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Infectious Diseases And Public Health > Viruses > Flashcards

Viruses Flashcards

0
Q

What are capsids made of?

A

Small peptide units called protomers or structural units.

They can be composed of one type of protein unit or many different sub units

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1
Q

List the purposes of viral capsid sand envelopes

A

Protection while traveling between hosts
Protection of their genetic material from pH extremes, temp extremes as well as the hosts immune response and proteolytic enzymes

Attachment to host cells - external molecules act to assist attachment but may also be markers for the immune response

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2
Q

What is a nucleocapsid?

A

The proteins of a capsid are arranged in either a helix or a icosahedral shape.

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3
Q

Name the types of viral capsid?

A

Helical and icosahedral
Helical - The genomic information is held in the groove of the capsid.
The term nucleocapsid refers to the intimate relationship between the nucleocapsid acid and the protein subunits
Example of a helical virus is the tobacco mosaic virus
Icosahedral - spherical in shape with units forming equilateral triangles using three protomers. Only accommodates small genomes
Can be very complex with 13 subunits per protomer
Example of a icosahedral virus is the adenovirus

Some can be neither helical or icosahedral. The t bacteriophage is very complex - see pg 17

Nucleosomes are membrane bound viruses which encompass their genome. Their exterior coat is covered in fibres - example pox virus

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4
Q

What is the function of the viral envelope?

A

The viral envelope surrounds the capsid and is often composed of host proteins. It can be loose fitting or tight. It often has the virus’s glycoproteins inserted into it which allow attachment but also are targets of the hosts immune system

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5
Q

Describe some features of the viral genome

A

The viral genome is small
Can have up to three reading frames therefore producing three different proteins from the same sequence
Can be single stranded, double stranded, linear or circular
Can be composed of DNA or RNA but not both

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6
Q

What is ambisense DNA?

A

Single stranded RNA with both positive and negative sections - found in some viral genomes

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7
Q

Give an example of a DNA virus

A

Small pox, slap cheek and herpes

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8
Q

The viral proteins encoded for by a viral genome can be split into three groups. Name them?

A

Structural proteins - surface proteins or protomers which form the capsid
Non structural proteins (regulatory proteins) - involved in viral genome transcription, such as enzymes etc
Neither structural or non structural - enzymes found in the virus’s capsid

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9
Q

Name the 3 main theories in the evolution of virus’s

A

Retrograde theory - viruses evolved from intracellular parasites by shedding all non essential genes - unlikely

Viruses evolved from cellular components composed of nucleic acids - for example plasmids and transposons which can replicate separately from a bacterial genome - possible

Viruses have evolved alongside cells and have always been here since the emergence of life

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10
Q

What is reverse transcriptase?

A

Reverse transcriptase is an RNA dependant DNA transcriptase. This enzyme transcribes DNA from an RNA template.

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11
Q

What is a virus receptor?

A

Surface molecules on host cells which allow attachment and entry of a virus into the cell

In order for some viruses to enter a cell there must be sequential interaction with more than one receptor before entry can be achieved, for example HIV virus required CD4 receptors as well as chemokine receptors before it can enter the host cell

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12
Q

Define tropism?

A

The preference of a virus for a specific species of host or specific cell type

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13
Q

Explain how an enveloped and a nun enveloped virus uncoats in a host cell….

A

Enveloped viruses fuse with the plasma membrane of the host cell or endocytic membranes and release capsids into the cytoplasm which contain nucleic acids.
Example HIV

Others enter via receptor mediated endocytosis. Example influenza

Unenveloped viruses can not fuse with the membranes and therefore have to disrupt the membrane of the host cell.
Example adenovirus which goes on to deliver its nucleic acid directly into the nucleus of the cell

The polio virus form pores in the membrane to deliver its nucleic acid
Example polio

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14
Q

In general terms, where do RNA and DNA viruses replicate their genetic material?

A

RNA - cytoplasm
DNA - nucleus

Some use their own enzymes to initiate replication, others use the host enzymes

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15
Q

What are early and late genes?

A

Early genes are the first to be transcribed and are usually non-structural such as genes for replication

Late genes are transcribed later and are structural such as genes which code for capsid production

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16
Q

Give examples of RNA and DNA viruses…

A

RNA - HIV, polio and measles

DNA - small pox and hepatitis B

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17
Q

How are viruses classified?

A

Baltimore classification system
Seven classes depending on the steps they take to produce mRNA which can then be translated by the host cell
See figure 3.12 on pg. 24

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18
Q

Describe the HIV virus

A

Retrovirus which replicates using reverse transcription
Single stranded positive RNA
Classed in the sub group lentivirus meaning slow as it causes disease over a long period of time
Affects the immune and nervous system

19
Q

Describe the structure of an HIV virus?

