What Is Virus ?

The virus

Viral proteases (high-dose) are known to damage cellular membranes. This is achieved by cleaving the cytoplasmic domain of the virus which is highly conserved in viruses. Once the virus particle has invaded the cell the virus particle becomes positively charged and attacks the membrane. Upon further penetration, the virus particle leaks into the cytoplasm, where it is associated with viral proteases and begins its digestion. Because the cytoplasmic domain is conserved, this process can be reversed, restoring the virus to its non-living state.

Structural features

The virus particle is about 80 nm long and usually contains three structural components: the RNA core, a glycoprotein envelope and a protein capsid shell. The viral capsid shell is essential for virus replication and only one type of capsid can infect a host cell. The other type infects other viruses and will be described later. The RNA core contains the genome of the virus and is thought to be a common origin of RNA viruses. This is because the RNA molecules produced by RNA viruses (such as influenza and HIV) are nucleic acids that appear to share a wide range of amino acid sequences with RNA viruses of other clades. As a consequence, the RNA core of viral RNA viruses has been used to clarify the evolutionary relationships between viruses of different clades. This has led to the re-description of the order in which clades of RNA viruses diverged. However, despite the remarkable similarity in nucleic acid sequence, not all RNA viruses produce infectious particles as the RNA genome of viruses such as influenza and HIV is large and the resulting capsid protein is too small.

Genome

The genomic DNA of viral RNA viruses is either circular, double stranded or single stranded and appears to comprise single A, single G or double stranded combinations of the three strands. A non-enveloped structure, the viral genome contains a single strand of RNA at the start and two strands at the end. Each strand of RNA corresponds to one of three stages of the RNA viral life cycle: the promoter, the transcriptional region and the transcriptional termination.

In the promoter region of the viral genome, the viral promoter is flanked by consensus transcriptional enhancers (CWEs). These CWEs are derived from the catalytic domain of the proteins of the viral DNA polymerase, that are required for RNA synthesis.

The CWEs are necessary for regulating the transcription of the viral genes and although this may be occurring at multiple levels of transcriptional activation, many CWEs function in a single regulatory manner, while others are almost exclusively required for activating transcription at specific post transcriptional promoters (and often, only in the presence of viral proteins).

In the N terminus of the viral genome, is the transcriptase enzyme, used to transcribe the viral RNA into new viral RNA. In contrast to many viral proteins, which encode phosphorylation sites, in the viral protein HIV, the HIV specific promoter has a phosphate group, meaning that viral RNA transcripts can only be transcribed when accompanied by an accompanying specific transcriptase enzyme. It has been proposed that the lack of phosphorylation sites in viral transcriptional regulators accounts for their adaptability, because they can function even when alternative promoter sequences are available.

Translation

Translation of the viral genome occurs when viral RNA molecules are transcribed into a larger molecule, the mature virion. Production of a mature virion from viral RNA is the main mode of viral replication. The size of the mature virion is known to be between 20 and 100 nm and it contains the infectious virion particles, or capsid. The virions of some RNA viruses are complex structures that can contain hundreds of viral particles. Other virions consist of only a single infectious particle or particle complexes of virions are formed when only part of the viral genome needs to be transcribed into a new mature virion.

Mature virions can contain a lipid matrix which protects the virion particles, a nucleocapsid and an envelope which encloses the virion. The envelop and the plasma membrane are covered by a lipoprotein membrane composed of phospholipids. In some viruses, the lipid matrix can be absent or is composed of a preformed lipid bilayer. The virion may be surrounded by a core membrane and in some cases, a lipid lid around the core, in which case the virion has been termed a lipid particle. However, most viral particles are very stable and do not require a lipid matrix or a lipid outer membrane to protect them.

Definitions

Pre-transcription

The genetic material of RNA viruses has not been sequenced, but has been inferred from sequences of the genome of most of the viruses and this has been done by methods such as phylogenetic analysis and restriction analysis. Some viruses have shown much variation in their genomes, which may have been due to variable host strains.[2]

Translation

The translation of the genetic material of RNA viruses has not been sequenced, but has been inferred from the number of