viral replication .pptx

Viral replication

1. Viral Replication Shah Faisal Jamal KMU-IPMS

2. Introduction Viral replication is the formation of biological viruses during the infection process in the target host cells The viral replication cycle is described in two different ways Growth Curve It shows the amount of virus produced at different times after infection Specific events of virus growth It shows the stepwise description of the specific events within the cell during virus growth

3. Viral Growth Curve When one virion infects a cell, it can replicate in approximately 10 hours to produce hundreds of virions within that cell but varies among the hosts The first event is the disappearing of virus particle (Solid line dropping) after entering the cell but the viral nucleic acid continues to function The time during which no virus is found inside the cell is known as eclipse period The latent period is the time from the onset of infection to the appearance of virus extracellularly. Alterations of cell morphology accompanied by marked derangement of cell function begin toward the end of the latent period, called cytopathic effect (CPE)

4. Specific events during the Growth Cycle Growth cycle is divided into three main events: Early Events Adsorption Penetration Uncoating Middle Events Early Transcription Early Translation DNA synthesis Late Transcription Late Translation Late Events Assembly Release of Virus and Lysis of infected cell

5. Attachment The proteins on the surface of the virion attach to specific receptor proteins on the cell surface through weak, noncovalent bonding. The specificity of attachment determines the host range of the virus. Some viruses have a narrow range, whereas others have quite a broad range. For example, poliovirus can enter the cells of only humans and other primates, whereas rabies virus can enter all mammalian cells. The organ specificity of viruses is governed by receptor interaction as well. Those cellular receptors that have been identified are surface proteins that serve various other functions. For example, herpes simplex virus type 1 attaches to the fibroblast growth factor receptor, rabies virus to the acetylcholine receptor, and human immunodeficiency virus (HIV) to the CD4 protein on helper T lymphocytes.

6. Penetration The virus particle penetrates by being engulfed in a pinocytotic vesicle, within which the process of uncoating begins. A low pH within the vesicle favors uncoating . Rupture of the vesicle or fusion of the outer layer of virus with the vesicle membrane deposits the inner core of the virus into the cytoplasm

7. Gene Expression and Genome Replication Viruses with a double-stranded DNA genome (e.g., papovaviruses such as human papillomavirus ) use host cell RNA polymerase to synthesize viral mRNA. Parvoviruses (Single stranded DNA virus) use host cell DNA polymerase to synthesize viral double-stranded DNA and host cell RNA polymerase to synthesize viral mRNA. Viruses with a single-stranded, negative-polarity RNA genome (e.g., orthomyxoviruses such as influenza virus) use a virion RNA polymerase to synthesize viral mRNA.

8. Gene Expression and Genome Replication Viruses with a double-stranded RNA genome (e.g., reoviruses ) use a virion RNA polymerase to synthesize viral mRNA. Some viruses with a single-stranded, positive-polarity RNA genome (e.g., retroviruses) use a virion DNA polymerase to synthesize a DNA copy of the RNA genome but a host cell RNA polymerase to synthesize the viral mRNA. Some viruses with a single-stranded, positive-polarity RNA genome (e.g., picornaviruses ) use the virion genome RNA itself as their mRNA.

9. Gene Expression and Genome Replication DNA viruses, with one exception, replicate in the nucleus and use the host cell DNA-dependent RNA polymerase to synthesize their mRNA. The poxviruses are the exception because they replicate in the cytoplasm, where they do not have access to the host cell RNA polymerase. They therefore carry their own polymerase within the virus particle. The genome of all DNA viruses consists of double-stranded DNA, except for the parvoviruses , which have a single-stranded DNA genome

10. Gene Expression and Genome Replication Most RNA viruses undergo their entire replicative cycle in the cytoplasm except retroviruses and influenza viruses, both of which have an important replicative step in the nucleus. Retroviruses integrate a DNA copy of their genome into the host cell DNA, and influenza viruses synthesize their progeny genomes in the nucleus. In addition, the mRNA of hepatitis delta virus is also synthesized in the nucleus of hepatocytes . The genome of all RNA viruses consists of single-stranded RNA, except for members of the reovirus family, which have a double-stranded RNA genome.

11. Gene Expression and Genome Replication RNA viruses fall into four groups with quite different strategies for synthesizing mRNA Single-stranded RNA of positive polarity: These viruses use their RNA genome directly as mRNA e.g. Polio virus Single-stranded RNA of negative polarity: mRNA must be transcribed by using the negative strand as a template e.g. measles virus and influenza virus Double-stranded RNA: Positive strand in double-stranded RNA cannot be used as mRNA because it is hydrogen-bonded to the negative strand so mRNA is transcribed from ds RNA e.g. Rotavirus single-stranded RNA of positive polarity (reverse transcriptase): that is transcribed into double-stranded DNA by the RNA dependent DNA polymerase carried by the virus e.g. Retrovirus

12. Gene Expression and Genome Replication After mRNA synthesis of either DNA or RNA viruses, it is translated by host cell ribosome’s into viral proteins, early proteins (Occurring before the replication) enzymes required for replication of the viral genome late proteins (occurring after replication) structural proteins of the progeny viruses The most important of the early proteins for many RNA viruses is the polymerase that will synthesize many copies of viral genetic material for the progeny virus particles. Some viral mRNAs are translated into precursor polypeptides or translated directly into structural proteins

13. Gene Expression and Genome Replication As the replication of the viral genome proceeds, the structural capsid proteins to be used in the progeny virus particles are synthesized. In some cases, the newly replicated viral genomes can serve as templates for the late mRNA to make these capsid proteins.

14. Replication of the viral genome is governed by the principle of complementarity , which requires that a strand with a complementary base sequence be synthesized; this strand then serves as the template for the synthesis of the actual viral genome.

15. Assembly and Release The progeny particles are assembled by packaging the viral nucleic acid within the capsid proteins Virus particles are released from the cell by either of two processes: Rupture of the cell membrane: this mainly occurs with non enveloped viruses Budding : enveloped viruses release by budding through the cell membrane except herpes virus which acquire their envelope from nuclear membrane

16. Lysogeny Some viruses can use lysogenic cycle instead of typical replicative cycle, in which the viral DNA becomes integrated into the host cell chromosome and no progeny virus particles are produced at that time One of the most important functions of lysogeny from a medical point of view is the synthesis of several exotoxins in bacteria, such as diphtheria, botulinum , cholera, and erythrogenic toxins, encoded by the genes of the integrated bacteriophage ( prophage ). Lysogenic conversion is the term applied to the new properties that a bacterium acquires as a result of expression of the integrated prophage genes

17. Lysogeny The choice between the pathway leading to lysogeny and that leading to full replication is made as early protein synthesis begins The choice depends on the balance between two proteins, the repressor produced by the c-I gene and the antagonizer of the repressor produced by the cro gene . If the repressor predominates, transcription of other early genes is shut off and lysogeny ensues If the cro gene product prevents the synthesis of sufficient repressor, replication and lysis of the cell result.

18. Lysogeny The integration of the viral DNA into the cell DNA occurs by the matching of a specific attachment site on the host cell DNA and the integration of the two DNAs mediated by a phage encoded recombination enzyme, this integrated viral DNA is called a prophage The integrated viral DNA is replicated along with the cell DNA, each daughter cell inherits a copy The prophage is not permanently integrated but can be induced to resume its replicative cycle by UV and certain chemical