The essential elements of the infective cycle of DNA bacteriophages consist of:
The virions attach to the surface of their host cell (a).
The proteins of the capsid inject the DNA core into the cell (b).
Once within the cell, some of the bacteriophage genes (the "early" genes) are transcribed (by the host's RNA polymerase) and translated (by the host's ribosomes, tRNA, etc.) to produce enzymes that will make many copies of the phage DNA and will turn off (even destroy) the host's DNA.
As fresh copies of phage DNA accumulate, other genes (the "late" genes) are transcribed and translated to form the proteins of the capsid (c).
The stockpile of DNA cores and capsid proteins are assembled into complete virions (d).
Another "late" gene is transcribed and translated into molecules of lysozyme. The lysozyme attacks the peptidoglycan wall (from the inside, of course).
Link to illustrated description of the action of lysozyme on the bacterial cell wall.
Eventually the cell ruptures and releases its content of virions ready to spread the infection to new host cells (e).
Hepatitis BThe genome of hepatitis B ("serum hepatitis") is also dsDNA, but its mode of replication is different from the other dsDNA viruses.
Once inside its host cell (a liver cell), the virion core enters the nucleus.
The viral DNA is transcribed (by the host's RNAP II) into molecules of mRNA.
These enter the cytoplasm where they are translated (again by host ribosomes, etc.) into the various proteins of the virus, including a viral reverse transcriptase.
These components are assembled into new viral cores, and in each
one molecule of mRNA is reverse transcribed into a single strand of DNA, which then serves as the template for the synthesis of the second strand.
2. Genome is single-stranded DNAExamples:
phiX-174, another famous bacteriophage (infects E. coli) that helped usher in the modern era of molecular genetics. Its single strand of DNA has 5,386 nucleotides and encodes 10 genes.
Link to a diagram (100 K) showing its life cycle.
Adeno-associated virus (AAV). This virus, which can only grow in cells infected with adenovirus, shows great promise as a safe and effective vector for introducing therapeutic genes into human patients.
Discussion
RNA Viruses
1. Negative-stranded RNA viruses: genome consists of one or more molecules of single-stranded "antisense" RNAExamples:
measles
mumps
respiratory syncytial virus (RSV), parainfluenza viruses (PIV), and human metapneumovirus. (In the U.S., these close relatives account for hundreds of thousands of hospital visits each year, mostly by children.)
rabies
Ebola
influenza
Link to a page devoted to influenza viruses.
Method of replication
In addition to its antisense RNA genome, the core of the virion contains an RNA replicase, which is an RNA-dependent RNA polymerase.
Once released in the host cell, this polymerase makes many complementary copies of the genome, which are "sense" and serve as messenger RNAs.
These are translated into the proteins needed to assemble fresh virions, e.g., capsid proteins and RNA polymerase. Note that this strategy
provides many copies of mRNA
depends on the virion having its own RNA replicase (because the host cell does not) (So, naked RNA molecules of these viruses are not infectious - in contrast to the next group: the positive-stranded RNA viruses)
2. Positive-stranded RNA: genome is a molecule of single-stranded "sense" RNAExamples:
polioviruses
rhinoviruses (frequent cause of the common "cold"; 99 different strains are known)
noroviruses (frequent cause of outbreaks of gastrointestinal illness — especially in "closed" settings like cruise ships and nursing homes)
coronaviruses (includes the agent of Severe Acute Respiratory Syndrome (SARS)
rubella (causes "German" measles)
yellow fever virus
West Nile virus
dengue fever viruses
equine encephalitis viruses
hepatitis A ("infectious hepatitis") and hepatitis C viruses
tobacco mosaic virus (TMV)
Method of replication
The "sense" RNA encodes an RNA replicase (an RNA-dependent RNA polymerase) that
is translated by the host machinery (ribosomes, etc.) into the enzyme, which
catalyzes the synthesis of large numbers of "antisense" replicative intermediates.
These serve as templates for the synthesis of large numbers of mRNA molecules that
are translated by the host cell machinery into the proteins needed to make fresh virions
are incorporated into the new virions.
3. Genome consists of several molecules of double-stranded RNAExamples:
reovirus
several plant viruses
Method of replication
The virus particle contains enzymatic machinery that transcribes each of the dsRNA molecules into a mRNA (complete with cap) and exports these into the cytosol of the infected cell.
4. RetrovirusesThese viruses contain a reverse transcriptase that copies their RNA genome into DNA.
Link to an illustrated discussion of the life cycle of retroviruses.Examples:
The Rous sarcoma virus (RSV)
HIV-1 and HIV-2, that cause AIDS
HTLV-1 and HTLV-2; about 3% of the people infected with HTLV-1 develop leukemia.
