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Flashcards in BM1022 2 Deck (22):

What is a virus

Viruses are obligate intercellular parasites that lack all the cellular mechanisms for self-replication


Structure of a virus

Their basic structure is simple comprises of one type of nucleic acid surrounded by a protein coat called a capsid

Some are surrounded by an envelope made from a combo of material from the virus and the host cell

A complete viral particle is Called virion


Nucleic acid in viruses

They can only contain one type of nucleic acid eitherDNA or RNA and that is either single stranded or double stranded



A protein coat called a capsid covers the nucleic acid of the virus. The repeating units of the capsid are called capsomeres and are often arranged in symmetrical patterns



Some viruses manufacture an envelope out of components of its host and various viral proteins and glycoproteins. This often complex structure sometimes acts to assist the virus in entering a host cell


Classification of a virus

They are grouped according to the composition and morphology, size and methods of replication
73 families

E.g Togavirus is an enveloped virus, Toga in Latin -covering )


Bacteria viruses

Viruses that infect bacteria are called bacteriophages or just "phages" - example of complex viruses


Cycles of infection


Phage attaches to specific receptor site of the host cell wall

Phage penetrated the cell wall and injects viral nucleic acid
Host protein synthesis ceases, viral nucleic acid and viral proteins synthesis commences
Viral components assemble to produce mature virions
Virions are released though a rupture or lysis of the host cell


Cycle of infection


Follows similar process of attachment, penetration and injection
The viral nucleic acid does not proceed to the synthesis of virions
Instead, the viral nucleic acid is incorporated into the host genome, where it remains latent - a latency of infection, or a prophage
This prophage is then passed on through generations of bacterial replication
When host cell is stressed by a spontaneous event e.g, UV light, the prophage may revert back to the lytic phase and cause virion assembly and release and thus rupture of the cell


6 steps of of viral infection in animals








Adsorption (animal infection)

Viral particles are absorbed into the animal cell after the virion attached to specific receptors on the site of the animal cell


Penetration (animal infection)

The whole virion is engulfed by the animal cell


Uncoating ( animal infection)

Host cell enzymes remove the envelope (if the virus has one) and the capsid, revealing the nucleic acid


Synthesis (animal infection)

Viral DNA is replicated in host cell nucleus using viral enzymes. Proteins are manufactured using host cell enzymes


Assembly (animal infection)

Proteins are assembled to form capsid and join with nucleic acids to form mature viral particles


Release (animal infection)

Viral particles are released, often resulting in lysis of the host cell, if the cell is enveloped it acquires one from the membrane of the host cell as it is released


Acute Lytic Infection
Influenza- influenza virus

This follows the classic path of a lytic infection
The infection produces an acute illness resulting from the consequences of the rupture of host cells, mostly respiratory epithelium
Flu virus A,B and C are examples of enveloped, helical RNA virus
This virus is able to change its genetic make up and thus avoid the immune response of the host and attempts to produce vaccine


Sub-clinical- German measles -rubella virus

Often a mild or sub-clinical infection passed via the respiratory route causing a skin rash
This virus is able to cross the placenta and cause severe foetal infection resulting in congenital rubella syndrome
Rubell virus is an icosahedral enveloped, RNA virus.


Latent- chicken pox/ shingles- varicella/zoster virus

The initial infection results in chicken pox, a mild respiratory illness that develops into a rash in children and often a more severe infection in adults
The virus infects nerve cells and after the primary infection may become latent until a time when the host is immuno-compromised
This is a iscohedral, enveloped, DNA virus


Chronic infections-hepatitis B

Inflammation of the liver
Acute hep B may develop into a sub clinical infection were infectious viral particles continue to be produced


Oncogenes viruses-cervical cancer - papilloma virus

Viruses that are implicated in causing cancer are called oncogenic viruses (onco-cancer)the virus enters a form of lysogeny with the host cell and the inserted nucleic acid in the host genome changes the characteristic of the cell producing uncontrolled growth
Unchecked cellular growth results in a tumour mass
Non enveloped, iscohedral, DNA viruses


Diagnosis of viral infections

Only able to replicate in living cells
Are cultured in living cells called a cell culture
Viral infections are often diagnosed by the identification of the specific antibody in an infected host