Structure of viruses and Chemical composition and classification of viruses

Capsomers

composed of protein subunits

Virion

the complete infectious unit of virus particle

Naked Viruses

Heat-resistant

Enveloped viruses

Sensitive to heat

Spikes

found in some and are composed of carbohydrates protein complexes that project from the surface of the envelope

Viral Nucleic acids

the genetic material of the virus that is either DNA or RNA but not both

Function of the viral core

1. Control the viral heredity

2. Control viral variation

3. Responsible for the infectivity

Viral proteins

1. structural proteins

2. non structural proteins

Structural proteins

capsid, envelop, matrix, and in extracellular and intracellular state.

Non-structure proteins

are absent in extracellular state but they form during viral reproduction like Enzyme and nucleic acid binding proteins

Function of structural proteins of virus particles

1. protection of the genome

2. delivery of the genome

Protection of the genome

Assembly of a stable protective protein shell, specific recognition and packaging of the nucleic acid genome, and interaction with host cell membranes to form the envelope

Delivery of the genome

1. Binds host cell receptors

2. Fusion with cell membrane

3. Uncoating of the genome

4. Transport of genome to the appropriate site

Subunit

single folded polypeptides chain

Structural unit

Unit from which capsids or nucleocapsids are built; one or more subunits

Nanometer

10(-9) meters = 0.0001 microns = 10 A

smallest virus

poliovirus at 30nm

Why are viruses described as metastable

they must protect the genome and come apart on infection

The tools of viral structure biology

1. Electron microscopy

2. X-ray microscopy

3. Cryo-electron microscopy (cryoEM)

4. Cryo - electron tomography

5. Nuclear Magnetic Resonance spectroscopy (NMR)

Electron microscopy

need to be stained with negative staining. resolution 50-77 A and detailed structure interpretation is impossible

X-ray crystallography

2-3 A

Function of Non structural proteins

provide regulation of viral genome expression and can inhibit host cell biosynthesis

Viral lipids

15-35% are in enveloped viruses

function of viral lipids

1. stabilization of viral shell

2. protection of inner virion shells and nucleic acid

3. deproteinization of virion (uncoating)

Integral membrane glycoprotiens ectodomain

attachment, antigenic sites, fusion

family name end in

viridae

genus name ends in

virus

Viral species

a group of virus sharing the same genetic information and ecological host

how are viruses classified

1. host cell

2. structure and composition (capsid)

3. mode of reproduction

4. nature of disease

5. genetic material

6. number of strands

7. enveloped

By the type of nucleic acid

baltimore method

by outer covering

1. capsid (HAV)

2. Envelope (COVID)

By the shape of the virus

1. Icosahedral (cubic)

2. Helical

3. Complex

Helical symmetry

coat protein molecules engage in identical, equivalent interactions with one another and the viral genome to allow construction of a large, stable structure from a single protein

Enveloped RNA viruses (-) ssRNA and helical capsid

Paramyxoviridae (measles and mumps)

Enveloped RNA viruses with (+) ssRNA and helical capsid

Coronavirdiea (SARS-Cov)

Icosahedral symmetry

Solid with 20 faces, each an equilateraltriangle

How are larger virus particles built

By adding more subunits

Complex capsids with two icosahedral protein layer

Reoviruses

By who they infect

1. Animals

2. Plants

3. Bacteria

By how they infect

1. DNA viruses

2. RNA Viruses

3. Retrovirus

DNA Viruses

Joins with the hosts DNA a and then transcribes to RNA

RNA viruses

Translate directly to protein with the use of hosts ribosomes

Retrovirus

Use enzyme reverse transcriptase to convert viral RNA to DNA and new DNA can become part of host

Classification by symptomatology

1. Dromotropic

2. Pneumotropic

3. Neurotrophic- CNS

4. Viscerotropic - organs

Dromotropic

Warts

Pneumotropic

Flu, parainfluenza, pneumonia

Neurotropic - CNS

Encephalitis

Viscerotropic - organs

Hepatitis, mumps

According to routes of transmission

1. Respiratory transmission

2. Faecal -oral transmission

3. Blood-borne transmission

4. Sexual transmission

5. Animal or insect vectors

Respiratory transmission

Influenza A virus

Faecal-Oral transmission

Enterovirus (Hepatitis A virus)

Blood- borne transmission

Hepatitis B and C viruses

Sexual Transmission

HIV

Animal or insect vectors

Yellow fever

Subunit

Structural unit

1940 :Helmulth Ruska used an EM to take the first picture of virus particles

X-ray crystallography

Cryo-electron microscopy (cryoEM)

Complex shape

Icosahedral

Helical symmetry

Sendai virus (paramyxovirus) nucleocapsid

Animal viruses with helical symmetry

Pentamers

Hexamers

Poliovirus (Pincornaviridae) 30nm

SV40 (polyomavirus) 50nm