General steps in virus replication

General steps in virus replication

1. Steps in Virus Replication
A SEMINAR ON-
Presented by-
Avijit Pramanik
AAH-MA-09-02
B.F.Sc, W.B.U.A.F.S.
M.F.Sc, CIFE, Mumbai

2. Characteristics of Bacteria and viruses

Growth on
artificial
media
Division by
binary fission
Whether they
have both DNA
and RNA
Whether they
have
ribosomes
Whether they
have
muramic acid
Their sensitivity
to antibiotics
Bacteria Yes Yes Yes Yes Yes Yes
Mycoplasma Yes Yes Yes Yes No Yes
Rickettsia No Yes Yes Yes Yes Yes
Chlamydia No Yes Yes Yes No Yes
Viruses No No No No * No No
* The arenavirus family (an RNA virus family) appears to package ribosomes 'accidentally'. The packaged ribosomes appear to play
no role in viral protein synthesis.

3. Viral Replication: Basic
Concepts
• Viruses are obligate intracellular parasites
• Viruses carry their genome (RNA or DNA) and
sometimes functional proteins required for
early steps in replication cycle
• Viruses depend on host cell machinery to
complete replication cycle and must
commandeer that machinery to successfully
replicate

4. • Replication cycle produces
- Functional RNA’s and proteins
- Genomic RNA or DNA and structural
proteins
•100’s-1,000’s new particles produced by each
cycle
- Referred to as burst size
- Many are defective
- End of ‘eclipse’ phase
• Replication may be cytolytic or non-cytolytic

5. 1- Adsorption (Attachment)
–Viruses must recognize and bind to specific cellular
receptors via particular envelope glycoproteins
2- Penetration
There are three different mechanisms
Unenveloped viruses enter the cell
either by direct translocation across the
plasma membrane or by endocytosis

6. Steps in Virus Replication
(Continued)
•Enveloped that have the ability to form syncytia
(multinucleated giant cell) enter the cell through
fusion of the virion envelope with the cell plasma
membrane (e.g. paramyxo and herpes viruses)
•The remaining enveloped viruses enter the cell
through endocytosis
•Endocytosis involves invagination of the cell
membrane to form vesicles in the cell cytoplasm.
•Infected viruses are then engulfed inside these
vesicles.

8. •Each vesicle fuses with a lysosome to form
lysosomal vesicle.
•The viral envelope fuses with the lysosomal
membrane and the viral nucleocapsid is expelled
into the cytoplasm.
3- Uncoating:
• Release of the viral genome from its protective
capsid to enable the viral nucleic acid to
replicate

9. 4- Transcription:
Synthesis of m-RNA
5- Translation:
The viral m-RNA is
translated on cell
ribosomes into
structural and non-
structural proteins.
6- Replication of the
viral genome:

10. Steps in Viral Replication: Basic Strategies
of Transcription and Translation
(Fourth and Fifth Steps)
(+) RNA Proteins
(-) RNA (+) RNA Proteins
RNA DNA RNA Proteins
DNA RNA Proteins

11. 7- Assembly:
New virus genomes and proteins are
assembled to form new virus particles.

12. 8- Release:
Enveloped
viruses are
released by
budding
Unenveloped
viruses are
released by
rupture of the
infected cells.