A

Exterior
Roughly spherical
Enveloped
Studded with about 100 spikes dispersed evenly
Spike formed by three copies of gp120 (glycosylated viral proteins) non covalently attached to a stalk composed of gp41(glycosylated viral proteins)
Surface also has MHC I and MHC II Immune system proteins from the host cell.

Interior
2 strands of positive RNA
Reverse transcriptase binds to the RNA as well as integrase
Nucleocapsid proteins p6 and p7 protect the RNA from host proteases
Viral proteases process the polyproteins into functional proteins

20
Q

What cell surface molecules are required for an HIV virus to bind?

A

CD4 and chemokine a

21
Q

Describe the replication process of HIV

A

Once inside the cell ss +ve RNA is transcribed to -ve ss DNA by viral reverse transcriptase
Reverse transcriptase then synthesises +ve ss DNA which combines with another ss to for double stranded +ve DNA
DNA is then transported from the cytosol into the nucleus via microtubules through the nuclear pore
Viral DNA is integrated into he hosts genome by the viral protein integrase to form provirus DNA
Provirus DNA is then used as a template for viral mRNA synthesis.
Viral protein Rev transports the mRNA out of the nucleus and into the cytosol where it is transcribed into viral proteins
These proteins congregate at the cell membrane where they mix with other viral proteins
Budding occurs and a new viral particle is formed
Once released from the cell, maturation takes place when proteases cleave the viral proteins into functional viral proteins.

22
Q

Describe the polio virus?

A

Small RNA virus
Single stranded +ve RNA
Affects the gut but can also spread to the blood and the central nervous system
Stable at a low pH therefore able to get past the stomach and into the intestine

23
Q

Describe the structure of the polio virus?

A

Small icosahedral capsid with 60 protomers

Each protomers composed of 4 viral proteins - VP1-4
See pg 16 for a diagram and EM of the virus

24
Q

Describe the replication strategy of the polio virus?

A

5 stages…
1- virus binds to cell surface receptor CD155 (immunoglobin like receptor) Schindler initiates the creation of a pore in the host cell membrane through which the +ve RNA enters he host cytoplasm

2- VPg protein is cleaved from the viral RNA. Some RNA is transcribed allowing replication of the genome, whereas some is translated into a single viral protein. Translation is initiated at the Internal Ribosome Entry Site (IRES) which is a different mechanism to the normal cap dependant initiation in the host cell. A protease is also translated from the viral genome which stops the normal cap dependant translation of the hosts genome, therefore stopping host protein production

Viral polypeptide is cleaved into three precursors….p1-3

3- the viral genome is replicated to create many copies which become templates for synthesis of -ve RNA which is the template for +ve viral RNA which is delivered back into the cytosol

4- some of this viral RNA is translated into more viral proteins

5- capsid proteins are synthesised and form new vision capsid containing new +ve viral genomes.
As the host cell is joe unable to produce its own proteins, the cell dies a matter of hours later and the viral capsids are released to infect other cells

25
Q

Describe the smallpox virus?

A

The small pox virus has been eradicated since 1977
It is a large animal virus
The most deadly human virus in history

26
Q

Describe the structure of a smallpox virus?

A

Large virus
Non symmetrical
Double stranded DNA which is fused at both ends forming a circular DNA strand
Replicates in the cytoplasm, unlike other DNA viruses
Therefore it carries it’s own polymerases and replication enzymes with it in the capsid as the hosts replication machinery is in the nucleus
Also carries it’s own capping and methylating enzymes
Still use the hosts energy supplies etc

27
Q

Describe the replication strategy of the smallpox virus?

A

Four stages to the replication strategy
1- attaches to the host cell and is endocytosed into the cytoplasm

2- partially uncoats allowing access for its own DNA dependant RNA polymerases to translate around 100 mRNA molecules into the cytoplasm. mRNA is processed by methylating and capping proteins to make it available for translation by the hosts machinery.
Early genes encode viral polymerase proteins and structural capsid proteins as well as uncoating enzymes

3- late genes begin to be transcribed while the early genes are no longer translated into a protein product
Not hey mainly encoded structural proteins for the capsid

4- visions are assembled, some are provided with a coat by the cells Golgi apparatus allowing it to leave he cell before it does. Most however are released when the cell lyses.

28
Q

Describe the measles virus?

A

Same family of virus that causes mumps

Bind to host cell surface receptors containing sialic acid (sugar)

29
Q

What receptors does the measles virus attach to on the host cell?

A

Bind to host cell surface receptors containing sialic acid (sugar)

30
Q

Describe the structure of the measles virus?

