell to make more viruses.
2. The surface of viruses that are made up of a few different types of
proteins can bind with host cells and limits the types of host the virus can
infect.
3. Upon this interaction, the viral DNA/RNA then travels into the host
cytoplasm and then the host nucleus where the viral genetic material
interacts with the host material.
4. After this interaction – viral mRNA is produced to create viral proteins.
5. Three classes of viral proteins:
a.
special enzymes needed for viral replication
b.
Inhibitory factors that stop host cell DNA. RNA, and protein
synthesis
c. structural proteins used in the construction of new virions
6. Most cells after the cell had made hundred to thousands of viruses have
been created rupture or lysese release the viruses at once.
7. The events of the virus – adsorption, penetration, replication and release
are called the lytic cycle of viral replication.
- An infection that results in a cell death is called a cytocidal infection.
a. Adsorption – the phage attaches to a specific receptor site on the
bacteria.
b. Penetration – The release of the genetic material into the host cell.
c. Replication – The synthesis of further virions.
d. Release – The release of the daughter viruses and the lysese of the host
cell.
8. Some phages can combine with their host DNA and is replicated from on
generation to the next. This association is called a lysogeny and then
waits till certain conditions in which it enters the lytic stage. Often
associated with temperate phages.
G. Four types of pages are used in biochemical and genetic research.
1. DNA phages of the T series – used in mRNA studies
2. Temperate phages – used in DNA cloning
3. Small DNA phages – used in viral life cycles. The first complete DNA
sequence was determined.
4. RNA phages – used in cell free protein synthesis.
H. Animal Viruses are classified by genome type and mRNA synthesis pathway.
1. How the mRNA ands the nucleic acid of the infected host is the basis of a
means to classify viruses.
2. The mRNA that can infect the cell is called the plus strand while the other
complementary strand is the negative strand.
3. There are 6 classes due to this system:
a. DNA viruses – class I and II – contain DNA
Class I has a double strand and class II has a single strand of DNA
b. RNA viruses – class III-VI –
Class III – double strand RNA – have segmented genomes
Class IV – contains a single plus strand of RNA
Class V – contains a single negative stand
Class VI – contains two plus strands – retroviruses – possible cancer
causing.
I.
Viral Vectors can be used to introduce specific genes into cells
1. Introduction of viral contains into a host cells is used for cloning – called
viral gene transduction.
2. This can also be used for gene therapy.
J. Viruses and cancer
1.
A tumor is a growth or lump of tissue resulting from neoplasia or
abnormal new cell growth.
2.
Cancer, a serious medical concern and focus of a immense amount of
research, is a malignant tumor that expands locally and by metastasis.
3.
At this time viruses have been linked to s different cancers of the human
body.
a. Epstein Barr
b. Hepatitis B
c. Cervical Cancer – two types
d. HTLV-1
e. HTLV-2
4. Viruses can cause cancer by introducing oncogenes into a cell and
inserting them into the genome.
- oncogene – cancer causing genes