Transcript virus - Microbiology

An Introduction
to the Viruses
Chapter 6
Copyright © The McGraw-Hill Companies, Inc) Permission required for reproduction or display.
Types of Microbes
Cellular
Prokaryotes
Acellular
Eukaryotes
Nucleus
Chromosome
Ribosomes
Viruses
Envelope
Capsid
Ribosomes
Nucleic
acid
AIDS virus
Cellwall Cell
membrane
Cell membrane
Bacteria and
archaea
Fungi, protozoa,
and helminthes
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Bacterial virus
Viruses and
bacteriophages
6.1 The Search for the Elusive Virus
• Viruses were too small to be seen with the first
microscopes
• The cause of viral infections was unknown for years
• Louis Pasteur first proposed the term virus
• 1890s
• Ivanovski and Beijerinck showed that a disease in
tobacco was caused by a virus
• Loeffler and Frosch discovered an animal virus that
causes foot –and-mouth disease in cattle
• Many years of experimentation showed what we know
today and by the 1950s virology had grown
6.2 The Position of Viruses in the
Biological Spectrum
• Can infect every type of cell
• Cannot exist independently from the host cell, so
aren’t considered living things
• However, since they can direct life processes they
are often considered more than lifeless molecules
• Referred to as infectious particles, either active or
inactive
• Obligate intracellular parasites
• Cannot multiply unless they invade a specific host
cell and instruct its genetic and metabolic
machinery to make and release new viruses
Properties of Viruses
6.3 The General Structure of Viruses
Size Range
• Smallest infectious agents
• Most are so small, they can only be seen with an electron
microscope
• Animal viruses
• Proviruses- around 20 nm in diameter
• Mimiviruses- up to 450 nm in length
• Viewing viruses
• Special stains and an electron microscope
• Negative staining outlines the shape
• Positive staining shows internal details
• Shadowcasting technique
The Scale of Viruses
• Too small for the light microscope
• Range in size from 20 to 200 nm (0.02-0.2 µm)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
BACTERIA CELLS
Rickettsia
0.3 µm
Viruses
450nm
1. Mimivirus
2. Herpes simplex 150 nm
3. Rabies
125 nm
4. HIV
110 nm
100 nm
5. Influenza
6. Adenovirus
75 nm
7. T2 bacteriophage 65 nm
8. Poliomyelitis
30 nm
9. yellow fever
22 nm
Protein Molecule
15 nm
10. Hemoglobin
molecule
Streptococcus
1 µm
(1)
(2)
E. coli
2 µm long
(10)
(9)
(8)
(7)
(3)
(6)
(4)
(5)
Viral Components: Capsids, Nucleic
Acids, and Envelopes
• Molecular structure- composed of regular, repeating
subunits that give rise to their crystalline appearance
• Contain only those parts needed to invade and
control a host cell
• Capsid
• Envelope- not found in all viruses
• Core- genetic material (DNA or RNA)
Basic Viral Structure
• External coating
• Capsid
• Envelope- in 13 of the 20 families of animal viruses
• If no envelope, called naked virus
• Core - Nucleic Acid
• DNA
• RNA
• The capsid and the nucleic acid together are called the
nucleocapsid
• Fully formed virus that is able to establish an infection in a
host cell- virion
Basic Viral Structure
Viral Capsid: The Protective Outer
Shell
• Constructed from identical subunits called
capsomers
• Made up of protein molecules
• Two different types
• Helical and Icosahedral
Helical Capsid
Icosahedral Capsid
• Three-dimensional,
20-sided figure with
12 evenly spaced
corners
• Although they all
display this
symmetry, there are
wide variations
Icosahedral Capsid
Complex Capsid
• Found in
Bacteriophages
• Viruses that infect
bacteria
• Shape is not
symmetrical
• Contain tail fibers and
capsid head
Concept Check
What is the protein shell that surrounds a viral
genome called?
A. Envelope
B. Capsid
C. Core
D. Cortex
The Viral Envelope
• Enveloped viruses take a bit of the host cell
membrane in the form of an envelope
• In the envelope, some or all of the regular
membrane proteins are replaced with viral
proteins
• Some proteins form a binding layer between the
envelope and the capsid
• Glycoproteins remain exposed as spikes essential for attachment
Viral Envelope
Functions of the Viral
Capsid/Envelope
• Protects nucleic acids
• Help introduce the viral DNA or RNA into a
suitable host cell
• Stimulate the immune system to produce
antibodies that can protect the host cells against
future infections
Concept Check
What is the lipid membrane that surround some
viruses called?
A. Capsid
B. Envelope
C. Outer membrane
D. Capsule