Viruses vary greatly in size and structure. They contain a protein capsid that protects the viral nucleic acid genome, which can be DNA or RNA. Capsids have different symmetries like icosahedral, filamentous or head-tail shapes. Viruses replicate inside host cells by attaching, entering, releasing their genome, replicating it, assembling new viral particles, and exiting via lysis or budding. Their ability to highjack host cell machinery allows replication but also determines their pathogenicity and transmission between hosts.

1. Structure & Replication of
Viruses

2. Size of Viruses
• Viruses vary considerably in size and shape.
• The smallest viruses are about 0.02 μm (20 nanometers), while the
large viruses measure about 0.3 μm (300 nanometers).
• Smallpox viruses are among the largest viruses; polio viruses are
among the smallest.

3. Viral Structure
• The complete single particle of virus is called virion.
• The protecting protein cat of virus is known as capsid, and the
smaller subunits of protein are termed as capsomeres.
• The central core consists of nucleic acid (DNA or RNA).
• Inside the host cells, RNA or DNA exists in the form of replicating
nucleic acid molecules devoid of protein coat.

4. Structure-Capsid
1. Capsid
• The capsid is composed of a large number of protein submits all of the same
shape.
• These units are known as capsomeres.
• The arrangement of capsomeres determines the shape of the virus particle.
• The combination of genome and capsid is called the viral nucleocapsid.
• The capsid protects the nucleic acid against the action of nuclease enzymes.

5. Structure-Capsid
• In some viruses capsid is further covered by a thin lipid membrane.
• Among viruses there are three different types of c apsid symmetry :
(i) Icosahedral
(ii) Filamentous
(iii) Head-tail

6. Structure-Capsid
i) Icosahedral – Icosahedral capsids have twenty faces, and are named
after the twenty-sided shape called an icosahedron.
ii) Filamentous – Filamentous capsids are named after their linear, thin,
thread-like appearance. They may also be called rod-shaped or
helical.

7. Structure-Capsid
iii) Head-tail –These capsids are kind of a hybrid between the
filamentous and icosahedral shapes. They basically consist of an
icosahedral head attached to a filamentous tail.

8. Structure-Capsid
.

9. Structure-Nucleic Acid
2. Nucleic Acid
• Viruses contain either DNA or RNA for their genetic information, which
may be single stranded or double stranded, linear or circular.
• Viruses containing DNA are called Deoxyviruses, whereas, having RNA
called Riboviruses.
• In general, all plant viruses have ss-RNA.
• Animal viruses have either single or (rarely) ds-RNA or ds-DNA.

10. Structure-Nucleic Acid
• The nucleic acid has genetic information for the synthesis of variety
of proteins.

11. Structure-Viral Protein
3. Viral Protein
• Proteins found in viruses may be grouped into the four categories:
i) Envelope protein
• Enveloped viruses contain glycoprotein which differ from virus to virus.
ii) Nucleocapside protein
• Viral capsids are made up totally of protein of identical subunits. E.g:
capsids contain single type of protein in TMV.

12. Structure-Viral Protein
iii) Core protein
• Protein found in the nucleic acid is known as core protein.
iv) Viral enzyme
• In animal viruses especially in the enveloped viruses, many virion
specific enzymes have been characterized. E.g: RNase, reverse
transcriptase in retrovirus.

13. Structure-Viral envelope
4. Viral envelope
• It is 10-15 μm thick, made up of protein, lipids and carbohydrate
• Lipid provide flexibility to the shape.
• Projections from the envelope are known as spikes.
• The spikes attached to the outer surface of the envelope are made up
of glycoproteins

14. Structure-Viral envelope
• The spikes sometimes contain essential elements for attachment of
the virus to the host cell.
• The virus of AIDS, the human immunodeficiency virus, uses its
spikes for this purpose.

15. Structure

16. Virus Replication
• As viruses are obligate intracellular pathogens they cannot replicate
without the machinery and metabolism of a host cell.
• Although the replicative life cycle of viruses differs greatly between
species and category of virus, there are six basic stages that are
essential for viral replication.

17. Virus Replication
1. Attachment
2. Penetration
3. Uncoating
4. Replication
5. Assembly
6. Virion release

18. Virus Replication
1. Attachment
• Viral proteins on the capsid or phospholipid envelope interact with
specific receptors on the host cellular surface.
• This specificity determines the host range (tropism) of a virus.

19. Virus Replication
2. Penetration
• The process of attachment to a specific receptor can induce
conformational changes in viral capsid proteins, or the lipid envelope,
that results in the fusion of viral and cellular membranes.
• Some DNA viruses can also enter the host cell through receptor
mediated endocytosis.

20. Virus Replication
3. Uncoating
• Shortly after penetration, uncoating of virus take place
• The viral capsid is removed and degraded by viral enzymes or host
enzymes releasing the viral genomic nucleic acid.

21. Virus Replication
4. Replication (Biosynthesis)
• After the viral genome has been uncoated, transcription or translation of
the viral genome is initiated.
• It is this stage of viral replication that differs greatly between DNA and
RNA viruses and viruses with opposite nucleic acid polarity.
• This process culminates in the de novo synthesis of viral proteins and
genome

22. Virus Replication
5. Assembly
• After de novo synthesis of viral genome and proteins, which can be
post transrciptionally modified, viral proteins are packaged with
newly replicated viral genome into new virions that are ready for
release from the host cell.
• This process can also be referred to as maturation

23. Virus Replication
5. Assembly
• The assembly occurs in nucleus or cytoplasm of host cell depending
upon types of virus.
• DNA virus assembled in nucleus except Poxvirus and RNA viruses
assembled in cytoplasm except Influenza virus and Reo virus.

24. Virus Replication
6. Virion release
• There are two methods of viral release:
a. lysis (naked viruses)
b. budding (enveloped viruses)
a) Lysis results in the death of an infected host cell, these types of viruses are
referred to as cytolytic. An example is variola major also known as smallpox.

25. Virus Replication
b) Budding
• Enveloped viruses, such as influenza A virus, are typically released
from the host cell by budding.
• It is this process that results in the acquisition of the viral phospholipid
envelope.
• These types of virus do not usually kill the infected cell and are termed
cytopathic viruses.

26. Virus Replication
Viral Replication

27. Structure
.