BM1022 2 Flashcards
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
Capsid
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
Envelope
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
LYTIC CYCLE
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
LYSOGENIC CYCLE
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
Penetration
Uncoating
Synthesis
Assembly
Release
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
