Replication of Virus
INTRODUCTION OF VIRUS
A virus (Latin for “poison”) is an obligate intracellular parasite that can only replicate inside a living host cell.
The major components of a virus are:
Nucleic acid core:
The nucleic acid core can either be DNA or RNA that makes up the genetic information (genome) of the virus.
- A capsid is the protein coat that encapsulates a virus and protects the nucleic acid from the environment.
- It also plays a role in how some viruses attach to a host cell. A capsid consists of one or more proteins that are unique to the virus and determine the shape of the virus.
- An envelope is a membrane bilayer that some viruses have outside their capsid.
- If a virus does not have an envelope, the virus is called a naked virus.
- Examples of diseases caused by naked viruses are chickenpox, shingles, mononucleosis, and herpes simplex.
- A naked virus is more resistant to changes and is less likely to be affected by conditions that can damage the envelope.
Environmental factors that can damage the envelope are:
• Increased temperature
• Freezing temperature
• pH below 6 or above 8
• Lipid solvents
• Some chemical disinfectants (chlorine, hydrogen peroxide, and phenol)
Naked viruses are more resistant to changes in temperature and pH.
Examples of diseases caused by naked viruses include poliomyelitis, warts, and the common cold.
SHAPES OF VIRUSES
A virus can have one of two structures. These are:
Helical virus: A helical virus is rod- or thread-shaped. The virus that causes rabies is a helical virus.
Icosahedral virus: An icosahedral virus is spherically shaped. Viruses that cause poliomyelitis and herpes simplex are icosahedral viruses.
- The easiest way to understand how viruses replicate is to study the life cycles of viruses called bacteriophages.
- Bacteriophages replicate by either a lytic cycle or a lysogenic cycle.
- The difference in these two cycles is that the cell dies at the end of the lytic cycle and remains alive in the lysogenic cycle.
- The first two scientists to observe bacteriophages were Frederic Twort of England and Felix d’Herelle of France in the early 1900s.
- The name bacteriophage is credited to d’Herelle and means “eaters of bacteria”.
- The lytic cycle of replication begins with the collision of the bacteriophage and bacteria, called attachment.
- The tail of the bacteriophage attaches to a receptor site on the bacterial cell wall.
- After attachment, the bacteriophage uses its tail like a hypodermic needle to inject its DNA (nucleic acid) into the bacterium. This is called penetration.
- The bacteriophage uses an enzyme called lysozyme in its tail to break down the bacterial cell wall, enabling it to inject its DNA into the cell.
- After the DNA is injected into the host’s bacterial cell’s cytoplasm, biosynthesis occurs.
- Bacteriophage uses the host bacterium’s nucleotides and some enzymes to make copies of the bacteriophage’s DNA.
- This DNA is transcribed to mRNA, which directs the synthesis of viral enzymes and capsid proteins.
- Viral enzymes catalyse reactions that make copies of bacteriophage DNA.
- The bacteriophage DNA will then direct the synthesis of viral components by the host cell.
- Next maturation occurs.
- The last stage is the release of virions from the host bacterium cell.
- The bacteriophage enzyme lysozyme breaks apart the bacterial cell wall (lysis) and the new virus escapes.
- The escape of this new bacteriophage virus will then infect neighboring cells and the cycle will continue in these cells.
The Lysogenic Cycle
- Some viruses do not cause lysis and destruction of their host cells which they infect.
- These viruses are called lysogenic phages or temporate phages.
- These bacteriophages establish a stable, long-term relationship with their host called lysogeny.
- The bacterial cells infected by these phages are called lysogenic cells.
- The most studied bacteriophage, which multiplies using the lysogenic cycle, is the bacteriophage Lambda.
- This bacteriophage infects the bacterium E. coli.
- When the bacteriophage Lambda penetrates an E. coli bacterium, the bacteriophage DNA forms a circle.
- The circle recombines with the circular DNA of the bacteria. This bacteriophage DNA is called a prophage.
- Every time the bacteria host cell replicates normally, so does the prophase DNA.
- On favorable condition the bacteriophage DNA can break out of the prophage and initiate the lytic cycle.