Viruses Structure and Classification Medicine Year 1 2024 PDF

College of Medicine Viruses Structure and Classification Balsam Qubais Saeed, PhD October 29th , 2024 Objectives ✓ Describe the basic properties of viruses ✓ know the structure and classification of viruses ✓ Able to differentiate between viruses, Viroids, and Prions ✓ Abl...

College of Medicine Viruses Structure and Classification Balsam Qubais Saeed, PhD October 29th , 2024 Objectives ✓ Describe the basic properties of viruses ✓ know the structure and classification of viruses ✓ Able to differentiate between viruses, Viroids, and Prions ✓ Able to differentiate between Enveloped viruses and non - Enveloped viruses Describe basic properties of viruses Structure and classification of viruses What is a virus? Genetic material Protein coat A virus is a small infectious agent that replicate only in the living cell of other organisms. Can infect human , animal , plant , and microorganisms include Bacteria A virus is an obligate intracellular parasite containing genetic material RNA or DNA surrounded by protein coat known as an envelope Virus particles can only be observed by an electron microscope The viral particles, also known as virions.  Properties of the virus ✓smallest infections agents- A virus is a very small ✓most viruses are between 20 –200 nm in size ✓Virus particles are not living ✓Do not move ✓viruses are not cellular ✓ acellular infectious organisms( Obligate intracellular parasites, which replicate inside host cells using host metabolic machinery and ribosomes to form components which assemble into particles called virions) ✓obligate intracellular parasites-can reproduce only in host cells. ✓can infect animals, plants, fungi, protists, bacteria ✓consist only of a nucleic acid either DNA or RNA, surrounded by a protein coat. (nucleocapsid) ✓Some viruses consist only nucleocapsid others have additional components ✓can not carry-on independent metabolism, Viruses cannot make energy or proteins independently  Virus A virus is a very small infectious agent that doesn't have independent metabolic function and is unable to replicate outside of its host. Rhinovirus, the most common cause of the common cold Influenza virus, the one that causes the flu HIV (human immunodeficiency virus), the one that leads to AIDS if left untreated Infectious particles of virus is known as Virion Virion complete infectious virus particle, includes a genome comprising one or a few molecules of either DNA or RNA surrounded by protein coat ✓Virions are complete virus infectious particles ✓Infectious particles of virus is known as virion ✓Assembled inside host cells, from virus-specified components ✓Virions do not grow ✓Transmit viral genomes from cell to cell and protecting the genome in harsh environments in which virus cannot replicate  Viroids & Prions Viroids are much simpler than viruses. Structurally, viroids are smaller in size ✓Viroids the smallest pathogens composed of single-stranded circular RNA (ssRNA genome) without protein covering mainly affect plants ✓Prions Infectious agents that are entirely protein composed of sialoglycoprotein – a combination of sialic acid and glycoprotein No nucleic acid Highly heat resistant “prions” refers to abnormal, pathogenic Affects nervous tissue and results in brien agents that are transmissible and are able to induce abnormal folding of specific normal cellular proteins called -kuru &Creutzfeldt-Jakob Disease(CJD)in humans prion proteins that are found most abundantly in the brain. The functions of these normal prion proteins are still not completely understood Virus facts generally, more resistant to some disinfectants than most bacteria. not affected by antibiotics Structure of viruses ✓Each one is made up of Genetic material–either DNA or RNA CAPSID–protective protein coat ✓Capsid + nucleic acid=Nucleocapsid ✓Some viruses have an additional coat–Envelope ✓envelope –derived from host cell membrane [lipids + proteins and glycoproteins]  Basic virus structure Naked DNA capsid virus Capsid Or Non- or + Nucleocapsid = protein Enveloped RNA virus Lipid membrane, Nucleocapsid + Enveloped virus glycoproteins  Capsid (coat) functions of the viral capsid: ✓Protect nucleic acid ✓Allow for release the nucleic acid DNA or RNA into a suitable host cell ✓Stimulate the immune system to produce antibodies that can protect the host cell against future infection. ✓Virus May be composed of a single protein that assembles itself into a symmetrical shape ✓Termed the capsid ✓Individual proteins within the capsid are termed capsomere Virus –envelope ✓Allows the virus to fuse with the appropriate membrane system in a new cell ✓Envelope may have virally specific protein projections  Enveloped viruses ✓The outer layer of the virion is envelope ✓Components Nucleocapsid + lipids + glycoproteins Properties of enveloped viruses ✓Envelope is sensitive to Drying Heat Detergents Acid ✓Consequences Must stay wet during transmission Transmission in large droplets and secretions Cannot survive in the gastrointestinal tract Do not need to kill cells in order to spread May require both a humoral and a cellular immune response ✓HIV, SASRS-CoV etc.  naked capsid viruses Non enveloped viruses ✓ The outer layer of the virion is the capsid ✓ component-DNA or RNA + Protein ✓ properties-Is environmentally stable Properties of naked capsid viruses ✓Capsid is resistant to Drying Heat Detergents Acids Proteases ✓ Consequences spread easily by dust, water, feco-oral routes Can survive in the gastrointestinal tract Retain infectivity on drying Survive well on environmental surfaces Humoral antibody response may be sufficient to neutralize infection ✓ Polio, Noro, Adeno etc. Classification of viruses classified as DNA viruses and RNA viruses. The nucleic acid may be single or double stranded, circular or linear, segmented or unsegmented. DNA viruses As their name implies, DNA viruses use DNA as their genetic material. common example of DNA viruses is herpesvirus. DNA viruses can affect both humans and animals and can range from causing benign symptoms to posing very serious health. RNA viruses The virus that possesses RNA as genetic material are called RNA viruses. hepatitis example of RNA virus. Classification of viruses On the basis of the presence of a number of strands Double-stranded DNA (dsDNA) Single-stranded DNA (ssDNA) Double-stranded RNA (dsRNA) Single-stranded RNA (ssRNA) Viral Structure Varies in size, shape 4 types: Virus symmetry Two major types of symmetry seen in virus particles ✓Helical, e.g., Rabies virus ( which the protein subunits and the nucleic acid are arranged in a helix) ✓Icosahedral, e.g., HIV (which the protein subunits assemble into a symmetric shell that covers the nucleic acid-containing core.) Few viruses have components of both icosahedral & helical Capsid symmetry Icosahedral Helical Naked capsid Enveloped Matrix Lipid Glycoprotein Classification of viruses On the Basis of Presence of Envelope The envelope is a lipid-containing membrane that surrounds some virus particles. It is acquired during viral maturation by a budding process through a cellular membrane Virus encoded glycoproteins are exposed on the surface of the envelope. These projections are called peplomers. Enveloped Virus Non-Enveloped Virus Classification of viruses Virus Classification by Capsid Structure Naked icosahedral: Enveloped icosahedral: Enveloped helical: Naked helical: Complex with many proteins: some have combinations of icosahedral and helical capsid structures. Classification of viruses On the Basis of Shapes of the Viruses Most of the animal viruses are roughly spherical with some exceptions. Rabies virus: Bullet shaped Ebola virus: Filamentous shaped Poxvirus: Brick shaped Adenovirus: Space vehicle shaped Brick shape On the Basis of the Type of Host The virus can be classified on the basis of the type of host. They are: Animal viruses Plant viruses Bacteriophage: Viruses which infect bacterial cells are known as bacteriophage or bacteria eaters. They contain DNA as genetic material. There are many varieties of bacteriophages. Usually, each kind of bacteriophage will attack only one species or only one strain of bacteria. Questions