Virus-Host Interactions
Uncoating
The uncoating of a nucleocapsid which release the viral RNA
Uncoating is an intracellular step during which viral nucleic acid and capsid are separated.
The virus genome is therefore released into the cell.
The virus genome therefore depending, might undergo reverse transcription where the RNA would be converted to DNA.
The DNA would then be integrated by the process of integration into the cell’s nuclear
Image above show a HIV’s DNA integrating into a nuclear of a white blood cell’s nuclear.
The DNA is mostly integrated into the cell together with a integrase.
The integrase would assist the viral DNA to be inserted into the cell’s own DNA.
Viral Replication
The replication of HIV virus mRNA.
Since part of the cell’s DNA contains the virus DNA, the cell would also produce virus mRNA together with its normal mRNA.
Different Class of virus replicate differently.
There are seven different class of classification.
Class I
This involves double-stranded DNA.
The replication is exclusively nuclear therefore its very dependent on host cell factors
The replication takes place in the cytoplasm, the viral genome contains all factors for genome replication and transcription.
Examples include Adenovirus, Herpesviruses and Poxiviridae.
These virus may cause cancer as they force the cells to undergo cell division.
Class II
This involves single-stranded DNA.
The replication of virus genome takes place in the nucleus
New single-stranded is made from double-stranded DNA formed.
Examples include Parvoviridae and Circoviridae
Class III
This involves double-stranded RNA
The replication of virus takes place all in the cytoplasm
The genome of the virus is fragmented, which mean each different genome code for a different protein.
The replication is monocistronic.
Class IV
This involves single-stranded positive RNA
There are 2 groups, group 1 and group2
Group1
The virus is with polycistronic mRNA, the genome RNA is responsible for forming the mRNA.
These mRNA are later translated into polyprotein.
Therefore from the same strand of RNA, the virus has different method to produce protein.
Group2
This involve virus with complex transcription process.
There is 2 round of transcription before the formation of genomic RNA.
Ribosome frame shifting and proteolytic could be used to produce protein from the same strand of RNA.
Class V
Contains 2 group.
Group1
Virus with non-segmented genome.
The transcription of the negative RNA is induced by RNA-dependent RNA polymerase which gives monocistronic mRNA.
Replication takes place within the cytoplasm.
Group2
Virus with segmented genome.
Replication takes place in nucleus.
RNA-dependent RNA polymerase produce monocistronic mRNA from each segmented genome.
The biggest different between both group is the location where replication takes place.
Class VI
This involve single-stranded positive RNA, but there is DNA intermediate.
It is diploid
The process of reverse transcription of viral RNA to dsDNA is done by viral RT
The dsDNA is integrated into the host cell’s genome
The viral RNA is not used as mRNA
Well studied examples are HIV
Class VII
This involves double stranded DNA, but there is RNA intermediate
Involves overlapping reading of frames
Examples include Hepadnaviriade
This class is not well studied
Assembly or maturation
The assembly of HIV in a white blood cell
Once the various viral subunits have been produced and processed, they must be separated for the final assembly into new virus. This separation, or cleavage, is accomplished by the viral enzyme.
The viral RNA is now repacked into a nucleocapsid again.
The structural subunits of the virus would mesh with a part of the cell membrane causing it to deform.
Release
This is the final step in virus replication, the genetic material enclosed in the nucleocapsid merges with the deformed cell membrane to form a new virus envelope. The virus is therefore released and readily attaches to another cell.
Regulation of expression
Transcriptional Control
· Involve the promotion of viral genome
· Involves the early and later activators / enhancers
· Involves late repressors
· The viral transciptases for RNA viruses is not very understood
Post-transcription control
· Involves the splicing of polycistronic mRNA in nucleus
· There is different rate of splicing
· Involves the control of mRNA from nucleus to cytoplasm
· There are regulatory sequences found on introns
Translational control
· There is different stability of mRNAs
· The secondary structures are close to initiation sequence
· There is problem due to overlapping reading of frames
Translational control – overlapping reading frames
· Internal ribosomal entry sites ( IRES )
· Frameshifting
· Pseudoknots
How could a virus enter a host?
· The skin when there are cuts, abrasions, open wounds
· The eyes (eyelid)
· The lungs when breathed in
· The small intestine, when virus is consumed
· The genitals, during sexual intercourse
Types of virus spread inside a host
· Systemic infection – infect several organs
· Haematogenous spread – spread through the bloodsteam
· Neural spread
Virus could be transmitted
· eating infected tissue (beef, mad cow disease)
· coughing / sneezing ( mucus / saliva )
· contaminated hands ( holding hands / unknowingly touching infected people or items and rub eyes or nose
· Saliva ( kissing , sharing of food )
· Blood ( sharing of needles )
· Sexual intercourse
· Faeces( only affect undeveloped countries as people relieve themselves in river and obtain drinking from river)
Virus induced injuries (CPE)
· Cell death
· Mutation ( 2 nucleus , changed in shape )
· Change in cell membrane permeability
(Others)
· Shut down of the cellular function of the cell which leads to cell death ( example neurons, shut down lead to a person being paralyzed)
· Immunopathological lesions ( HIV, the white blood cell are unable to function. Enhancement in immune response causing haemorrhagic fever, dengue )
Videos
This shows the HIV replication cycle.
This shows how a HIV virus enters a host cell.
This video shows a flash animation of the virus replication cycle. (without audio)
References:
