Are they living

Viruses are fragments (strands) of DNA or RNA that are encased within a protein coat and that have been detached from the genome of bacteria or eukaryotes and have ability to replicate themselves within cells.

Viruses are considered to be non-living because they lack all of the necessary biochemical features that would allow them to replicate on their own, consequently, they are not considered to be organisms. Their genetic materials consist of DNA or RNA, but not both

Viruses being strands of nucleic acid, scientist regard them as chemical matter than as living organisms; however they are regarded as being living by virtue of infecting and reproduce when they enter cells and use the cellular machinery of their host

Viral genome means gene or generic material of virus, it may be DNA or RNA further more that RNA can directly give to protein or it first goes to DNA and then make RNA.

The interesting fact of viral genome is that it contains Information for only replication and synthesis of its cot protein. all other Information like that for polymerases and other protein is used of the host so it is very small genome

The shape of virus is determined by the organization of protein subunits that make up the capsid.

Viral capsids are generally Helix, Icosahedron or complex combination for both.

Helical capsid

A capsid which is a hollow cylinder surrounding nucleic acid of Virus (RNA) resembling a long rod with capsomers arranged to form coiled up appearance: e.g. Paramyxovirus, Orthomyxovirus, Retrovirus, Tobacco mosaic virus.

Icosahedra [Polyhedral]

A protein coat in cubic symmetry shaped virus with many side often of a regular polyhedron with 20 triangular faces and12 corners e.g.-Adenovirus, Poliovirus, Herpes virus

Baltimore Classification System

Viruses are classed into 7 types of genes, each of which has its own families of viruses, which in turn have differing replication strategies themselves

David Baltimore, a Nobel Prize-winning biologist, devised a system called the Baltimore Classification System to classify different viruses based on their unique replication strategy

There are seven different replication strategies based on this system (Baltimore Class I, II, III, IV, V, VI, and VII). The seven classes of viruses are listed here briefly and in generalities:

Class 1: Double-stranded DNA viruses

Class 2: Single-stranded DNA viruses

Class 3: Double-stranded RNA viruses

Classes 4 & 5 below are Single-stranded RNA viruses

Class 4: Single-stranded RNA viruses – Positive-sense

Class 5: Single-stranded RNA viruses – Negative-sense

Class 6: Positive-sense single-stranded RNA viruses that replicate through a DNA intermediate

Class 7: Double-stranded DNA viruses that replicate through a single-stranded RNA intermediate

Enters the host nucleus before it is able to replicate.

Some of these viruses require host cell polymerases to replicate their genome, while others, such as adenoviruses or herpes viruses, encode their own replication factors.

Replication of the viral genome is highly dependent on a cellular state permissive to DNA replication and, thus, on the cell cycle.

The virus may induce the cell to forcefully undergo cell division, which may lead to transformation of the cell and, ultimately, cancer. An example of a family within this classification is the Adenoviridae

There is only one well-studied example in which a class 1 family of viruses does not replicate within the nucleus. This is the Poxvirus family, which comprises highly pathogenic viruses that infect vertebrates. One example is the smallpox virus

Like most viruses with RNA genomes, double-stranded RNA viruses do not rely on host polymerases for replication to the extent that viruses with DNA genomes do.

Double-stranded RNA viruses are not as well-studied as other classes. This class includes two major families, the Reoviridae and Birnaviridae. Replication is monocistronic and includes individual, segmented genomes, meaning that each of the genes codes for only one protein, unlike other viruses, which exhibit more complex translation

These viruses consist of two types, however both share the fact that replication is primarily in the cytoplasm, and that replication is not as dependent on the cell cycle as that of DNA viruses. This class of viruses is also one of the most-studied types of viruses, alongside the double-stranded DNA viruses.

Viruses that fall under this category include ones that are not as well-studied, but still do pertain highly to vertebrates. Two examples include the Circoviridae and Parvoviridae.

They replicate within the nucleus, and form a double-stranded DNA intermediate during replication. A human Circovirus called TTV is included within this classification and is found in almost all humans, infecting them asymptomatically in nearly every major organ.

The positive-sense RNA viruses and indeed all genes defined as positive-sense can be directly accessed by host ribosomes to immediately form proteins. These can be divided into two groups, both of which reproduce in the cytoplasm:

Viruses with polycistronicmRNA where the genome RNA forms the mRNA and is translated into a polyprotein product that is subsequently cleaved to form the mature proteins. This means that the gene can utilize a few methods in which to produce proteins from the same strand of RNA, all in the sake of reducing the size of its gene.

