NTA Level 5 • Semester 1 • PST05103

Introduction Virus

Pharmaceutical Microbiology • Source Session/Topic 24

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Session 24: Introduction Virus

Total Session Time: 120 minutes

Pre-requisites

• Human anatomy and physiology

Students Learning Tasks

By the end of this session students are expected to be able to:

• Define common terms used in virology

• Describe general structure and properties of viruses

• Classify viruses according to their genetic and morphological

properties

• Describe viral-host-cell interaction and replication

• List various drug targets in virus

Resources Needed:

• Flip charts, marker pens, and masking tape

• Black/white board and chalk/whiteboard markers

SESSION OVERVIEW

SESSION CONTENTS

STEP 1: Presentation of Session Title and Learning Tasks (5 minutes)

READ or ASK students to read the learning tasks and clarify

ASK students if they have any questions before continuing

STEP 2: Common Terms Used in Virology (10 minutes)

• Virology

o Virology is the study of viruses. Their structures and activities

including infections caused by viruses.

• Virus

o This is a very small acellular particle that can only replicate

inside cells of another organism. A virus is capable of infecting

cells and potentially cause disease

• Virion

o Is complete virus particle.

• Capsid

o Is the protein shell or coat that encloses the nucleic acid of a

virus

• Capsomeres

o Morphological units seen in electron microscopes that make up the

capsid

• Envelope

o A lipid-containing membrane that surrounds some viruses. It is

acquired during maturation by a budding process through the cell

membrane.

• Peplomers

o Are virus-encoded glycoproteins that are exposed on the surface f

the envelope

• Nucleocapsid

o This is the protein-nucleic acid complex representing the packed

form of the viral genome.

STEP 3: Characteristics of Viruses (10 minutes)

|Activity: Buzzing (5 minutes) |

| |

|ASK students to pair up and buzz on the following question for 2 |

|minutes |

| |

|What are the characteristics of viruses? |

| |

|ALLOW few pairs to respond and let other pairs to add on points not |

|mentioned |

| |

|WRITE their response on the flip chart/board |

| |

|CLARIFY and SUMMARIZE by using the content below |

• Characteristics of viruses include;

o They infect prokaryotic and eukaryotic cells

o They are acellular

o They contain either DNA or RNA surrounded by a protein coat

o Viruses are obligate intracellular entities

o They cannot reproduce outside a host cell

▪ Viruses are cultivated in living cells e.g. bacteria

▪ Viruses that infect bacteria are called bacteriophage

▪ Some viruses multiply in cytoplasm of host cell while others

multiply in the nucleus of host cells

o They are very small (the largest virus is the size of a small

bacteria)

o Some viruses have envelope outside the capsid while other are

naked

STEP 4: Viral Structure (25 minutes)

• A complete viral particle (virion) consists of;

o Nucleic acid

▪ A virus contains only one type of nucleic acid (DNA or RNA)

that can be linear or circular, segmented or one molecule.

• The viral DNA

o single-stranded DNA (ssDNA)

o double-stranded DNA (dsDNA)

o single-stranded RNA (ssRNA)

▪ The RNA can be of positive sense (+) which is

the same as Mrna or a negative sense (-) which is

complementary to mRNA

• double-stranded RNA (dsRNA)

o Protein coat (capsid):

▪ The nucleic acid is surrounded by a protein coat (the capsid)

▪ The capsid is formed from protomers which collectively form

capsomeres. The capsomere is the basic unit of the capsid

▪ The capsid protects the virion in the external environment

▪ It also helps in transfer of nucleic acid between host cells

▪ The capsid and the nucleic acid are collectively called

Nucleocapsid

▪ Viral Symmetry

• Capsid is constructed in a highly symmetrical manner

o Icosahedral

▪ The capsid is composed of triangular faces made

from capsomeres (hexons)

▪ The virus appears spherical e.g. poliovirus

o Helical

▪ Virus is hollow, cylindrical in shape

▪ The virus can be rigid or flexible e.g. tobacco

mosaic virus (TMV)

o Complex

▪ Contains several types of symmetry in one virus

o Viral envelope

▪ Some viruses have a lipid layer (the envelope) surrounding

the nucleocapsid

▪ Viruses with the envelope are called enveloped viruses while

those not containing the envelope are called non enveloped (or

naked) virues

▪ Contains proteins projecting from the envelope. These

proteins help with attachment of the virus to the host cells

STEP 5: Classification of Viruses (30 minutes)

• Viruses are not included in the five kingdom classification

• Universal classification of viruses group viruses into orders,

families, sub families, genera and species.

• More than 24 families cause disease in human)

• The nomenclature and classification of viruses do not use the

conventional taxonomic groups

o Suffix endings

▪ Genus –ends in ‘virus’ e.g. coronavirus

▪ Sub-family –ends in ‘virinae’

▪ Family –ends in ‘viridae’

▪ Order –ends in ‘virales’

▪ Species –contains name of host, the word virus etc.

