4. life cycle and replication of viruses

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… The host cell must be permissive and the virus must be compatible to host cell . The host cell must not degrade the virus . The viral genome must possess the information for multiplying utilizing the normal metabolism of host cell . 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. Lytic cycle 2. Lysogenic cycle

4. Following are the basic stages that are essential for viral replication: Attachment Penetration Uncoating Replication Assembly Maturation Virion release

5. Steps in virus replication:

6. 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 Rhinovirus binds to ICAM-1 Epstein Barr virus binds to C3 complement receptor.

7. 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.

8. 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 .

9. 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.

10. 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.

11. 6. Maturation: After  de novo  synthesis of viral genome and proteins, which can be post- transrciptionally modified. V iral 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.

12. 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.

13. 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/

14. Thank you