The virions attach to the surface of their host cell (a).
The proteins of the capsid inject the DNA core into the cell (b).
Once within the cell, some of the bacteriophage genes (the "early" genes) are transcribed (by the host's RNA polymerase) and translated (by the host's ribosomes, tRNA, etc.) to produce enzymes that will make many copies of the phage DNA and will turn off (even destroy) the host's DNA.
As fresh copies of phage DNA accumulate, other genes (the "late" genes) are transcribed and translated to form the proteins of the capsid (c).
The stockpile of DNA cores and capsid proteins are assembled into complete virions (d).
Another "late" gene is transcribed and translated into molecules of lysozyme. The lysozyme attacks the peptidoglycan wall (from the inside, of course).
Link to illustrated description of the action of lysozyme on the bacterial cell wall.
Eventually the cell ruptures and releases its content of virions ready to spread the infection to new host cells (e).
Hepatitis BThe genome of hepatitis B ("serum hepatitis") is also dsDNA, but its mode of replication is different from the other dsDNA viruses.
Once inside its host cell (a liver cell), the virion core enters the nucleus.
The viral DNA is transcribed (by the host's RNAP II) into molecules of mRNA.
These enter the cytoplasm where they are translated (again by host ribosomes, etc.) into the various proteins of the virus, including a viral reverse transcriptase.
These components are assembled into new viral cores, and in each
one molecule of mRNA is reverse transcribed into a single strand of DNA, which then serves as the template for the synthesis of the second strand.
2. Genome is single-stranded DNAExamples:
phiX-174, another famous bacteriophage (infects E. coli) that helped usher in the modern era of molecular genetics. Its single strand of DNA has 5,386 nucleotides and encodes 10 genes.
Link to a diagram (100 K) showing its life cycle.
Adeno-associated virus (AAV). This virus, which can only grow in cells infected with adenovirus, shows great promise as a safe and effective vector for introducing therapeutic genes into human patients.
Discussion
RNA Viruses
1. Negative-stranded RNA viruses: genome consists of one or more molecules of single-stranded "antisense" RNAExamples:
measles
mumps
respiratory syncytial virus (RSV), parainfluenza viruses (PIV), and human metapneumovirus. (In the U.S., these close relatives account for hundreds of thousands of hospital visits each year, mostly by children.)
rabies
Ebola
influenza
Link to a page devoted to influenza viruses.
Method of replication
In addition to its antisense RNA genome, the core of the virion contains an RNA replicase, which is an RNA-dependent RNA polymerase.
Once released in the host cell, this polymerase makes many complementary copies of the genome, which are "sense" and serve as messenger RNAs.
These are translated into the proteins needed to assemble fresh virions, e.g., capsid proteins and RNA polymerase. Note that this strategy
provides many copies of mRNA
depends on the virion having its own RNA replicase (because the host cell does not) (So, naked RNA molecules of these viruses are not infectious - in contrast to the next group: the positive-stranded RNA viruses)
2. Positive-stranded RNA: genome is a molecule of single-stranded "sense" RNAExamples:
polioviruses
rhinoviruses (frequent cause of the common "cold"; 99 different strains are known)
noroviruses (frequent cause of outbreaks of gastrointestinal illness — especially in "closed" settings like cruise ships and nursing homes)
coronaviruses (includes the agent of Severe Acute Respiratory Syndrome (SARS)
rubella (causes "German" measles)
yellow fever virus
West Nile virus
dengue fever viruses
equine encephalitis viruses
hepatitis A ("infectious hepatitis") and hepatitis C viruses
tobacco mosaic virus (TMV)
Method of replication
The "sense" RNA encodes an RNA replicase (an RNA-dependent RNA polymerase) that
is translated by the host machinery (ribosomes, etc.) into the enzyme, which
catalyzes the synthesis of large numbers of "antisense" replicative intermediates.
These serve as templates for the synthesis of large numbers of mRNA molecules that
are translated by the host cell machinery into the proteins needed to make fresh virions
are incorporated into the new virions.
3. Genome consists of several molecules of double-stranded RNAExamples:
reovirus
several plant viruses
Method of replication
The virus particle contains enzymatic machinery that transcribes each of the dsRNA molecules into a mRNA (complete with cap) and exports these into the cytosol of the infected cell.
4. RetrovirusesThese viruses contain a reverse transcriptase that copies their RNA genome into DNA.
Link to an illustrated discussion of the life cycle of retroviruses.Examples:
The Rous sarcoma virus (RSV)
HIV-1 and HIV-2, that cause AIDS
HTLV-1 and HTLV-2; about 3% of the people infected with HTLV-1 develop leukemia.