• Viruses are classified based on the following criteria;

o Nucleic acid type: DNA viruses and RNA viruses

o Nature of the nucleic acid: linear, circular, segmented, non-

segmented genome or polarity i.e. positive sense or negative sense

of the strands

o Enzyme production

▪ Reverse transcriptase

▪ Haemaglutinin enzyme (H) –required for adsorption and

penetration of orthomyxoviruses

▪ Neuraminidase (N) –required for invation and release of

influenza viruses

▪ Polymerase enzymes, examples

• RNA-dependent RNA polymerase in RNA viruses

• DNA polymerase

• RNA-dependent DNA polymerase (reverse transcriptase)

o Morphology

▪ Helical viruses

▪ Icosahendral viruses

▪ Complex viruses

▪ Presence or absence of envelope

• Enveloped viruses

• Naked viruses

Table 28.1: Major families of human viruses

| |

|DNA Viruses |

| | |ral | |chickenpox, shingles, |

| | |ral | |meningitis |

| | |ra | |cervical cancer |

|e | |ral | | |

| | |ral | | |

| |

|RNA Viruses |

| | |ral | | |

|e |(+) |ral | | |

| |(+) |ral | | |

| |(+) |ral | | |

| |(+) |ral | |hepatitis C, West Nile |

| |(+) | | | |

| |(-) | | |fevers |

| |(-) | | |syndrome |

|dae |(-) | | | |

|ae |(-) |ral | | |

| |(-) | | | |

| |(+) | | | |

| |(+) |ral | |leukemia |

STEP 6: Viral-host cell Interaction and Viral Replication (15 minutes)

• Viruses replicate in living cells. They depend on host cell machinery

for replication.

• Generally, viral replication involves the following steps;

o Attachment

▪ First step where the virus uses surface proteins to interact

with specific receptors on the target cell surface.

▪ The viral-specific receptors are necessary for virus to

infect the cell

o Penetration

▪ Enveloped viruses e.g. HIV, penetrate cells through fusion of

the viral envelope with the host cell membrane

▪ Non-enveloped viruses penetrate cells by endocytosis

o Uncoating

▪ This is also known as disassembly step)

▪ This step is different in different taxonomic class of the

virus involved

▪ It involves removal of the protein coat (capsid) and expose

the DNA or RNA to allow multiplication of the viral genome

o Transcription and Translation

▪ The virus uses host cell machinery to replicate and make

functional and structural proteins

o Assembly and release

▪ Involves assembly of newly formed viral nucleic acid and

structural proteins to form the nucleocapsid of the virus

▪ Non-enveloped viruses mature in the cytoplasm e.g.

picornavirus or the nucleus e.g. adenoviruses with

disintegration of the cell and release of virions

▪ Enveloped viruses complete their cycle (maturation) as the

virion exists the cell, and the host cell undergoes lysis

STEP 7: Drug Targets in Viruses (15 minutes)

• Drugs that are used in treatment of viral infections are generally

called antiviral drugs or antivirals

• Antiviral drugs inhibit active replication of viruses

o Specific antivirals have specific names e.g. antiretrovirals are

used for infections caused by retroviruses

• Targets for antiviral drugs include;

o Fusion and penetration of virus to specific host cell

▪ Interferes with the ability of viruses to bind to the cells

▪ The inhibit binding of virus to the viral-specific receptors

on host cells e.g. Enfuvirtide

o Uncoating

▪ Interferes with release of viral genome into the host cell

e.g. amantadine (for influenza A)

o Integration of viral genome into host genome

▪ Integrase inhibitors e.g. Dolutegravir

o Inhibition of replication of viral (DNA or RNA) genome

▪ Polymerase inhibitors; inhibit synthesis of viral DNA or RNA

e.g. reverse transcriptase inhibitors such as Lamivudine and

Tenofovir used in AIDS, Acyclovir used for herpes infections

o Inhibition of assembly of new virions

▪ Protease inhibitors; inhibit post-translational modification

of proteins. These drugs inhibit modification and assembly of

newly formed proteins with nucleic acid to form new virions

e.g. Nevirapine used for treatment of AIDS

o Inhibition of packaging and release of new virions

▪ E.g. neuraminidase inhibitors

STEP 6: Key Points (5 minutes)

• Viruses are particles that are capable of replicating while only in

living cells

• Viruses differ from bacteria in that viruses are acellular i.e. they

do not have cellular structures

• Viruses are classified by several criteria including morphology and

nature of genome

• Viral replication takes place inside living host cells and involves

attachment, penetration, uncoating, synthesis of macromolecules,

assembly and release of new viral particles

• Antiviral drugs target various steps of viral replication

STEP 7: Evaluation (5 minutes)

• What are the characteristics of viruses?

• What are drug targets in viruses?

• How do viruses replicate?

References

Hugo and Russell (2011), Pharmaceutical Microbiology 8th Edition, Willey-

Blackwel publications

Karen C. Carroll et al (2013); Jawetz, Melnick and Adelberg’s Medical

Microbiology 26th Ed. McGraw Hill Co. Inc.

Greenwood et al (2012); Medical Microbiology, 18th edition Churchill

Livingstone

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