Chapter 19: Viruses
A Borrowed Life
-Viruses invade host cells by injecting their genetic information into them and hijacking metabolic machinery for production of more viruses to further the infection.
-Compared to eukaryotic and even prokaryotic cells, viruses are smaller and less complex.
-Virus: infectious particle consisting of little more than genes packaged in a protein coat.
-Because viruses cannot reproduce or carryout metabolism without a host cell, they are not considered living.
-Viruses are used by researchers to transfer genes in gene therapy studies.
19.1: A virus consists of a nucleic acid surrounded by a protein coat
The Discovery of Viruses: Scientific Inquiry
-Mayer: discovered that he could transmit tobacco mosaic disease from one plant to another by rubbing extracted sap from diseased leaves onto heathy ones.
-failed to find a microbe to blame for this, concluded this particular microbe was unusually small
-Ivanowsky: filtered sap to remove bacteria, but leaves still infected with disease after sap rubbed on healthy leaves
-concludes bacteria small enough to pass through filter or produced a toxin that passed through filter
-Beijerinck: infectious agent in sap could replicate within the host plant, could not be cultured on media in lab
-concludes caused by particle much smaller and simpler than a bacterium.
-Stanley: crystallized the infectious particle, now known as tobacco mosaic virus.
-viruses can be seen with electron microscope.
Structure of Viruses
-Viruses are infectious particles that consist of a nucleic acid enclosed in a protein coat and, for some viruses, surrounded by a membranous envelope.
Viral Genomes
-Viral genomes may consist of double-stranded or single-stranded DNA, or double-stranded or single-stranded RNA.
-viruses containing DNA are called DNA viruses.
-viruses containing RNA are called RNA viruses.
-The genome is organized as a single linear or circular molecule
-viruses can contain between 3 and 1000 genes.
Capsids and Envelopes
-Capsid: protein shell surrounding the viral genome.
-may be rod-shaped, polyhedral or more complex in shape.
-built from a large number of capsomere protein subunits, generally there are very few different types of capsomere proteins composing the capsid.
-Viral Envelope: accessory structure derived from plasma membrane of host cell that surround the capsid
-because they are derived from host cell mmb, consist of phospholipids and membrane proteins.
19.2: Viruses Replicate only in host cells
- Viruses lack metabolic enzymes and equipment for making proteins, such as ribosomes.
-Each particular virus can infect only a limited number of host species, this is called the host range.
-this is the result of the evolution of recognition systems by the virus.
-viruses usually recognize host by a "lock and key" fit between viral surface proteins and specific receptor molecules on the outside of cells.
-certain viruses have very broad host ranges, while others are limited to one species
-Viral infection of multicellular eukaryotes is usually limited to particular tissues.
- (i.e. : AIDS virus binds cells in immune system, human cold viruses bind only cells in the upper respiratory tract)
General Features of Viral Replicative Cycle
-A virus begins when virus binds host cell and viral genome enters cell.
-the mechanism of entry varies among viruses
-Once the viral genome is inside, the proteins it encodes can commandeer the host, reprogramming the cell to copy the viral genome and manufacture viral proteins.
-the host provides the nucleotides, enzymes, ribosomes, tRNAs, amino acids, ATP, etc.
-Many DNA viruses use the DNA pol of the host cell to synthesize new genomes
-RNA viruses use virally encoded RNA polymerases that can use RNA as a template. (uninfected cells make no such enzymes)
-After viral nucleic acid molecules and capsomeres are produced, they spontaneously assemble into new viruses within the host cell.
-The simplest of virus replicative cycles ends with the exit of hundreds or thousands of viruses from a host cell, which often damages or destroys the cell.
-The viral progeny that exit the cell have the potential to infect additional cells, spreading the viral infection.
Replicative Cycles of Phages
-Some double-stranded DNA viruses can replicate by two mechanisms: the lytic cycle and the lysogenic cycle
1-phage binds to the surface of the cell and injects their linear DNA genome.
2 -within the host, the phage DNA forms a circle
3-From this point it depends on which replicative mode is employed by the phage.....
- Lytic Cycle: a phage replicative cycle that culminates in death of the host cell, the term refers to the last stage of infection, during which the bacterium lyses (breaks open) and releases the phage progeny.
