Viruses

Biology:

Kingdoms Research Project

Mr. Jones

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EM of Enveloped Virus with receptor molecules

Viruses: Overview & Concepts

Structure Adaptations Reproduction &

Development Diversity Phylogeny Importance

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TEM: Ebola Virus

Definition: Virus

Sub microscopic entity consisting of a single nucleic acid (N.A) surrounded by a protein coat, or capsid, capable of replication only within a living host.

Obligate Intracellular Parasite

Are Viruses Alive? Characteristics of life What Viruses Don’t...

– Respire (produce energy)– Metabolize (make biological molecules)– Grow & Develop

What Viruses Do...– Reproduce- obligate to a host– Adapt/Evolve- change through time

Viral Morphology (Structure)

Basic Structure: 2 principle structures– Protein Capsid

• Outer covering protecting core & giving shape

– Inner Nucleic Acid Core (RNA or DNA)• Used in viral classification

– *Envelope or Lipoprotein Coat • Enveloped viruses- similar to their eukaryotic

host cell’s plasma membrane• Not present in all viruses

Basic Enveloped Viral Structure

Nucleic Acid

Receptor Molecules

ProteinCapsid

Lipid Envelope

VirionAssociatedPolymerase(enzyme)

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Influenza Virus following lytic cycle

Structure: Capsid Protein Arrangement Protein Arrangement Determines:

– 1. Viral shape- images on next slide• Polyhedral (HPV, Polio, Adenovirus)• Helical (Tabacco Mosiac Virus)• Enveloped (Influenza, HIV)• Binal (baceriophages)

– 2. What host a virus can infect– 3. Method virus will infect host

Typical Viral Shapes

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Viral Size Extremely Small...

– most range 5nm-300nm– as small as ribosomes– between 4-900 genes

Yet Complex...– Icosahedrals like polio &

adenoviruses can have up to 20 symmetrical sides

Learn more in the Class ProjectDYK?- 1 drop of blood can contain as many as6 billion viruses!

Bacteriophages infect bacteria

Virus Classification

Classified by Shape & Nucleic Acid Class DNA viruses – stable, do not mutate as

rapidly– Single-stranded or double-stranded– Smallpox, Hepatitis B

RNA viruses – mutate rapidly, unstable– Single-stranded or double-stranded– HIV, Rhinovirus (common cold)

Viral Classification Chart

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Naming Viruses

Once named by...– Common names such as...

• Diseased they cause (rabies, poliovirus)• Organ or tissue they infect (adenovirus: from

adenoid tissue where common cold is found)

Universal System(s)– Families with Genus name + “virus” &

species name + “viridae” (herpesviridae)

Viral Adaptations

Viruses evolve quickly Have several methods of infecting a host Viruses can survive for long periods of time

both inside & outside of a host Recognition & Attachment

– Virus receptor molecule to cell receptor site– Viral species are specific to host

• Smallpox- affects only humans • Polio- affects only certain human nerve cells

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Bacteriophases invading E.coli

Viral Adaptations

Transmission– Air, fluids (blood, fecal, sexual), animal vectors

(insects, mice, monkeys) Methods of entering & leaving host

– Direct attachment to receptor molecule (Polyhedral viruses)

– Endocytosis with cell membrane (Enveloped viruses)

– Attachment by tail fibers (Binal viruses)

Viral Reproduction

Reproductive Overview: A Virus...– infects a host through a reception

mechanism– alters host ribosomes, DNA, or RNA– uses host metabolism to replicate N.A.

• Host cell makes & assemble new viruses• Host cell lyses (burst) leading to cell death

– All host cell’s eventually enter a Lytic Cycle

Viral Reproduction Diagram

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Viral Diversity

There are 22 classes of bacteria. 400 identified plant viruses causing up

to 1,000 identified diseases Eastimated:

– 1000’s of animal viruses causing as many as 10,000 diseases

– 1031 bacteriophages on Earth alone

Viral Diversity

Reproductive Cycles– Lytic & Lysogenic Cycles

Provirus– viral DNA integrated into host DNA– remains dormant or inactive

Retroviruses– contain enzyme reverse transcriptase

• RNA is transcribed into DNA (ex. HIV)

Lytic Cycle

Virus attaches to host cell’s membrane and injects its nucleic acid into the host cell.

The viral nucleic acid takes over protein synthesis, creating new viruses.

The host cell lyses releasing the newly formed viruses.

Cycle repeats exponentially

Cell lyses

Before attachment

Attachment/Reception

Penetration of N.A.

Replication of viral proteins

Viral assembly

Lysogenic Cycle

Viral N.A. are integrated into host’s DNA becoming a provirus

Provirus is replicated with host cell replication (mitosis)

Infect cell’s turn lytic, killing host cells Lysogenic Cycle Video

– http://www.youtube.com/watch?v=_J9-xKitsd0

Lysogenic Cycle

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Adaptations- Comparing Cycles

Lytic– Kills host cells– Mutates quickly &

ramdomly• Often without benefit

to the virus

– Rapid• Typical lytic cycle- 30

minutes to produce 200 viruses

Lysogenic, Provirus, & Retrovirus– Uses cell’s reproduction

& lysis– NA integration into cell

DNA– remain dormant for

periods of time – Slower

• High transmission rate• Slower but more

effective mutation

Viral Phylogeny

Viruses likely evolved from the host themselves

Some may have evolved in protists such as amoebas

HIV phylogeny & evolution– Emergent Viruses- host to human

HIV Phylogeny- SIV to HIV

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Importance

Viruses drive evolution– Adapted organisms survive– Controls populations– Mutate genetics of host

• Both for the good & bad

Gene Therapy & Viral Vectors– Using viruses to transmit healthy genes to people

with genetic disorders & diseases

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Importance No Cure- most drugs affect enzymes, cell

membranes, or reproductive cycle; virus have almost none of these traits

Vaccines & immunization – Cowpox & Smallpox (eradicated)

HIV can infect 10 billion cells per day, the body can only replace 1.8

AIDS Prevalence – http://www.avert.org/worldstats.htm– http://kwanzaakeepers.com/africa-aids-death-count/africa-a

ids-death-count.htm• One sub-Saharan African is infected with HIV every 9 seconds. One

person dies from AIDS every 13 seconds.