Virology: Study of viruses.

Capsid: The protein shell, or coat, that encloses the nucleic acid or genome.

Capsomeres: is one of the protein subunits that come together to form the capsid.

Defective virus: is a viral particle which can not replicate on its own because it lacks one of the essential genes needed for replication or assembly

Helper virus – a virus that provide the missing function or gene product for the defective virus to produce its particle or virion

Envelope: A lipid-containing membrane that surrounds some virus particles.

It surrounds the viral capsid and is derived from the host cells.

Nucleocapsid: It is a structural combination of the viral genome ( nucleic acid ) and the protein capsid

Viral subunit(protomer): is a building block of the viral capsid, usually a single protein molecule

Virion: The complete virus particle.

They are small in size ranging 20-300 nm in diameter.

They are akaryotic particles (neither eukaryotes nor prokaryotes).

They contain either DNA or RNA and not both in their genome.

They exhibit living properties when inside the living cells. They are incapable of independent reproduction unless they are in the living cell.

They are non-motile.

They can be grown in cell cultures.

Viruses are known to infect unicellular and multicellular organisms such as bacteria, and algae and all higher plants and animals

It is made of three basic units

Viral core/genome (RNA or DNA, not both).

Capsid, the viral coat that encloses the genome

Envelope made of glycoprotein and lipids.

Not all viruses have envelopes.

Part

Function

Envelope

(Outermost coat of the virus made of lipid and proteins)

Support glycoproteins or spikes used for attachment to the host cell

Gives stability to the virus

Protects the inner parts of the virus

Capsid

(Protein coat which covers the viral genome, it is made up of repeating subunits called capsomeres)

Gives the virus a symmetric appearance that is useful for classification purposes

Viral core

(Contains genetic materials either DNA or RNA not both)

Replication

Viruses are classified based on the following categories

According to morphology.

According to type of nucleic acid.

According to mode of transmission.

Symmetry of the capsid

According to taxonomy

Presence or absence of an envelope as the outer coat

Enveloped viruses.

Non enveloped viruses.

Influenza virus.

Respiratory syncytial virus.

Herpes virus.

Measles virus.

Mumps virus.

Poliovirus.

Hepatitis A virus and E

Enteroviruses.

Human Papilloma virus.

Adenovirus.

ii.Symmetry of capsid

Viral symmetry of capsid, refers to the protein (capsomers) arrangement of the capsid which gives the viral capsid its shape

The capsomers within the capsid may be arranged and making the following viral capsid shapes.

Icosahedral symmetry.

Helical symmetry.

Complex symmetry.

Orthomyxoviridae (e.g. Influenza virus, Respiratory syncytial virus)

Bunyaviridae (e.g. Rift valley fever virus)

Rhabdoviridae (e.g. Rabies virus)

Paramyxoviridae (e.g. Measles virus, Mumps virus)

Filoviridae (e.g. Ebola virus, Marburg virus)

NOTE

Almost all RNA viruses have helical symmetry of capsid.

The icosahedral symmetry, its spherical or polyhedral shape

Examples

Picornaviridae (e.g. Poliovirus,rhinovirus, ).

Reoviridae (e.g. Reovirus).

Togaviridae (e.g. Chikungunya virus).

Flaviridae (e.g. dengue virus, zika virus, yellow fever virus

Adenoviridae (e.g. Adenovirus).

Papillomaviridae (e.g. Papillomavirus).

Parvoviridae (e.g. Parvovirus).

Poxviridae (e.g. Smallpox virus).

Flaviviridae (e.g. Yellow fever virus).

Retroviridae (e.g. HIV).

Arenaviridae (e.g. Lassa virus).

Coronaviridae (e.g. Corona virus).

There are two (2) classes of viruses according to the type of nucleic acid:

RNA viruses.

DNA viruses.

These genetic materials can be single stranded or double stranded.

Also, these genetic materials can be positive sense or negative sense.

75% of all viruses

Examples of RNA viruses include

Single stranded RNA viruses are

Measles virus.

Mumps virus.

HIV.

Ebola virus.

Double stranded RNA virus is

Retrovirus. (HIV)

Blue tongue virus

Almost all are single stranded EXCEPT Family Reoviridae is double stranded.

Togaviridae

Chikungunya virus.

Rubella virus.

Flaviviridae

Dengue fever virus.

Yellow fever virus.

Hepatitis C virus.

Retroviridae

Human Immunodeficiency viruses 1 and 2.

Human T-cell leukemia viruses 1 and 2.

Paramyxoviridae

Measles virus.

Mumps virus,

Parainfluenza viruses type 2 and 4.

Parainfluenza viruses type 1 and 3.

Respiratory syncytial viruses.

Orthomyxoviridae

Influenza viruses.

Coronaviridae

Coronavirus.

Filoviridae

Ebola virus.

Marburg virus.

Bunyaviridae

Bunyavirus.

Hantavirus.

Arenaviridae

Lassa virus.

Double stranded

Reoviridae.

Example: Rotavirus.

