Structure of Viruses

All viruses are composed of:– Nucleic acids:

Virus’s genetic material DNA or RNA Code solely for reproduction

– Capsid: Protein coat 1 or 2 layers thick Determines shape Determines infection process Houses nucleic acids

Structure of Viruses

Some viruses:– Viral envelope:

Outside of protein coats Made of phospholipids (like a membrane) Flu and HIV

Viral Shapes

Determined by capsid (protein coat)

Viral Shapes

Polyhedral: polio

Viral Shapes

Cylindrical: tobacco mosaic

Viral Shapes

Studded capsid: HIV, influenza

Viral Shapes

Polyhedral head with protein tails: bacteriophage

Viral Attachment

A cell’s membrane won’t let just any ol’ particle in

Viral Attachment

Amazing forms viruses take mimic molecules a host cell wants and needs to take in

Viruses “trick” the host cell into letting the virus in

Viral Attachment

Specificity of membrane receptors forces viruses to be specific shapes

Usually a virus can infect only a single species– Sometimes single cell type within single species

Viral Reproduction

After attaching virus incorporates its nucleic acid into the host cell (method depends on viral shape)– Inject just the nucleic acid– Whole virus barges in inside a vacuole which then

bursts

Viral Reproduction

Lytic cycle Lysogenic cycle Retroviruses

Viral Reproduction

Lytic cycle Lysogenic cycle Retroviruses

Step 1: virus attaches

Step 2: virus inserts nucleic acid

Step 4: virus takes over cell’s metabolism and viral parts are constructed

Step 5: viral partsare assembled intonew viruses

Step 6: host cell bursts, releasingnewly-constructed viruses intothe environment toinfect other hostcells

Step 3: host cell’s DNAis destroyed

Viral Reproduction

Lytic cycle Lysogenic cycle Retroviruses

Viral Reproduction

Lytic cycle Lysogenic cycle Retroviruses

Step 1: virus attaches

Step 2: virus insertsnucleic acid

Step 3: viral DNA is incorporated into host cell’s forming provirus

Step 4: Provirus isreplicated when hostcell replicates

Step 5: lysogenic phase continues

Step 1: virus attaches

Step 2: virus insertsnucleic acid

Step 3: viral DNA is incorporated into host cell’s forming provirus

Step 4: Provirus isreplicated when hostcell replicates

Step 5: lysogenic phase continues

Step 6: provirus pops out of DNA and enters lytic cycle

Step 1: virus attaches

Step 2: virus insertsnucleic acid

Step 3: viral DNA is incorporated into host cell’s forming provirus

Step 4: Provirus isreplicated when hostcell replicates

Step 5: lysogenic phase continues

LYTIC CYCLE

Step 6: provirus pops out of DNA and enters lytic cycle

Lysogenic Cycle

Viruses that do not burst host cell are referred to as temperate viruses

Lysogenic Cycle

Classic lysogenic virus: Herpes Simplex I– Causes cold sores– Virus is always present in cells– Symptom-free during lysogenic cycle– Cold sores appear during lytic cycle

Lysogenic Cycle

Viruses that bud: influenza– Release from host cell without bursting– Form of exocytosis

Viral Reproduction

Lytic cycle Lysogenic cycle Retroviruses

Viral Reproduction

Lytic cycle Lysogenic cycle Retroviruses

Retroviruses

Only contain RNA (not DNA) RNA is made from DNA but not the other

way around How can a virus that contains RNA take over

the host cell’s DNA?

Retroviruses

Retroviruses also inject reverse transcriptase– Enzyme– Makes double-stranded DNA copy of viral RNA to

incorporate into host cell’s DNA

Lysogenic cycle then proceeds

normally

Retroviruses

Viruses that are quietly active: HIV– HIV is a retrovirus– HIV enters then produces only small numbers of

viruses within host cell (lysogenic-type of cycle)– Viruses are released by budding (without bursting the

cell)– Eventually it will enter a lytic cycle

causing the disease: AIDS

Viral Diseases

Viruses cause illness by attacking and destroying cells

Illnesses caused by viruses include:– Cold, flu, smallpox, AIDS, chickenpox, measles,

Hepatitis, West Nile

Control of viruses

Vaccination: individuals are infected with active, but relatively harmless strain of virus– Makes some people sick– Smallpox virus, flu

Antibiotics do nothingnothing for a person infected with a virus!

