Transcript Foundations of Biology - Geoscience Research Institute

Exodus 15:26
26 … If thou wilt diligently hearken to the
voce of the LORD thy God, and wilt do
that which is right in his sight, and wilt
give ear to his commandments, and keep
all his statutes, I will put none of these
diseases upon thee, which I have brought
upon the Egyptians: for I am the LORD
that healeth thee.
©1999 Timothy G. Standish
Genetics of Viruses
Timothy G. Standish, Ph. D.
©1999 Timothy G. Standish
What Are Viruses?
Viruses are tiny (20 - 400 nm) particles
composed of a nucleic acid core (either
DNA or RNA) surrounded by a protein coat
(capsid) and sometimes a membrane
envelope (derived from its host cell)
Viruses are incapable of reproduction
independent of host cells whose cellular
machinery is taken over by the virus and
used as a factory for producing new viruses
©1999 Timothy G. Standish
Discovery of Viruses
Mayer
The first virus discovered was the tobacco
mosaic virus (TMV) which causes tobacco
leaves to be mottled and stunts growth
1883 A. Mayer (German) - Discovered that
tobacco mosaic disease could be transmitted
from one plant to another via the sap
Mayer could not find bacteria in the sap
He theorized that a bacterium too small to be
seen with a microscope caused tobacco
mosaic disease
©1999 Timothy G. Standish
Discovery of Viruses
Ivanowsky
1890s D. Ivanowsky (Russia) filtered sap
through ceramic filters designed to remove
bacteria and found that the filtered sap still
caused tobacco mosaic disease
Ivanowsky theorized that the disease had two
possible causes:
1 Bacteria small enough to pass through his
filters
2 A toxin that passed through the filters
©1999 Timothy G. Standish
Discovery of Viruses
Beijerinck
1897 Martinus Beijerinck (Holland)- Demonstrated
that the mosaic disease agent could reproduce
First sap was filtered, then placed on leaves
When these leaves developed mosaic disease, their
sap was used to cause disease in other plants for
several generations
At no point was the ability to cause disease
diminished
This eliminated the possibility of a toxin alone being
the cause
©1999 Timothy G. Standish
Discovery of Viruses
Beijerinck
Beijerinck reasoned that if it was a small bacteria it
could be cultured using nutrient media
This proved to be impossible as the agent would
only reproduce in the tobacco plant
Bacteria should be killed by ethanol, but ethanol did
not destroy the ability to cause mosaic disease
Because of this, the idea of a very small bacterium
was rejected and replaced with a new theory:
Mosaic disease is caused by an agent much smaller
and simpler than bacteria
©1999 Timothy G. Standish
Discovery of Viruses
Stanley
1935 Wendell Stanley (US) - Crystalized the
Tobacco Mosaic Virus (TMV)
After the advent of electron microscopy viruses
could be viewed directly
©1999 Timothy G. Standish
Viral Structure
Viruses are made of a maximum of three basic
parts:
1 Genetic material:
– DNA viruses – Single- or double-stranded DNA
– RNA viruses – Single- or double-stranded RNA
2 Capsid - A protein coat made up of one or a few
capsomere proteins
3 Envelope - Not all viruses have an envelope
which is derived from the host cell’s membrane
The envelope may contain proteins which assist
in attachment and entry into new host cells
©1999 Timothy G. Standish
Phage Reproduction:
The Lytic Cycle
Infection
Destruction of
the bacteria’s
DNA
Production of
viral parts
Lysis
Packaging
Replication of
the viral
genome
©1999 Timothy G. Standish
Phage Reproduction:
The Lysogenic Cycle
Infection
Circularization
of phage DNA
Many generations
of bacteria
Exit of phage
On to the
lytic cycle
Temperate
phage
Integration of
phage DNA into
the bacterial
genome
©1999 Timothy G. Standish
Are Viruses Alive?
Viruses require cells to reproduce and do not
have cells of their own, thus they do not fit
the Cell Theory which states that being made
out of cells is a characteristic of living things
Are viruses “missing links” between living
and non-living things?
No, viruses require cells to reproduce, so
cells had to come first, then viruses; thus
viruses cannot serve as a “missing link”
between living and non-living things
©1999 Timothy G. Standish
©1999 Timothy G. Standish