Structure of virus

1. Structure of Virus
Prepared by ,
Mr. Snehal Patel
Sumandeep Vidyapeeth

2. Structure of virus
• Virus are not cellular and therefore they do
not have nucleus, cytoplasm or cell
membrane.
• Virus is composed of nucleic acid surrounded
by a coat that protect it from the
environment.

3. Structure of virus

4. Capsid and envelope
• The nucleic acid of virus is surrounded by a protein
coat called capsid.
• Each capsid is composed of protein subunits called
capsomers.
• In some viruses, the capsid is covered by an
envelope which usually consists of some
combination of lipids, proteins and carbohydrates.
• Depending on the virus, envelops may or may not be
covered by spike, which are carbohydrate protein
complexes that project from the surface of envelop.
• Some viruses are attached to the host cell by means
of spikes.

5. • Viruses whose capsids are not covered by an envelop
are known as naked viruses or non –enveloped viruses.
The capsid of a naked virus protects the nucleic acid
from nuclear enzymes in biological fluids and promotes
virus attachment to susceptible host cell.

6. Nucleic acid
• The core contains nucleic acid.
• The virus contains only one type of nucleic
acid. It may be either DNA or RNA.
• If virus contains DNA is called Deoxyvirus. If it
contains RNA is called Ribovirus.
• The nucleic acid may be single stranded or
double stranded.

7. Life cycle of Bacteriophages
• Bacteriophages exhibit two different types of
life cycle.
1. Lytic or virulent cycle.
2. Temperate or avirulent or lysogenic cycle

8. 1. Lytic or virulent cycle.
• The replication of virulent phage occurs in five
stages.
1. Attachment or adsorption
2. Penetration
3. Biosynthesis of phages components
4. Maturation
5. Release of progeny phage particle

10. 1. Attachment or adsorption
• The first step in infection of a host bacterial
cell by a phage is adsorption.
• Phage particles come into contact with
bacterial cells by random collision.
• A phage attaches to the surface of a
susceptible bacterium by its tail.

11. 2. Penetration
• Attachment is followed by injection of DNA
(nucleic acid) into bacterial cell.
• The phage DNA is injected into the bacterial
body through the hollow core.
• Penetration may be facilitated by the presence
of phage tail of lysozymes which breaks a
portion of the bacterial cell wall for the entry of
the phage DNA.

12. 3. Biosynthesis of phages components
• After infection and penetration of DNA,
transcription of a part of the viral genome
produces early mRNA molecule, which are
translated into a set of early proteins.
• This translated proteins leads to formation of
Phage head and tails.

13. 4. Maturation
• Phage DNA , head and tail protein are
synthesised separately in a bacterial cell.
• The DNA is condensed into a compact
polyhedron and packaged into the head and
finally, the tail structures are added.
• The process of assembly of the phage from its
components is called maturation.

14. 5. Release of progeny phage particle
• The release of progeny phage particle takes by
sudden explosion or bursting (lysis) of the
bacterial cell wall.
• Lysozyme synthesized within the cell causes the
bacterial cell wall to break down.
• The released bacteriophages infect other
susceptible bacterial cells.
• Each cycle of phage reproduction may require 20
to 60 minutes and a single phage infection may
produce 200 or more progeny.

15. Lysogenic cycle
• In contrast to virulent phages, temperate
phages don’t causes lysis of the host cell.
• Following entry into host cell, the temperate
phage nucleic acid becomes integrated with
the bacterial chromosomes.
• The integrated phage nucleic acid is known as
prophage.