4- viral genes immediately turn the host cell into a phage producing factory.
5- the cell soon lyses and releases the virus progeny
-The phages that are released from the cell can then infect other healthy cells
-Successive cycles can destroy an entire bacterial population in a short time.
- Phages that only replicate by this mechanism are called virulent.
- Bacteria have defenses from these phages:
- natural selection favors bacterial mutants with surface proteins that are no longer recognized by phages as receptors. (However, natural selection also selects for phage mutants that can bind the altered receptors)
- phage DNA is also recognized by bacterial restriction enzymes and cut up into pieces, making it so the phage
cannot replicate.(However, natural selection also selects for phage mutants that are resistant to restriction enzymes)
-Viruses invade host cells by injecting their genetic information into them and hijacking metabolic machinery for production of more viruses to further the infection.
-Compared to eukaryotic and even prokaryotic cells, viruses are smaller and less complex.
-Virus: infectious particle consisting of little more than genes packaged in a protein coat.
-Because viruses cannot reproduce or carryout metabolism without a host cell, they are not considered living.
-Viruses are used by researchers to transfer genes in gene therapy studies.
19.1: A virus consists of a nucleic acid surrounded by a protein coat
The Discovery of Viruses: Scientific Inquiry
-Mayer: discovered that he could transmit tobacco mosaic disease from one plant to another by rubbing extracted sap from diseased leaves onto heathy ones.
-failed to find a microbe to blame for this, concluded this particular microbe was unusually small
-Ivanowsky: filtered sap to remove bacteria, but leaves still infected with disease after sap rubbed on healthy leaves
-concludes bacteria small enough to pass through filter or produced a toxin that passed through filter
-Beijerinck: infectious agent in sap could replicate within the host plant, could not be cultured on media in lab
-concludes caused by particle much smaller and simpler than a bacterium.
-Stanley: crystallized the infectious particle, now known as tobacco mosaic virus.
-viruses can be seen with electron microscope.
Structure of Viruses
-Viruses are infectious particles that consist of a nucleic acid enclosed in a protein coat and, for some viruses, surrounded by a membranous envelope.
Viral Genomes
-Viral genomes may consist of double-stranded or single-stranded DNA, or double-stranded or single-stranded RNA.
-viruses containing DNA are called DNA viruses.
-viruses containing RNA are called RNA viruses.
-The genome is organized as a single linear or circular molecule
-viruses can contain between 3 and 1000 genes.
Capsids and Envelopes
-Capsid: protein shell surrounding the viral genome.
-may be rod-shaped, polyhedral or more complex in shape.
-built from a large number of capsomere protein subunits, generally there are very few different types of capsomere proteins composing the capsid.
-Viral Envelope: accessory structure derived from plasma membrane of host cell that surround the capsid
-because they are derived from host cell mmb, consist of phospholipids and membrane proteins.
19.2: Viruses Replicate only in host cells
- Viruses lack metabolic enzymes and equipment for making proteins, such as ribosomes.
-Each particular virus can infect only a limited number of host species, this is called the host range.
-this is the result of the evolution of recognition systems by the virus.
-viruses usually recognize host by a "lock and key" fit between viral surface proteins and specific receptor molecules on the outside of cells.
-certain viruses have very broad host ranges, while others are limited to one species
-Viral infection of multicellular eukaryotes is usually limited to particular tissues.
- (i.e. : AIDS virus binds cells in immune system, human cold viruses bind only cells in the upper respiratory tract)
General Features of Viral Replicative Cycle
-A virus begins when virus binds host cell and viral genome enters cell.
-the mechanism of entry varies among viruses
-Once the viral genome is inside, the proteins it encodes can commandeer the host, reprogramming the cell to copy the viral genome and manufacture viral proteins.
-the host provides the nucleotides, enzymes, ribosomes, tRNAs, amino acids, ATP, etc.
-Many DNA viruses use the DNA pol of the host cell to synthesize new genomes
-RNA viruses use virally encoded RNA polymerases that can use RNA as a template. (uninfected cells make no such enzymes)
-After viral nucleic acid molecules and capsomeres are produced, they spontaneously assemble into new viruses within the host cell.
-The simplest of virus replicative cycles ends with the exit of hundreds or thousands of viruses from a host cell, which often damages or destroys the cell.
