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
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.
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.