Viruses with complex transcription, for which sub genomic mRNAs, ribosomal frame shifting and proteolytic processing of polyproteins may be used.

All of which are different mechanisms with which to produce proteins from the same strand of RNA.

Examples of this class include the families Coronaviridae, Flaviviridae, and Picornaviridae.

The negative-sense RNA viruses and indeed all genes defined as negative-sense cannot be directly accessed by host polymerases to immediately form proteins. Instead, they must be transcribed by viral polymerases into a “readable” form, which is the positive-sense reciprocal. These can also be divided into two groups:

Viruses containing no segmented genomes for which the first step in replication is transcription from the negative-stranded genome by the viral RNA-dependent RNA polymerase to yield monocistronic mRNAs that code for the various viral proteins. A positive-sense genome copy that serves as template for production of the negative-strand genome is then produced. Replication is within the cytoplasm.

Viruses with segmented genomes for which replication occurs in the nucleus and for which the viral RNA-dependent RNA polymerase produces monocistronic mRNAs from each genome segment. The largest difference between the two is the location of replication.

Examples in this class include the families Orthomyxoviridae, Bunyaviridae, Filoviridae, and Rhabdoviridae (which includes rabies).

A well-studied family of this class of viruses includes the retroviruses.

One defining feature is the use of reverse transcriptase to convert the positive-sense RNA into DNA.

Instead of using the RNA for templates of proteins, they use DNA to create the templates, which is spliced into the host genome using integrase.

Replication can then commence with the help of the host cell’s polymerases. A well-studied example includes HIV.

Viruses are classified based on the number of criteria which include

Size, morphology and capsid symmetry of the virus particle

Type of nucleic acid and mode of replication

Mode of transmission

Symmetry of the capsid

Helical symmetry

Icosahedra symmetry

Complex symmetry

Envelope as the outer coat

Enveloped viruses

None enveloped viruses (naked viruses)

In replication of viruses with helical symmetry, identical protein subunits (protomers) self-assemble into a helical array surrounding the nucleic acid, which follows a similar spiral path.

Such nucleocapsids form rigid, highly elongated rods or flexible filaments shaped like a spiral stare case.

Examples of the helical symmetry viruses are measles virus, mumps viruses, rabies viruses, respiratory syncytial virus

An icosahedron is a polyhedron having 20 equilateral triangular faces and 12 vertices

Overall appearance of such viruses is spherical

There are exactly 60 identical subunits on the surface of an icosahedron

Examples of viruses with icosahedral symmetry are Adenoviruses, herpes simplex virus, cytomegalovirus, varicela zoster virus, hepatitis B virus and papiloma virus

An envelope is an outer (bounding) lipoprotein bilayer membrane possessed by many viruses

Some viruses contain lipid as part of a complex outer layer, but these are not usually regarded as enveloped unless a bilayer unit membrane structure is clearly demonstrable

Most of the enveloped viruses are helical in shape

Most of the enveloped helical viruses are RNA viruses

Example influenza viruses, measles virus, mumps virus, rabies virus, respiratory syncytial virus

Enveloped viruses which are icosahedral in shape are also DNA viruses. These include Herpes simplex virus, Varisella zoster virus, and cytomegalovirus

Viruses which are not enveloped are all icosahedra in shape

RNA viruses which are not enveloped include Poliovirus, Hepatitis A virus, and Enteroviruses

DNA viruses which are not enveloped include Adenoviruses, Papilloma viruses and Polyoma viruses

Based on the nucleic acid viruses are classified into

RNA viruses

DNA viruses

RNA viruses, comprising 70% of all viruses, vary remarkably in genome structure

Because of the error rate of the enzymes involved in RNA replication, these viruses usually show much higher mutation rates than do the DNA viruses

The viral RNA may be single-stranded (ss) or double-stranded (ds), and the genome may occupy a single RNA segment or be distributed on two or more separate segments (segmented genomes)

The proteins necessary for the construction of complete virions are always made via the information coded in the host messenger RNA (mRNA)

RNA viruses can either have positive sense or negative sense genome

Virus with positive sense strand can function as messenger RNA (mRNA), while a negative sense strand cannot function as mRNA protein translation.

Positive sense viral RNA alone can replicate if injected into cells, since it can function as mRNA and initiate translation of virus-encoded proteins.

Negative sense RNA, on the other hand, has no translational function and cannot per se produce viral components without the help of the host cell messenger RNA.

Examples of double stranded RNA viruses include, reoviridae family (Reoviruses spp)

Examples of single stranded RNA viruses include, Retroviridae (example HIV), Rhabdoviridae (example Rabies virus), Orthomyxoviridae (example Influenza virus), Filoviridae (example Murbug and Ebola Viruses), Paramyxoviridae (examples Measles viruses, Mumps viruses, Respiratory syncytial viruses and Parainfuenza viruses), Pircornaviridae (examples Polioviruses, Hepatitis A, Enteroviruses)

Most DNA viruses contain a single genome of linear double stranded DNA (dsDNA).

However the papovaviruses (papillomaviruses, polyoma and vacuolating agents), have circular DNA genomes.

dsDNA serves as a template both for mRNA and for self-transcription.

Examples of double stranded DNA (dsDNA) viruses include Herpadnaviridae (Hepatitis B virus), Poxviridae (Small pox virus), Herpesviridae (Herpex simples viruses, Varisela zoster viruses, cytomegaloviruses)

Examples of Single stranded DNA(ssDNA) viruses include Parvoviridae (B19 virus)

Viral transmission depend on type of cells they infect

Viruses infect the cells of humans, domestic and wild animals, plants and bacteria

Viruses are host-specific while some are capable of infecting cells of several different host

Human viral diseases transmission is categorized into

Man as natural host or most important maintenance host

Arthropods (mosquitoes, sand flies, ticks) and vertebrate animals (rodents, birds, monkeys) as natural or most important host and humans as only accidental or secondary host.

Viruses for which man is the natural host include: rotaviruses, poliovirus, hepatitis viruses, rubella virus, influenza virus, rhino viruses, pappiloma viruses and several herpes virus.

Viruses for which arthropod and vertebrate animals are natural or main reservoir hosts include: rabies viruses, virus for viral hemorrhagic fever and the large of group of arthropod borne viruses (yellow fever, encephalitis and rift valley

By direct contact e.g. herpes virus, hepatitis B virus (STI)

By ingestion of viruses (faecal-oral) e.g. enteroviruses, rotaviruses, and hepatitis A virus

By inhaling viruses in airborne droplets or dust particle e.g. influenza viruses, measles viruses, adenoviruses, respiratory synctial virus and rhinoviruses.

By mother to child during pregnancy or birth (Transplacental) e.g. cytomegalovirus, rubella virus

By contact with an article, such as floor mat e.g. papillomavirus (wart producing virus) or towel contaminated with a virus that cause eye infection

By vector e.g. housefly bedbugs help in the transfer of hepatitis virus from one person to another on their bodies

By the bite of an infected mosquito, sandfly, tick, or midge.

By the bite of an animal host e.g. rabies virus.

By contact with materials e.g. vegetation, food, or articles contaminated with excretions of infected animals’ esp. rodents.

Infection occurs: if viruses enter damaged skin, is inhaled in aerosols, or is ingested.

By the direct transfer of viruses from one person to another, e.g Ebola, Marburg, and Lassa viruses

Contact

Examples Herpes viruses, HIV, Hepatitis viruses, Ebola virus, Rabies virus, Marburg virus

Viruses under this group are transmitted through contact with virus from the vesicle, blisters. This may be via sexual contact, during birth, kissing

Inhalation

Examples measles viruses, varicella zoster virus, mumps virus, variolla virus, rubella virus, respiratory syncytial virus, Influenza virus

Fecal Oral Route

Examples enteroviruses

Vector

Examples yellow fever viruses

Transfusion

Examples HIV, Hepatitis B

Viruses are inert in the extracellular environment

They replicate only in living cells, being parasites at the genetic level

The viral nucleic acid contains information necessary for programming the infected host cell to synthesize virus-specific macromolecules required for the production of viral progeny (new virus particle)

Attachment and adsorption

Uncoating (viral genome is injected to the host nucleus/cytoplasm)

Viruses must synthesize mRNA by using the host cell mechanisms

RNA viruses produce mRNA by several different routes

04.Viral mRNA is then translated in the host cytoplasm to produce viral proteins

05.Replication of viral DNA or RNA occurs in the host nucleus except for pox viruses where it takes place in the cytoplasm

06.Assembly of the viral proteins and genome

During the replicative cycle, numerous copies of viral nucleic acid and coat proteins are produced

07.Release of the viral particles

This takes place on the host cell plasma membrane

In order for viruses to reproduce, they must infect a cell. Viruses are not technically alive: they are like a brain with no body. In order to make new viruses, they must hi-jack a cell, and use it to make new viruses. Just as your body is constantly making new skin cells, or new blood cells, each cell often makes new proteins in order to stay alive and to reproduce. Viruses hide their own DNA in the DNA of the cell, and then, when the cell tries to make new proteins, it accidentally makes new viruses as well. HIV mostly infects cells in the immune system.