-The viral progeny that exit the cell have the potential to infect additional cells, spreading the viral infection.
Replicative Cycles of Phages
-Some double-stranded DNA viruses can replicate by two mechanisms: the lytic cycle and the lysogenic cycle
1-phage binds to the surface of the cell and injects their linear DNA genome.
2 -within the host, the phage DNA forms a circle
3-From this point it depends on which replicative mode is employed by the phage.....
- Lytic Cycle: a phage replicative cycle that culminates in death of the host cell, the term refers to the last stage of infection, during which the bacterium lyses (breaks open) and releases the phage progeny.
4- viral genes immediately turn the host cell into a phage producing factory.
5- the cell soon lyses and releases the virus progeny
-The phages that are released from the cell can then infect other healthy cells
-Successive cycles can destroy an entire bacterial population in a short time.
- Phages that only replicate by this mechanism are called virulent.
- Bacteria have defenses from these phages:
- natural selection favors bacterial mutants with surface proteins that are no longer recognized by phages as receptors. (However, natural selection also selects for phage mutants that can bind the altered receptors)
- phage DNA is also recognized by bacterial restriction enzymes and cut up into pieces, making it so the phage
cannot replicate.(However, natural selection also selects for phage mutants that are resistant to restriction enzymes)
- Some bacteria coexist with phages in a state called lysogeny.....
-Lysogenic Cycle: allows replication of the phage genome without destroying the host.
4- the DNA molecule is incorporated into a specific site on the host chromosome by viral proteins that break both circular DNA molecules and join them to each other.
-The viral DNA is known as a prophage when it is incorporated into the bacterial chromosome
-one prophage gene codes for a protein that prevents transcription of most other prophage genes,
making the phage genome mostly silent.
5- The viral DNA is replicated with the host DNA, and when the host divides, the virus DNA is present in the offspring in prophage form.
-Sometimes a few prophage genes are expressed during lysogeny.
- these genes may alter the host's phenotype.
-diphtheria,botulism and scarlet fever all cause the host bacteria to produce toxins, which make people ill.
-Lysogenic Cycle: allows replication of the phage genome without destroying the host.
4- the DNA molecule is incorporated into a specific site on the host chromosome by viral proteins that break both circular DNA molecules and join them to each other.
-The viral DNA is known as a prophage when it is incorporated into the bacterial chromosome
-one prophage gene codes for a protein that prevents transcription of most other prophage genes,
making the phage genome mostly silent.
5- The viral DNA is replicated with the host DNA, and when the host divides, the virus DNA is present in the offspring in prophage form.
-Sometimes a few prophage genes are expressed during lysogeny.
- these genes may alter the host's phenotype.
-diphtheria,botulism and scarlet fever all cause the host bacteria to produce toxins, which make people ill.
-Phages capable of both replicative cycles are called temperate.
- temperate phages can be induced to exit the bacterial chromosome and initiate a lytic cycle.
Replicative Cycles of Animal Viruses
-viruses that cause illness in humans an animals can only replicate inside host cells, as well.
-One key variable is the nature of the viral genome, double-stranded or single-stranded DNA or RNA.
- this is the basis for common classification of viruses.
-Single- stranded RNA viruses are further broken down into 3 classes based on how the RNA genome functions inside a host cell.
-Few bacteriophages have an envelope or RNA genome, but many animal viruses have both.
-Viral envelopes:
-a viral envelope is an outer membrane, used to enter a host cell.
- on the exterior of the viral envelope are viral glycoproteins that bind to specfic receptor molecules on the surface of the host cell.
-see process outlined below
- temperate phages can be induced to exit the bacterial chromosome and initiate a lytic cycle.
Replicative Cycles of Animal Viruses
-viruses that cause illness in humans an animals can only replicate inside host cells, as well.
-One key variable is the nature of the viral genome, double-stranded or single-stranded DNA or RNA.
- this is the basis for common classification of viruses.
-Single- stranded RNA viruses are further broken down into 3 classes based on how the RNA genome functions inside a host cell.
-Few bacteriophages have an envelope or RNA genome, but many animal viruses have both.
-Viral envelopes:
-a viral envelope is an outer membrane, used to enter a host cell.
