INTRODUCTION TO VIRUSES
A virus is an infectious agent containing genome which is either RNA or DNA and a protein capsid designed to protect the genome. Many viruses have additional structure like envelope which is protein containing lipid bi-layer. The sort of nucleic acid in the virus is the most fundamental and important of properties of virus. The nucleic acid may be single stranded (ss RNA ss DNA) or double stranded (ds DNA or ds RNA).
The Single stranded RNA genome are subdivided into the positive (+) polarity which is, of messenger RNA sense that can be used as template for protein synthesis. Negative (-) polarity or antisense which is complementary to the mRNA sense and so they can not be used as temperate for protein synthesis directly.
THE REPLICATION CYCLES OF VIRUSES
The initial attachment of a virus to the host cell is with the interaction between specific molecular structure on the surface of the virus and receptor molecules in the host membrane that can recognize the structure. The receptor molecules on the host cell membrane are specific for the family of the viruses and they are the molecular structures that usually carry out normal cell functions. The receptors for the viruses are present only on specific cells or are unique for one animal species. So, the absence or presence of the host cell receptors is so important determination for the susceptibility or resistance of a species to a given virus. If we can genetically manipulate the specific receptor affinity for the viruses, we will be able to attack or kill or change the desired targeted cells.
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The two mechanisms the virons enter the cells crossing the cell membrane are the receptor-mediated endocytosis: the viron binds the cell surface receptor and the cell membrane invaginates enclosing the virion in and endocytotic vesicle (endosome). The virion then enters the cytoplasm by various mechanisms depending upon the viruses. It is facilitated by one or more viral molecules in general.
Membrane Fusion: Some enveloped viruses enter the cell by fusion with their envelopes with the membranes of host cells. Glycoproteins of the envelope can promote this and viral membrane then still remains associated with the plasma membrane of the cell and just the nucleocapsid is released into the cells. HIV viruses enter the cells by this fashion.
This is the stepwise process of disassembly of the viron that enables the expression of the viral genes that carry out viral replications. Most of the steps occur inside the cells and depend on cellular enzymes and in rare occasions, newly synthesized viral proteins are needed to complete the process. The loss of one or more structural components of the viron will lead into the loss of ability to infect another cells reflecting as the eclipse period of the growth curve.
DNA virus replication: There is a wide macromolecular event variation between families of viruses for the replication processes depending primarily upon the viral genome sizes. The smaller the viral genome, the more the virus must depend on the host cell to replicate. Also the mechanisms of replications for ss DNA viruses and ds DNA viruses are different.
RNA virus replications
Type 1: RNA viruses with a single stranded genome of (+) polarity that replicates with complementary (-) strand intermediate. In this, the infecting parental RNA serves as both mRNA and later as a template for synthesis of the complementary (-) strand.
Type 2: Viruses with ssRNA genome of (-) polarity which replicate with a complementary (+) strand intermediate. (-) polarity genomes have two functions, one is to provide information for protein synthesis and the second is to serve as template for replication. But they can not accomplish without prior construction of complementary (+) strand intermediate.
Type 3: Viruses with ds RNA genome: dsRNA genome is segmented, with each segment coding for one polypeptide. But, the eukaryotic cells do not have the enzyme to transcribe dsRNA. So, mRNA transcripts are produced by virus-coded, RNA dependent RNA polymerase (transcriptase) located in the sub viral core particle. This particle contains dsRNA genome and associated viral protein, including the transcriptase. In replications, the (+) RNA transcripts are not only used for translation, but also as templates for complementary (-) strand synthesis, resulting in the formation of dsRNA progeny.
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Type 4: Viruses with a genome of ssRNA of (+) polarity that is replicated with with a DNA intermediate: the conversion of a (+) strand RNA to a double-stranded DNA is accomplished by an RNA-dependent DNA polymerase, commonly known as “reverse transcriptase”, which is contained in the virion. The resulting dsDNA becomes integrated into the cell genome by a viral integrase's action. Viral mRNA abd progeny (+) strand RNA genomes are transcribed srom this integrated DNA by the host cell RNA polymerase.
ASSEMBLY AND RELEASE OF PROGENY VIRUSES:
The assembly of the nucleocapsids generally occurs in the cytoplasm for most RNA viruses and in the nucleus for most DNA viruses where the viral nucleic acid replications take place.
- LIPPINCOTT's ILLUSTRATED REVIEWS of MICROBIOLOGY 2007 EDITION, Unit-Viruses, 233-243
- KAPLAN USMLE TEXT BOOK (MICROBIOLOGY) 2009 EDITION
- HARRISON'S PRINCIPLES OF INTERNAL MEDICINE 2006 EITION