Structure of Viruses

All viruses are composed of:– Nucleic acids:

Virus’s genetic material DNA or RNA Code solely for reproduction

– Capsid: Protein coat 1 or 2 layers thick Determines shape Determines infection process Houses nucleic acids

Structure of Viruses

Some viruses:– Viral envelope:

Outside of protein coats Made of phospholipids (like a membrane) Flu and HIV

Viral Shapes

Determined by capsid (protein coat)

Viral Shapes

Polyhedral: polio

Viral Shapes

Cylindrical: tobacco mosaic

Viral Shapes

Studded capsid: HIV, influenza

Viral Shapes

Polyhedral head with protein tails: bacteriophage

Viral Attachment

A cell’s membrane won’t let just any ol’ particle in

Viral Attachment

Amazing forms viruses take mimic molecules a host cell wants and needs to take in

Viruses “trick” the host cell into letting the virus in

Viral Attachment

Specificity of membrane receptors forces viruses to be specific shapes

Usually a virus can infect only a single species– Sometimes single cell type within single species

Viral Reproduction

After attaching virus incorporates its nucleic acid into the host cell (method depends on viral shape)– Inject just the nucleic acid– Whole virus barges in inside a vacuole which then

bursts

Viral Reproduction

Lytic cycle Lysogenic cycle Retroviruses

Viral Reproduction

Lytic cycle Lysogenic cycle Retroviruses

Step 1: virus attaches

Step 2: virus inserts nucleic acid

Step 4: virus takes over cell’s metabolism and viral parts are constructed

Step 5: viral partsare assembled intonew viruses

Step 6: host cell bursts, releasingnewly-constructed viruses intothe environment toinfect other hostcells

Step 3: host cell’s DNAis destroyed

Viral Reproduction

Lytic cycle Lysogenic cycle Retroviruses

Viral Reproduction

Lytic cycle Lysogenic cycle Retroviruses

Step 1: virus attaches

Step 2: virus insertsnucleic acid

Step 3: viral DNA is incorporated into host cell’s forming provirus

Step 4: Provirus isreplicated when hostcell replicates

Step 5: lysogenic phase continues

Step 1: virus attaches

Step 2: virus insertsnucleic acid

Step 3: viral DNA is incorporated into host cell’s forming provirus

Step 4: Provirus isreplicated when hostcell replicates

Step 5: lysogenic phase continues

Step 6: provirus pops out of DNA and enters lytic cycle

Step 1: virus attaches

Step 2: virus insertsnucleic acid

Step 3: viral DNA is incorporated into host cell’s forming provirus

Step 4: Provirus isreplicated when hostcell replicates

Step 5: lysogenic phase continues

LYTIC CYCLE

Step 6: provirus pops out of DNA and enters lytic cycle

Lysogenic Cycle

Viruses that do not burst host cell are referred to as temperate viruses

Lysogenic Cycle

Classic lysogenic virus: Herpes Simplex I– Causes cold sores– Virus is always present in cells– Symptom-free during lysogenic cycle– Cold sores appear during lytic cycle

Lysogenic Cycle

Viruses that bud: influenza– Release from host cell without bursting– Form of exocytosis

Viral Reproduction

Lytic cycle Lysogenic cycle Retroviruses

Viral Reproduction

Lytic cycle Lysogenic cycle Retroviruses

Retroviruses

Only contain RNA (not DNA)

Retroviruses

RNA is made from DNA but not the other way around

How can a virus that contains RNA take over the host cell’s DNA?

Retroviruses

Retroviruses also inject reverse transcriptase– Enzyme– Makes double-stranded DNA copy of viral RNA to

incorporate into host cell’s DNA

Lysogenic cycle then proceeds

normally

Retroviruses

Viruses that are quietly active: HIV– HIV is a retrovirus– HIV enters then produces only small numbers of

viruses within host cell (lysogenic-type of cycle)– Viruses are released by budding (without bursting

the cell)– Eventually it will enter a lytic cycle

causing the disease: AIDS

Viral Diseases

Viruses cause illness by attacking and destroying cells

Illnesses caused by viruses include:– Cold, flu, smallpox, AIDS, chickenpox, measles,

Hepatitis, West Nile

Control of viruses

Vaccination: individuals are infected with active, but relatively harmless strain of virus– Makes some people sick– Smallpox virus, flu

Antibiotics do nothingnothing for a person infected with a virus!