- on the exterior of the viral envelope are viral glycoproteins that bind to specfic receptor molecules on the surface of the host cell.
-see process outlined below
-The viral envelope is usually derived from the host cell's plasma membrane, though the molecules that make it up are specified by the viral genes.
-this replicative cycle does not kill host cells.
-Some envelopes are not derived from plasma membrane:
- i.e. herpes virus derive their membranes from the host's nuclear envelope, that is then shed and traded for one made from
the golgi's membrane.
-RNA as Viral Genetic Material:
-There are 3 types of single-stranded RNA genomes found in animal viruses.
- class IV: can directly serve as mRNA and thus can be translated into viral protein immediately after infection.
-class V: the RNA genomes serves instead as a template for mRNA synthesis.
-the RNA genome is transcribed into complimentary RNA strands, which function both as mRNA and as templates for the
synthesis of additional copies of genomic RNA
-class VI: retroviruses, equipped with enzyme called reverse transcriptase which transcribes an RNA template into DNA.
-HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome).
-retroviruses contained 2 single strands of RNA and 2 molecules of reverse transcriptase. (see life cycle below)
- provirus: integrated viral DNA
-this replicative cycle does not kill host cells.
-Some envelopes are not derived from plasma membrane:
- i.e. herpes virus derive their membranes from the host's nuclear envelope, that is then shed and traded for one made from
the golgi's membrane.
-RNA as Viral Genetic Material:
-There are 3 types of single-stranded RNA genomes found in animal viruses.
- class IV: can directly serve as mRNA and thus can be translated into viral protein immediately after infection.
-class V: the RNA genomes serves instead as a template for mRNA synthesis.
-the RNA genome is transcribed into complimentary RNA strands, which function both as mRNA and as templates for the
synthesis of additional copies of genomic RNA
-class VI: retroviruses, equipped with enzyme called reverse transcriptase which transcribes an RNA template into DNA.
-HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome).
-retroviruses contained 2 single strands of RNA and 2 molecules of reverse transcriptase. (see life cycle below)
- provirus: integrated viral DNA
-Evolution of viruses:
-Although viruses cannot replicate or carry out metabolic activities independently, their use of the genetic code shows an evolutionary connection with the living world.
-There are viruses that infect every type of living organism
-Because viruses need a host to replicate and metabolize, it is likely that viruses evolved after the first cells appeared.
-It is theorized that viruses originated from naked bits of cellular nucleic acids that moved from one cell to another possibly
through injured cell surfaces.
- The evolution of genes coding for capsid proteins may have allowed viruses to bind cell membranes, thus facilitating
the infection of uninjured cells.
-Candidates for the original sources of viruses are plasmids and transposons because they are all mobile genetic elements.
-plasmids are small, circular DNA molecules found in bacteria and in the unicellular eukaryotes called yeasts.
-plasmids exist apart from and can replicate independently of the bacterial chromosome, and are occasionally
transferred between cells
-transposons are DNA segments that can move from one location to another within a cell's genome.
-Some viral genes are essentially identical to genes of the host.
-Recent sequencing has also shown that the genetic sequences of some viruses are very similar to seemingly distant viral relatives.
- some animal viruses share
-Although viruses cannot replicate or carry out metabolic activities independently, their use of the genetic code shows an evolutionary connection with the living world.
-There are viruses that infect every type of living organism
-Because viruses need a host to replicate and metabolize, it is likely that viruses evolved after the first cells appeared.
-It is theorized that viruses originated from naked bits of cellular nucleic acids that moved from one cell to another possibly
through injured cell surfaces.
- The evolution of genes coding for capsid proteins may have allowed viruses to bind cell membranes, thus facilitating
the infection of uninjured cells.
-Candidates for the original sources of viruses are plasmids and transposons because they are all mobile genetic elements.
-plasmids are small, circular DNA molecules found in bacteria and in the unicellular eukaryotes called yeasts.
-plasmids exist apart from and can replicate independently of the bacterial chromosome, and are occasionally
transferred between cells
-transposons are DNA segments that can move from one location to another within a cell's genome.
-Some viral genes are essentially identical to genes of the host.
-Recent sequencing has also shown that the genetic sequences of some viruses are very similar to seemingly distant viral relatives.
- some animal viruses share