Several different kinds of cells have proteins on their surface that are called CD4 receptors. HIV searches for cells that have CD4 surface receptors, because this particular protein enables the virus to bind to the cell. Although HIV infects a variety of cells, its main target is the T4-lymphocyte (also called the “T-helper cell”), a kind of white blood cell that has lots of CD4 receptors. The T4-cell is responsible for warning your immune system that there are invaders in the system.Once HIV binds to a cell, it hides HIV DNA inside the cell’s DNA: this turns the cell into a sort of HIV factory and replicates itself.

A virus consists of an outer envelope of protein, fat and sugar wrapped around a set of genes (in the case of HIV, genetic information is carried as RNA instead of DNA) and special enzymes.HIV has proteins on its envelope that are strongly attracted to the CD4+ surface receptor on the outside of the T4-cell. When HIV binds to a CD4+ surface receptor, it activates other proteins on the cell’s surface, allowing the HIV envelope to fuse to the outside of the cell.Entry can be blocked by entry inhibitors.

HIV’s genes are carried in two strands of RNA, while the genetic material of human cells is found in DNA. In order for the virus to infect the cell, a process called “reverse transcription” makes a DNA copy of the virus’s RNA.After the binding process, the viral capsid (the inside of the virus which contains the RNA and important enzymes) is released into the host cell. A viral enzyme called reverse transcriptase makes a DNA copy of the RNA. This new DNA is called “proviral DNA.”Reverse transcription can be blocked by: Nucleoside Reverse Transcriptase Inhibitors (NRTIs), and Non-Nucleoside Reverse Transcriptase Inhibitors (NNRTIs).

The HIV DNA is then carried to the cell’s nucleus (center), where the cell’s DNA is kept. Then, another viral enzyme called integrase hides the proviral DNA into the cell’s DNA. Then, when the cell tries to make new proteins, it can accidentally make new HIVs.Integration can be blocked by integrase inhibitors

Once HIV’s genetic material is inside the cell’s nucleus, it directs the cell to produce new HIV.The strands of viral DNA in the nucleus separate, and special enzymes create a complementary strand of genetic material called messenger RNA or mRNA (instructions for making new HIV).Transcription can be blocked by antisense antivirals or transcription inhibitors (TIs), new classes of drugs that are in the earliest stage of research

The mRNA carries instructions for making new viral proteins from the nucleus to a kind of workshop in the cell. Each section of the mRNA corresponds to a protein building block for making a part of HIV.As each mRNA strand is processed, a corresponding string of proteins is made. This process continues until the mRNA strand has been transformed or “translated” into new viral proteins needed to make a new virus.

The final step begins with the assembly of new virus. Long strings of proteins are cut up by a viral enzyme called protease into smaller proteins. These proteins serve a variety of functions; some become structural elements of new HIV, while others become enzymes, such as reverse transcriptase.Once the new viral particles are assembled, they bud off the host cell, and create a new virus. The virus then enters the maturation stage, which involves the processing of viral proteins. Maturation is the final step in the process and is required for the virus to become infectious.With viral assembly and maturation completed, the virus is able to infect new cells. Each infected cell can produce a lot of new viruses.Viral assembly can be blocked by Protease Inhibitors (PIs). Maturation, a new target of companies developing anti-HIV drugs, may be blocked using Maturation Inhibitors

Viruses are classified according to

Morphology, size and capsid symmetry

Nucleic acid type

Most RNA viruses have helical capsid symmetry and DNA viruses have icosahedral capsid symmetry

For viruses to replicate viral proteins must be synthesized by the host cell protein synthesizing machinery

All human viruses with helical nucleocapsid poses an envelope

What are the criteria used to classify viruses?

What are differences between enveloped and none enveloped viruses?

What are the important steps involved during viral replication?

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