Virus replication

Virus replication

Contents

• Life cycle of virus

• Lytic cycle

• Lysogenic cycle

• Life cycle of animal virus

• Replication of DNA virus

• Replication of RNA virus

Intended Learning objectives

At the end of this lecture, student will be able to:

• Describe the lytic and lysogenic life cycle of a virus

• Differentiate between the lytic cycle of a bacteriophage and animal virus

• Explain the multiplication of DNA and RNA virus

Viral multiplication

• For a virus to multiply, it must invade a host cell and take over the host's metabolic machinery.

• A single virion can give rise to several or even thousands of similar viruses in a single host cell. This process can drastically change the host cell and usually causes its death.

• In a few viral infections, cells survive and continue to produce viruses indefinitely.

A viral one-step growth curve - No new infective virions are found in a culture until after biosynthesis and maturation have taken place

Multiplication of Bacteriophages

Bacteriophages can multiply by two alternative mechanisms:

• The lytic cycle -ends with the lysis and death of the host cell

• The lysogenic cycle - the host cell remains alive in the lysogenic cycle

Lytic cycle of bacteriophage  

1.   Attachment

2.   Penetration

3.   Biosynthesis - Synthesis of Phage Nucleic Acids and Proteins

4.   Maturation - The Assembly of Phage Particles

5.   Release of Phage Particles

Replication of animal virus

1. Adsorption of Virions

- Through a random collision of the virion with a plasma membrane receptor site protein, frequently a glycoprotein

2. Penetration 

Entry of naked animal virus

Entry of enveloped animal virus

Entry of enveloped virus by endocytosis

3. Uncoating

• The removal of the capsid and release of viral nucleic acid

4. Replication and transcription of DNA

5. Synthesis and Assembly of Virus Capsids

6. Virion Release

• Naked virions appear to be released most often by host cell lysis.

• Enveloped virus - budding

Release of virus by budding

Summary

• Virus can multiple by lytic or lysogenic cycle

• Steps in lytic cycle – attachment, penetration, biosynthesis, assembly and release

• Lysogeny involves the formation of a prophage

• DNA virus replicates in nucleus

• RNA virus replicates in cytoplasm 

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