Important
words and concepts from Chapter 18, Campbell & Reece, 2002 (1/29/2005):
by Stephen T. Abedon (abedon.1@osu.edu)
for Biology 113 at the Ohio State University
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Course-external links are
in brackets Click [index] to access site index Click here to
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(1) Chapter title: Microbial Models: The
Genetics of Viruses and Bacteria
(a)
[microbial models: the genetics
of viruses and bacteria, the genetics of viruses and
bacteria (Google Search)] [index]
(2) Relevance of
microorganisms
(a)
Microorganisms are the most important component of environmental health
(b)
Microorganisms cause diseases
(c)
Microorganisms can help heal as well as prevent disease
(d)
Microorganisms have numerous commercial/industrial applications
(e)
Mitochondria and chloroplasts are microorganisms
(f)
Microorganisms serve as model systems
(g)
Microorganisms are extremely abundant
(h)
Microorganisms are Fun!
(i)
See Figure 18.1, Comparing
the sizes of a virus, a bacterium, and a eukaryotic cell
(4)
(a)
Different kinds of microorganisms include
(i)
Bacteria
(ii)
Viruses
(iii)
Fungi
(iv)
Algae
(v)
Protozoa
(b)
This chapter will consider mostly the first two types on this list,
bacteria and viruses
(c)
[types of microorganisms
(Google Search)] [index]
VIRUSES
(5) Virus distinguishing features
(a)
Viruses are smaller than bacteria (typically, at least)
(b)
Viruses are obligate intracellular
parasites (some bacteria are also)
(c)
Viruses are structurally simpler than cellular organisms
(d)
Viruses possess a relative dearth of metabolic machinery
(e)
Many viruses have unusual genomes
(f)
There exists a relative dearth of antiviral "antibiotics"
(g)
Viruses go through an acellular stage
(h)
[virus distinguishing features
(Google Search)] [index]
(a)
Viruses tend to vary in terms of their
(i)
Genome type
(iii)
Host range
(iv)
Life cycles
(b)
[viral characteristics
(Google Search)] [index]
(a)
The genomes of viruses are typically much smaller
than the genomes of cellular organisms
(b)
Virus genomes are also not always composed dsDNA
(c)
Depending on virus, genomes can be
(i)
dsDNA
(ii)
ssDNA
(iii)
dsRNA
(iv)
ssRNA
(d)
Virus genomes can also take on a variety of configurations, depending
on the virus including
(i)
Linear
(ii)
Circular
(iii)
Segmented (more than one DNA molecule, each holding a different gene
or genes)
(iv)
Diploid (most viruses are haploid, though)
(e)
See Table 18.1, Classes of
animal viruses, grouped by type of nucleic acid
(f)
[virus genome types
(Google Search)] [index]
(a)
Defining characteristic of viruses is their
protected extracellular state
(b)
Protection is achieved via a capsid
(c)
In addition, an envelope may be present, surrounding the capsid
(d)
See Figure 18.2, viral
structure
(e)
[capsid, enveloped virus (Google Search)] [index]
(9)
Capsid (capsomers)
(a)
A capsid is a protein shell that surrounds and protects a virus genome
while the virus is in the extracellular state
(b)
The proteins that make up the capsid are called capsomers
(c)
Note that capsids can come in a variety of shapes and levels of
complexity
(i)
Helical
(ii)
Polyhedral
(iii)
Complex
(d)
See Figure 18.2, viral
structure
(e)
[capsid, capsomer OR capsomere
(Google Search)] [index]
(a)
Some viruses are additionally surrounded by an
envelope
(b)
An envelope is a lipid bilayer
located external to the capsid
(c)
Envelopes are derived from host-cell lipid bilayers
(d)
In addition to host membrane proteins, envelopes contain virus-coded
proteins
(e)
These virus proteins are involved in host-cell attachment and genome
uptake
(f)
In non-enveloped viruses,
the capsid proteins are responsible for facilitating host-cell attachment and
genome uptake
(g)
See Figure 18.2, viral
structure
(h)
[enveloped virus (Google Search)] [index]
(a)
All viruses are limited in the host cells they may
successfully infect
(b)
One term that describes this limit is host range
(c)
Many viruses are limited to only a single host species
(d)
Other viruses have broader host ranges, being capable of successfully
infecting more than one host species
(e)
Many viruses are additionally limited in the cell types they are able
to infect within a host
(f)
One determinant of the host range of a virus is the
"lock-and-key" fit between the virus capsid or envelope
proteins and virus receptors, the latter of which typically consist of
host-proteins (or carbohydrates) found on the surface of cells
(g)
[virus host range (Google Search)] [index]
(12)
Bacteriophage (phage)
(a)
Note that host range often plays a role in the
naming of viruses
(b)
One category, ones which infect only bacteria, are
called bacteriophages (a.k.a., phage or phages for short)
(c)
[bacteriophage or phage
(Google Search)] [bacteriophage ecology group
(MicroDude)] [index]
(a)
Viruses have varied life cycles, some of which
are very complex
(b)
A life cycle, in general, is a series of events that an organism goes
through from birth through reproduction
(c)
The simplified virus life cycle consists of
(i)
Adsorption to a host cell
(ii)
Uptake of the virus genome into the
cell
(iii)
Transcription
of virus genes
(iv)
Translation of the resulting virus mRNAs
(v)
Replication of the virus genome
(vi)
Packaging of the new virus genomes into capsids
(vii)
Progeny-virus release from the host cell
(d)
See Figure 18.3, A
simplified viral reproductive cycle
(e)
Life cycles we will consider in more detail include
(i)
The lytic life
cycle
(ii)
The lysogenic life
cycle
(iii)
The life cycle of an enveloped animal virus
(iv)
The life cycle of a retroviruses
(f)
[viurus life cycle, life cycle (Google Search)] [index]
(a)
A lytic life cycle requires the destruction of the host cell before
progeny release may occur
(b)
This host-cell destruction is called lysis
(c)
See Figure 18.4, The lytic
cycle of phage T4
(d)
[virus life cycle (Google Search)] [index]
(15)
Lysogenic life cycle (prophage, provirus, temperate virus)
(a)
In a lysogenic life cycle virus progeny are neither produced nor
released
(i)
Temperate virus = a virus capable of going through a lysogenic cycle
(ii)
Prophage = a bacteriophage whose
genome has integrated into its host's genome during lysogenic
growth
(iii)
Provirus = equivalent to prophage but more generally applicable (e.g., to animal
viruses)
(b)
Note that a temperate virus must have some alternative life cycle, one that results in progeny production and
release
(c)
Typically this alternative life cycle is a lytic one (e.g.,
phage lambda, a.k.a., l)
(d)
See Figure 18.5, the lysogenic
and lytic reproductive cycles of phage l, a temperate
virus
(e)
[lysogenic cycle, prophage, provirus, temperate virus (Google Search)] ["Lytic, Lysogenic,
Temperate, Chronic, Virulent, Quoi?" (Bacteriophage Ecology Group)] [index]
(a)
Prophages can carry bacterial virulence factors (genes that produce toxins, for example)
(b)
Often these factors are required for the bacteria to cause
disease
(c)
Examples include those bacteria responsible for shigatoxigenic E.
coli, diphtheria, botulism, and scarlet fever
(d)
[prophage and disease
(Google Search)] [index]
(a)
Certain animal viruses are capable of entering a
proviral state
(b)
This state allows the virus to remain within the host without inducing
a host anti-viral immune response
(c)
Such viruses include the herpesviruses and retroviruses
(d)
[provirus and disease
(Google Search)] [index]
(a)
Enveloped viruses do not go through a lytic cycle
(b)
Instead they produce and release progeny "chronically", i.e.,
without necessarily first destroying the host cell
(c)
Progeny release often occurs simultaneous with envelope acquisition
(d)
See Figure 18.6, The
reproductive cycle of an enveloped virus
(e)
[enveloped virus (Google Search)] [index]
(19)
Retroviruses (reverse transcriptase)
(a)
One type of enveloped animal virus is the
retrovirus
(b)
Retroviruses are named for their RNA genomes which are converted to DNA
in the course of the viral intracellular life cycle
(c)
The enzyme that accomplishes this feat is called
reverse transcriptase
(d)
Interestingly, once converted to DNA, the virus then makes new viral
genomes as well as mRNAs simultaneously, as the same molecule
(e)
An example of a retrovirus is human immunodeficiency virus (HIV)
(f)
See Figure 18.7, HIV, a
retrovirus
(g)
[retrovirus, reverse transcription,
reverse transcriptase
(Google Search)] [index]
BACTERIA GENETIC VARIATION
(a)
Evolutionary adaptation is dependent on genetic variation