Virus replication

Virus replication


• Life cycle of virus

• Lytic cycle

• Lysogenic cycle

• Life cycle of animal virus

• Replication of DNA virus

• Replication of RNA virus

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


• 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

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

of animal virus

1. Adsorption of

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

2. Penetration 

Entry of naked animal

Entry of enveloped
animal virus

Entry of enveloped
virus by endocytosis

3. Uncoating

• The removal of the capsid and release of viral nucleic

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


• 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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