Most have linear double stranded DNA.

Examples of DNA viruses include

Single stranded DNA viruses is

B19 virus.

Double stranded DNA viruses are

Herpes viruses.

Hepatitis B virus.

Double stranded

Herpesviridae

Herpesviruses.

Poxviridae

Smallpox virus (Vaccinia virus and Variola virus).

Hepadnaviridae

Hepatitis B virus.

Single stranded

Parvoviridae

Parvovirus 19 (B 19 virus).

Double stranded

Papovaviridae

Papillomaviruses.

Adenoviridae

Adenovirus.

Contact

This may be via sexual contact, during birth, kissing.

Viruses are transmitted through contact with virus from the vesicle, blisters.

Examples

Herpes viruses.

HIV.

Ebola virus.

Rabies virus.

Corona virus

Hepatitis B and c

Inhalation

Examples

Measles viruses

Respiratory syncytial virus

Influenza virus

Rhinovirus

Coronavirus

Adenovirus

Fecal oral route

Examples

Enteroviruses like Poliovirus,

Hepatitis A virus,

Hepatitis E virus.

Rotavirus

Norovirus

Vector

Examples

Yellow fever virus.

Dengue fever virus.

Zika virus

Chikungunya virus

West Nile virus

Rift valley virus

Blue tongue virus

Tick-born encephalitis virus

Crimean –congo hemorrhagic fever virus

Blood transfusion

Examples

HIV.

Hepatitis B.

West nile virus

Cytomegalovirus

Zika virus

Murburg virus

Hepatitis c

Ebola virus

Viruses are acellular organisms and do not fit in the biological classification of organisms, the ICTV is used

The taxonomic classification of viruses is a structured system used to organize and name viruses based on their genetic makeup, replication method, morphology, and host range. This system is governed by the International Committee on Taxonomy of Viruses (ICTV).

The ICTV, classifies virus basing on

viral genome, structure, and biological properties

ICTV Hierarchical taxonomic ranks (similar to biological taxonomy)

Rank

Example

Realm

Riboviria

Kingdom

Orthornavirae

Phylum

Negarnaviricota

Class

Monjiviricetes

Order

Mononegavirales

Family

Filoviridae

Genus

Ebolavirus

Species

Zaire ebolavirus

And the other type of classification is Baltimore classification which explains how the virus replicate by making mRNA in protein synthesis

Baltimore devides all the virus into seven groups (i-vii)

An oncovirus is a virus that can cause cancer by transforming normal cells into malignant ones.

These viruses are classified as Group 1 carcinogens by the International Agency for Research on Cancer (IARC) due to their direct or indirect role in tumor development

These are viruses which are associated with cancer formation.

Also known as Oncoviruses.

ONCOGENIC VIRUS

ASSOCIATED CANCER

Human papillomaviruses

Genital tumors e.g. Cancer of cervix

Oropharyngeal carcinoma

Epstein-Barr virus

Nasopharyngeal carcinoma

Burkitt lymphoma

Hodgkin disease

B-cell lymphoma

Human herpesvirus 8

(Kaposi`s sarcoma associated herpesvirus)

Kaposi sarcoma

Hepatitis B virus

Hepatitis C virus

Hepatocellular carcinoma

Human T- lymphotropic virus

Adult T-cell leukemia

Human immunodeficiency virus

AIDS-related malignancies

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).

The following are general steps for replication of a virus

Attachment and adsorption

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

Viruses must synthesize mRNA by using the host cell mechanisms

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

Replication of viral DNA or RNA occurs in the host nucleus

Some viruses like pox viruses where it takes place in the cytoplasm

Assembly of the viral proteins and genome

Release of the viral particles

Stage

Description

Attachment

Virus binds to specific receptors on the host cell surface

Entry

Virus enters the host cell via fusion or endocytosis

Uncoating

Viral capsid is removed, releasing the viral genome

Integration

Viral genome integrates into the host DNA (in lysogenic or retroviral cycles)

Replication

Viral genome is copied using host cell machinery

Assembly

New viral particles are assembled from proteins and replicated genomes

Release

Viruses exit the host cell by lysis or budding to infect new cells

adenine (A)

cytosine (C)

guanine (G)

thymine (T),

and uracil (U)

Paring in DNA

Adenine –Thymine

Guanine –Cytosine

Paring in RNA

Adenine –Uracil

Guanine-cytosine

Viruses are the smallest infectious agents and contain only one type of nucleic acid (DNA or RNA).

Known viruses are highly diverse, varying in size, shape, and genetic content.

Only some types possess a lipid envelope.

Viruses are classified into groups, designated virus families, based on common properties, such as virion morphology, genome structure, virus protein properties, and strategies of replication.

Viruses are obligate intracellular parasites and multiply only in living cells.

48

List five (5) general characteristics of viruses.

Describe the general structure of a virus.

Mention five (5) RNA enveloped viruses.

Outline steps involved in the replication cycle of a virus.

Enumerate five (5) oncogenic viruses.