Virus general characteristics

Biology, genetics, and morphology, and physicochemical properties of virus components came into the scene.

Around , the introduction of extremely sensitive and high-sequencing technologies made dramatic changes. The world saw a multitude of novel viruses, most of them known by their genomic hereditary formation.

The classification of the virus according to their genome hereditary sequence may be in consideration as the ongoing phase of virus taxonomy or classification. As you know viruses are infectious agents with living and nonliving characteristics. A virus can infect animals, plants, and other microorganisms. Viruses that infect only bacteria are known as bacteriophages. Viruses that infect fungi are known as mycophages.

And, the virus that attacks other viruses is known as virophages. Characteristics of viruses can be living as well as non - living. The Living Characteristics of Viruses are:. They reproduce at a fast rate, only in living host cells. They can mutate. The Non - Living Characteristics of Viruses are:. They are not cells, contain no cytoplasm or cellular organelles.

New viruses are synthesized and assembled within the infected host cell. It can be a single-stranded or double-stranded RNA. The diameter of the helical capsid is determined by characteristics of its protomeres, while its length is determined by the length of the nucleic acid it encloses.

Complex -e. This group comprises all those viruses which do not fit into either of the above two groups.

Adsorption -Viruses can enter cells via phagocytosis, viropexis or adsorption. Adsorption is the most common process and the most highly specific process.

It requires the interaction of a unique protein on the surface of the virus with a highly specific receptor site on the surface of the cell. Penetration -This occurs by one or more processes.

Uncoating -During this stage cellular proteolytic enzymes digest the capsid away from the nucleic acid. This always occurs in the cytoplasm of the host cell. The period of the replication cycle between the end of the uncoating stage and maturation of new viral particles is termed the eclipse. Thus during the eclipse stage, no complete viral particles can be viewed within the cell.

Replication of nucleic acid. Replication of viral nucleic acid is a complex and variable process. The specific process depends on the nucleic acid type.

DNA virus replication -with the exception of the poxviruses, all DNA viruses replicate in the nucleus. In some cases one of the DNA strands is transcribed in others both strands of a small part of the DNA may be transcribed step 4 into specific mRNA, which in turn is translated step 5 to synthesize virus-specific proteins such as tumor antigen and enzymes necessary for biosynthesis of virus DNA.

This period encompasses the early virus functions. Host cell DNA synthesis is temporarily elevated and is then suppressed as the cell shifts over to the manufacture of viral DNA step 6.

As the viral DNA continues to be transcribed, late virus functions become apparent. Messenger RNA transcribed during the later phase of infection step 6 migrates to the cytoplasm and is translated step 7. Proteins for virus capsids are synthesized and are transported to the nucleus to be incorporated into the complete virion step 8. Assembly of the protein subunits around the viral DNA results in the formation of complete virions step 9 , which are released after cell lysis.

They then undergo a typical replication cycle. RNA virus replication -with the exception of the orthomyxoviruses and retroviruses, all RNA viruses replicate in the cytoplasm of the host cell. The exact process varies with the species of virus. The single-stranded RNA that is released after uncoating will act as either: a the mRNA to synthesize viral-coded proteins; or b a template to synthesize mRNA; or c a template to synthesize double stranded RNA, which is then used as a template to synthesize mRNA; or d a template to synthesize double-stranded DNA, which is then utilized as a template to synthesize mRNA.

This latter process occurs only with the retroviruses oncornaviruses. The replication of poliovirus, which contains a single-stranded RNA as its genome, provides a useful example. The infected bacterium is referred to as a lysogen or lysogenic bacterium.

In this state, the virus enjoys a stable relationship with its host, where it does not interfere with host cell metabolism or reproduction. The host cell enjoys immunity from reinfection from the same virus. Exposure of the host cell to stressful conditions i. This event triggers the remaining steps of the lytic cycle, synthesis, maturation, and release, leading to lysis of the host cell and release of newly formed virions.

So, what dictates the replication type that will be used by a temperate phage? If there are plenty of host cells around, it is likely that a temperate phage will engage in the lytic cycle of replication, leading to a large increase in viral production. If host cells are scarce, a temperate phage is more likely to enter lysogeny, allowing for viral survival until host cell numbers increase. The same is true if the number of phage in an environment greatly outnumber the host cells, since lysogeny would allow for host cells numbers to rebound, ensuring long term viral survival.

One of the best examples of this is for the bacterium Corynebacterium diphtheriae , the causative agent of diphtheria. The diphtheria toxin that causes the disease is encoded within the phage genome, so only C. Eukaryotic viruses can cause one of four different outcomes for their host cell. The most common outcome is host cell lysis, resulting from a virulent infection essentially the lytic cycle of replication seen in phage.

Some viruses can cause a latent infection , co-existing peacefully with their host cells for years much like a temperate phage during lysogeny. Some enveloped eukaryotic viruses can also be released one at a time from an infected host cell, in a type of budding process, causing a persistent infection. Lastly, certain eukaryotic viruses can cause the host cell to transform into a malignant or cancerous cell, a mechanism known as transformation.

Viruses and Cancer There are many different causes of cancer, or unregulated cell growth and reproduction. Some known causes include exposure to certain chemicals or UV light. There are also certain viruses that have a known associated with the development of cancer. Such viruses are referred to as oncoviruses.

Oncoviruses can cause cancer by producing proteins that bind to host proteins known as tumor suppressor proteins , which function to regulate cell growth and to initiate programmed cell death, if needed.

If the tumor suppressor proteins are inactivated by viral proteins then cells grow out of control, leading to the development of tumors and metastasis, where the cells spread throughout the body. Skip to content Viruses are typically described as obligate intracellular parasites , acellular infectious agents that require the presence of a host cell in order to multiply. Virus Characteristics Viruses can be extremely simple in design, consisting of nucleic acid surrounded by a protein coat known as a capsid.

Virus Structure Viral nucleocapsids come in two basic shapes, although the overall appearance of a virus can be altered by the presence of an envelope, if present.

Virus Replication Cycle While the replication cycle of viruses can vary from virus to virus, there is a general pattern that can be described, consisting of five steps: Attachment — the virion attaches to the correct host cell.

Penetration or Viral Entry — the virus or viral nucleic acid gains entrance into the cell. Assembly — viruses are produced from the viral components. Release — newly formed virions are released from the cell. Attachment Outside of their host cell, viruses are inert or metabolically inactive. Penetration or Viral Entry Many unenveloped or naked viruses inject their nucleic acid into the host cell, leaving an empty capsid on the outside.

Assembly The complexity of viral assembly depends upon the virus being made.