Max Sanam

 Understand stages in animal virus replication  Compare and contrast the multiplication cycle of DNA and RNA-containing animal viruses  Understand how viral proteins are synthesized

 Attachment  Entry  Uncoating  Biosynthesis  Maturation or Assembly  Release

 The receptor sites of animal cells are proteins and glycoproteins of the plasma membrane  The attachment sites of animal viruses are distributed over the surface of the virus  In many of the enveloped virus, such as influenza virus--- spikes  The receptor for a particular virus can vary from person to person  Some people lack for cellular receptor (called P antigen) for parvo virus B19

 Viruses enter into eukaryotic cells by pinocytosis. An active celluar process where a cell’s plasma mebrane contiously fold inward to form vesicles  Enveloped viruses can enter by an alternative method called fusion, in which the viral envelope fuses with the plasma membrane and releases the capsid into cell’s cytoplsma (for example, HIV)

 Separation of the viral nucleic acid from its protein coat, the capsid  The capsid is digested when when the cell attempts to digest the vesicle’s contents  Uncoating may result from the cation of lysosome enzymes of the host. These enzymes degrade the proteins of the viral capsid

 Generally, DNA containing viruses replicate their DNA in the nucleus of the host cells by using viral enzymes, and  They synthesize their capsid and other proteins in the cytoplasm by using host cell enzymes  Then the proteins migrate into the nucleus and are joined with the newly synthesized DNA to form virions.  These virions are transported along the ER to the host cell’s membrane for release

 The multiplication of RNA viruses is essentially the same as that of DNA viruses, except that several different mechanisms of mRNA fromation occus among different groups of RNA viruses  RNA viruses multiply in the host cell’s cytoplasm

 The first step in viral maturation is the assembly of the protein capsid  The capsid may be enclosed by an envelope consisting protein, lipid, and carbohydrate  The envelope develops arround the capsid by a process called budding  Budding does not immediately killed the host cell, in some case the host cell survive  The assembled capsid containing NA pushes through the PM. As a result, a portion of the PM, now the envelope, adheres to the virus  Nonenveloped virus are released through rupture in the host cells PM; results in the death of the host cell

O=P-O OPhosphate Group Group N Nitrogenous base ( A, G, C, or T ) ( A, G, C, or T ) CH2 O C1C1 C4C4 C3C3 C2C2 5 Sugar Sugar (deoxyribose)

Gene Expression – Transcription & Translation: Overview

 Double-stranded DNA viruses, such as herpesviruses, papovaviruses and adenoviruses, which replicate in the nucleus of the cell, have a relatively direct replication strategy.  The viral DNA is transcribed by cellular DNA dependent RNA polymerase (transcriptase) to form mRNA.  In contrast, the single-stranded DNA viruses, parvoviruses and circoviruses, which also replicate in cell nuclei, utilize cellular DNA polymerase to synthesize double-stranded DNA. This is then transcribed to mRNA by cellular transcriptases.

Stages in the replication of a herpes virus, an enveloped double- stranded DNA virus.

 Reoviruses and birnaviruses, double-stranded RNA viruses, have segmented genomes. Transcription occurs in the cytoplasm under the direction of a viral transcriptase.  The negative-sense strand of each segment is transcribed to produce individual mRNA molecules.  In contrast, the genomes of positive-sense, single-stranded RNA viruses can act directly as mRNA after infection (Fig. 49.3).

 The enzymes necessary for genome replication in these viruses are produced after infection by direct translation of virion RNA.  This RNA can bind directly to ribosomes and is translated to yield a single polyprotein which is then cleaved to yield both functional and structural proteins.  Because direct translation can occur, naked RNA extracted from such viruses is infectious.

Stages in the replication of a picornavirus, a non- enveloped, positive-sense, single-stranded RNA virus

 Negative-sense single-stranded RNA viruses possess an RNA-dependent RNA polymerase.  The naked RNA of these viruses, unlike that of the positive-sense singlestranded RNA viruses, cannot initiate infection.  After infection by the virion, the genomic RNA functions as a template for transcription of positive-sense mRNA and also for virus replication, utilizing the same polymerase.

 The positive-sense RNA subsequently serves as the template for synthesis of negative-sense genomic RNA.  Most single-stranded, negative-sense RNA viruses replicate in the cytoplasm of the cell.  Notable exceptions are orthomyxoviruses and Borna disease virus which replicate in the nucleus.

Stages in the replication of a rhabdovirus, an enveloped, negative-sense, single-stranded RNA virus.

 The genome of retroviruses consists of positive-sense, single-stranded RNA which does not function as messenger RNA. Instead, a single-stranded DNA copy is produced by RNA-dependent DNA polymerase (reverse transcriptase) using the viral RNA as a template.  As the second strand of DNA is formed, the parental RNA is removed from the RNA-DNA hybrid molecule. The double-stranded DNA is integrated into the host cell genome as a provirus (Fig. 49.5).  The integrated DNA provirus, which may be incorporated into cellular chromosomes at a number of sites, can be transcribed to new viral RNA.