A

Similar to the structure of the influenza virus and attaches to the same receptors
Helical, enveloped capsid
Negative strand RNA genome
Similar replication strategy to influenza

31
Q

Describe the replication strategy of measles?

A

Similar to influenza
3 stages
1- entry into host and attachment to a host cell
Attachment to sialic acids on cell surface. Fuses with cell membrane and releases nucleocapsid into the cytoplasm
It carries it’s own RNA dependant RNA polymerases to replicate its own genome and transcribes its 6 genes. The genes nearest the 3’end are most important for infection and are transcribed in greater numbers
Host cell machinery transcribes the viral proteins
The -ve genome is translated into a +ve genome in the form of a replicative intermediate

2- the replicative intermediate is the template for the formation of viral -ve genome compiles which form new virions

3- the nucleocapsid a are formed in the cytoplasm and released from the cell by budding

32
Q

Describe the HBV virus?

A

HBV is the hepatitis B virus

It causes liver disease

33
Q

Describe the HBV virus structure?

A

Enveloped icosahedral virus
Partially double stranded DNA genome
Contains its own DNA dependant DNA polymerase which has reverse transcriptase ability

34
Q

Describe the replication strategy of the HBV virus?

A

Three stages

1- infects mainly liver cells. Several possible receptors that the virus and bind to by unknown mechanisms
The genome is carried to the nucleus of the host cell where the +ve DNA strand is spliced together to form a circular DNA molecule which acts as a template for the synthesis of 4 +ve RNA strands
One is used for genome replication, the other three are used for viral protein synthesis.

2- the viral capsid is formed and contains the viral RNA along with a newly synthesised polymerase which translates it into DNA

3- the capsid is surrounded by a membrane and studded with viral proteins before being released from the cell. The cell does not die as a result of the infection .

The damage to the liver is thought to be as a result of the hosts immune response as opposed to the virus itself

35
Q

What is a cytocidal virus?

A

A virus which kills the host cell after infection

36
Q

What are syncitia?

A

Giant cells which have been formed by virally infected cells fusing together, can form multinucleate cells
Occurs in measles and HIV

37
Q

What are inclusion bodies?

A

Granular areas where viral components are being synthesised or assembled. This can be in the cytosol, the nucleus or the organelles.
Granular inclusion areas can damage the cells structure

38
Q

How do viruses disrupt normal cell function?

A

Inhibit the synthesis of the hosts DNA
The rate of viral protein synthesis may outcompete the hosts protein synthesis
Compromise the integrity of the plasma membrane
Also the integrity of the inner membranes therefore increasing pH and the dissipation of ion gradients
Viral proteins can be inserted into the membrane which cause the immune system to attack it
Host cells can also be transformed because of the insertion of the viral genome into its own genome, this can lead to cancerous cells

39
Q

Define the incubation period?

A

The time between infection and the onset of symptoms

40
Q

What are the effects of HIV?

A

HIV is present in bodily fluids
Affects helper T cells of the immune system

Acute phase 1-12 weeks after infection
Symptoms of flu
Brief peak of virus in the blood stream called viraemia followed by a brief drop in concentration of t helper cells - also referred to as CD4 cells
There is then an asymptomatic period of 5-10 years where there are few symptoms
The t helper cells then begin to decline rapidly and the patient becomes very susceptible to opportunistic pathogens and tumours.

41
Q

Describe the effects of a polio infection?

A

Incubation period of 1-3 weeks
Can cause paralysis due to destruction of motor neurons
Most common route of transmission is faecal-oral
Initial infection site is the tonsils and lymphoid tissues
Can spread to the CNS causing meningitis
Most common in children under 5

42
Q

Describe the effects of a measles infection?

A

Highly contagious
Spread by respiratory droplets
Particularly common in under 5’s
10 day incubation period
Causes viraemia
Symptoms are cough, runny nose, fever, kopliks spots in the mouth and a pussy rash lasting 5-10 days
No specific treatment
There can be serious complications such as immunodeficiency and secondary infection
In some cases the virus enters the brain and causes inflammation

A very rare subsequent infection called subacute sclerosing panencephalitis occurs 6-8 years after a measles infection
It is a fatal neurodegenerative disease caused by prion proteins

43
Q

Describe the effects of the HBV virus?

A

Causes an inflammation of the liver
Routes of transmission are same as HIV, sexual, blood and vertical transmission
Six viruses that cause hepatitis, HBV being the most common.
Death is as a result of the immune response to the infected hepatocytes as opposed to the effect of the virus itself

44
Q

Describe the Ebola virus

A

Viral haemorrhagic fever
Muscle pain, circulatory shock, haemorrhage and death
First recorded in 1976
Ebola is a zoonose although the reservoir species has not been identified - likely to be fruit bats

Infectious Diseases And Public Health (12 decks)

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