1. Hallmarks that distinguish a living from a nonliving system
(self-feeding, self-replication, differentiation, chemical signaling,
and evolution). Why are virus not considered living by some
2. Properties of viruses:
- chemistry- composed of a nucleic acid and protein
- genome- viruses have either a DNA or a RNA genome.
- DNA viruses- depending on virus, genome can be either
double (ds) or a single stranded (ss) structure. DNA may be
either a circular or linear molecule.
- RNA viruses- depending on virus, RNA can be ss or ds.
With one possible exception, all ds-RNA viruses are animal
viruses, and all have a linear genome. In some cases, animal
viruses, the genes are on different RNA molecules. These
viruses are referred to as segmented viruses.
- protein- a protective protein coat, called a capsid
structure, surrounds the nucleic acid. The capsid is composed
of identical protein subunits called capsomers. The nucleic
acid surrounded by the capsid is called the nucleocapsid
structure of the virion. In the case of some animal viruses, a
lipid layer may surround the nucleocapsid. These viruses are
called enveloped viruses.
- size and shape- viruses are an order of magnitude smaller than
their host cell. The largest virus, the vaccinia virus, is as
largest as the smallest bacterium (Chlamydia), while the
smallest virus can be as small as the bacterial ribosome (10 nm).
Shape- linear, spherical (polyhedral), or complex (having a linear
and polyhedral component).
- lack ability for self replication- in order for viruses to
replicate, they must do so at the expense of a host cell. Thus,
viruses are referred to as obligate intracellular parasites. What
aspect to bacteria of the genus Rickettsia and
Chlamydia share with viruses?
3. General step in replication of a virus:
- attachment or adsorption to host cell.
- cell entry- in the case of all phage and some animal viruses
entry is the result of the nucleic acid entering the cell. this is
called penetration. With enveloped animal viruses the
membrane may be removed during entry or immediately after virus
enters cytoplasm. This process is called uncoating.
- synthesis of viral component parts- Following entry and/or
uncoating, viral genome must be replicated, phage genome must be
expressed, i.e., transcription of phage genes, and translation of
phage mRNA to make phage proteins. These events are called
biosynthesis of viral component parts.
- assembly of viral component parts- during the maturation
step, viral component parts are assembled into infectious
particles. In most cases, this occurs after all component part are
made, and occurs by a self-assembly process.
- release- viral particle must be released. This can
occur as a result of cell lysis, rupture, or release without
damage to host cell.
A viral infection may result in a productive infection, meaning
that viral particles are produced, or a latent infection. In a
latent infection the viral genome becomes part of host chromosome,
and no virus particles are produced. The host cell properties may
be altered in specific ways, and at some future time a latent
infection can lead to a productive infection. In the case of
bacteria, a cell harboring phage DNA in its chromosome is called a
lysogenic cell, while the integrated phage genome is called
a prophage. The process of inserting phage DNA into the
bacterial chromosome is called lysogeny, and phage with
this ability are referred to as temperate bacteriophage. No
examples of a RNA phage that inserts itself into genome of host
(into host DNA)! In the case of a group of animal viruses called
retroviruses, of which the HIV is an example, the viral RNA
genome, which is ss, is converted to a ssDNA copy, then to a
double stranded form, the viral genome, as is the case for HIV,
becomes integrated into the DNA of certain cells of the human
body! Not all retroviruses do this last step.
4. Examples of how certain phage replicate:
- ds-DNA phage- phage T4 Host cell is E. coli Fig.
- phage receptors
- phage typing
- phage differential gene expression
- latent period
- burst size
- ss-DNA phage- fX-174 and the
filamentous phage M13 (ds replicative intermediate).
- ss-RNA phage (sense or + stranded phage)- MS2
phage. See replication diagram.
5. Lysogeny: Phage l
(lambda). Fig. 13.6
6. Lysogenic conversion (new cell properties acquired as a
consequence of carrying or harboring a prophage)- examples
See Table 13.3
7. Mechanisms to resist phage infection- lack or modify phage
receptor and/or make a restriction endonucleases that recognizes
specific DNA sequences of phage genome. Restriction endonucleases,
have ability to recognize specific nucleotide sequences within a DNA
molecule, and cut the DNA. Cells protects own DNA by modifying a
nucleotide within sequence recognized by a specific restriction
endonuclease. In general, only one restriction endonuclease and
modifying enzyme per species.
- What properties/characteristics allow you to distinguish
between a virus and a cell?
- Describe the chemical composition of a bacteriophage.
- Describe the events in the life cycle of bacteriophage
- Compare and contrast the replication of a ds-DNA phage with
that of a ss-DNA phage.
- how do ss-RNA phage replicate?
- How does phage lambda differ in its replication cycle from
- What is lysogenic conversion?
- How can bacteria avoid a phage infection?
- You should be familiar with the following words: host range,
lytic/ temperate bacteriophage, capsid, nucleocapsid, eclipse and
latent period, burst size, a lysogenic cell, and prophage.
Examples of lysogenic
- Cells immune to further infection by a similar phage due to
presence of repressor protein.
- Establishes pathogenic state in certain bacteria.
- In Corynebacterium diphtheriae prophage carries a
gene which encodes for a protein toxin, diphtheria toxin, which
is deleterious to the eucaryotic cell (blocks eucaryotic
- In Clostridium botulinum the prophage encodes for a
neurotoxin, which causes blurred vision and respiratory
paralysis which can lead to death.
- The erythrogenic toxins produced by strains of
Streptococcus pyogenes (damages blood capillaries under
the skin producing a red skin rash)
- Staphylococcal enterotoxin ( stimulates discharge of large
amounts of fluids and ions from cells lining intestinal track,
diarrhea) and pyrogenic toxin produced by Staphylococcus
aureus, and the capsule produced by Streptococcus
pneumoniae are all phage encoded.