Replication and life cycle of viruses

1. Life cycle and replication of viruses
Dhole N. A.
Department of Botany,
Digambarrao Bindu ACS College, Bhokar

2. • Virus are the obligate intra cellular particles, they replicate inside host
cell only.
• For a specific virus to replicate within a specific host cell, certain
condition must be fulfilled and which are…
1.The host cell must be permissive and the virus must be compatible
to host cell.
2.The host cell must not degrade the virus.
3.The viral genome must possess the information for multiplying
utilizing the normal metabolism of host cell.
4.The virus must be able to use the metabolic capability of host cell to
produce new progeny virus particles containing replicated copy of
viral genome.

3. • Two types of life cycles observed in viruses. Initially the details of life
cycle studied in bacteriophages.
1. Lytic cycle
2. Lysogenic cycle

6. Following are the basic stages that are essential for viral replication:
1. Attachment
2. Penetration
3. Uncoating
4. Replication
5. Assembly
6. Maturation
7. Virion release

7. Steps in virus replication:

8. 1. Attachment:
• This is the first step in virus infection in which interaction of virion
with a specific receptor site on the surface of host cell occurs.
• The receptors sites are normal cell surface components of host cell
such as protein, polysaccharides or lipoprotein-polysaccharide
complex to which virus attach.
• For eg. HIV binds to CD4 cell receptor of T-lymphocytes
1.Rhinovirus binds to ICAM-1
2.Epstein Barr virus binds to C3 complement receptor.

9. 2. Penetration:
The entry of virus into host cell may involves;
• Transfer of only genome across cytoplasmic membrane
• Transport of entire virus across cytoplasmic membrane by
endocytosis
• Fusion of viral envelope with cytoplasmic membrane of host cell.

10. 3. Uncoating:
• Shortly after penetration, uncoating of virus take place.
• Uncoating is defined as release of viral genome from capsid and is
accessible to enzymes required to translate, transcribe and replicate
it.
• The uncoating process vary from virus to virus.
• Transcription of viral genome is usually the next step in all virus
except in those virus whose genome acts directly as mRNA (eg.
Picorna virus).
• RNA viruses that carry minus(-) stranded RNA first transcribe their
DNA to plus (+) stranded RNA that function as mRNA.
• The transcription is catalyzed by viral RNA polymerase released during
uncoating.

11. 4. Replication:
• The biosynthesis process of virus replication can be divided into early
event and late events.
• Early event:
• In most virus, only part of nucleic acid is initially transcribed into mRNA.
• The early mRNA codes for early proteins (enzymes) required for nucleic acid
replication
• After nucleic acid replication, many copy of progeny nucleic acids formed.
• Late event:
• Late mRNA is transcribed from progeny genome.
• Late mRNA codes for structural proteins by the process of translation. The
translation process always occurs in cytoplasm of host cell, even if the mRNA
synthesized in nucleus, it enter cytoplasm for translation.

12. 5. Assembly:
• When critical number of various viral components have been
synthesized, they assembled into mature virus.
• 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.

13. 6. Maturation:
• 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.

15. 7. Virion release:
• There are two methods of viral release: lysis or budding.
• 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.
• 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.

16. References:
• http://www.columbia.edu/itc/hs/medical/pathophys/id/2004/lecture
/notes/viral_rep_Hammer.pdf
• https://www.cliffsnotes.com/study-guides/biology/microbiology/the-
viruses/viral-structure-and-replication
• https://www.onlinebiologynotes.com/virus-replication-outcomes-
and-steps/
• https://www.immunology.org/public-information/bitesized-
immunology/pathogens-and-disease/virus-replication
• https://www.khanacademy.org/science/biology/biology-of-
viruses/virus-biology/a/bacteriophages
• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7149704/

17. Thank you