Chapter 19: Viruses Yes we are finally going to talk about Ebola… A Slideshow by Miss Valerie

Virus “A borrowed life” Particle consisting of genes in a protein coat Non-living because they need to infect others to replicate DNA

Discovery Found by looking at Tobacco Mosaic disease Adolf Mayer rubbed infected sap on healthy leaves and spread disease Speculated that it was a tiny bacteria 1893 – Dimitri Ivanowsky passed infected leaves through bacteria removing filter and sap still produced disease Martinus Beijerinck – ruled out bacteria and saw agent could replicate 1935 – American scientist (go America!) Wendell Stanley found virus with electron microscope

Structure Genomes may consist of double-stranded DNA, single- stranded DNA, double-stranded RNA, or single-stranded RNA Grouped by type of nucleic acid May have anywhere from 4 to a thousand genes (still little compared to bacteria) The protein shell is called a capsid Made up of different protein subunits called capsomeres Viruses have different shapes depending on protein arrangement Rod-shaped (helical), Polyhedral (icosadral), other

Accessory Structures Help viruses infect hosts Viral Envelope – derived from membranes of the host cell (contain host cell phospholipids and membrane proteins) How might this help a virus like the influenza virus? Bacteriophages/phages: infect bacteria, complex capsids Elongated icosahedral heads with a protein tail and anchoring fibers

Viruses NEED a Host Viruses are obligate intracellular parasites -like something out of a sci-fi movie, viruses can only “survive” and replicate by taking over host cells Without a host cell they are just packaged genes Can only infect certain host species ( host range ) For example you can’t give your dog a cold or measles Some diseases are cross-species – H1N1, Bird flu, etc.

General Viral Replication Virus binds to host cell Virus enters cell and is uncoated, releasing its genetic material and capsid proteins Host enzymes transcribe viral genome into viral mRNA, which in turn make more capsid proteins Viral genomes and capsid proteins self-assemble into new virus particle, which then exit the cell to infect more cells

Replicative Cycle of Phages Phages are best understood virus 2 methods that double-stranded DNA viruses can replicate Lytic Cycle – host cell dies Lysogenic Cycle – host lives Some can use both methods – temperate phages

The Lytic Cycle Virulent phage – replicates only using lytic cycle A few cycles can destroy an entire bacteria population Steps: Attachment – tail fibers bind to receptor sites Entry of phage DNA and degradation of host DNA – phage injects host cell with DNA Synthesis of viral genomes and proteins Assembly – parts for new phages get put together Release – enzyme damages cell wall, cell bursts and phages explode out

Why hasn’t every bacteria been destroyed in a phage explosion? Natural selection favors bacterial mutants with phage resistant receptors Sometimes the cell realizes the DNA is foreign before it can start replicating Restriction enzymes cut up the foreign DNA There is another way phages reproduce…..

The Lysogenic Cycle Turns host cell into a phage DNA carrying and reproducing monster Steps: Phage attaches to host cell and injects DNA Phage decided to enter lysogenic cycle Phage DNA integrates into bacterial chromosome, becoming a prophage Bacteria reproduces normally creating daughter cells with prophage When it gets a certain environmental signal the phage switches over to lytic cycle

Animal Viruses Lots of variation among viruses DNA vs. RNA RNA has 3 classes based on their functions in a host cell. Animal viruses can be more complex than bacterial phages Can have both RNA and envelopes

How Viral Envelopes Affect Replication Envelopes used to enter host cell Viral glycoproteins bind to receptor sites on surface of host cell STEPS: 1.Binding of glycoproteins to receptors 2.Capsid and viral genome enter the cell; Capsid is digested by enzymes releasing genome 3.Viral genome acts as template for synthesis of complentary strand 4.Viral Genome is copied 5.RNA also works like mRNA making capsid proteins and glycoproteins 6.Vesicles transport glycoproteins to cell membrane 7.Capsid assembles around viral genome 8.New virus buds from host cell

RNA as the Genetic Material Broadest variety of RNA genomes are animal viruses Each class has different function in host cell… use enzymes to accomplish tasks: Viral class IV can directly serve as mRNA – translated into viral protein right after infection Class V – RNA acts as template for mRNA synthesis Class VI - most complicated replication… retroviruses

Retroviruses Work backwards -Use their RNA as a template for DNA using Reverse Transcriptase Enveloped viruses with two identical molecules of single- stranded RNA and two molecules of reverse transcriptase Replication: 1. Virus enters host cell 2.Reverse transcriptase makes DNA strand complementary to viral RNA 3.Reverse transcriptase makes second strand of DNA complementary to first 4.New Double stranded DNA incorporated into cells DNA (provirus) 5.Cell starts making viral proteins and enzymes 6.Get packaged in vesicles and put themselves together then bud off from host

HIV More than 35 million people are now living with HIV of those 3.2 million are under the age of 15 Since the beginning of the pandemic, nearly 78 million people have contracted HIV and close to 39 million have died of AIDS-related causes Human immunodeficiency virus – attacks cells in the immune system (specifically T4 cells) Destroys these cells when switches to lytic cycle Remains in lysogenic cycle for many years building up virus in body In advanced stages of infection AIDS can be diagnosed… AIDS is a syndrome caused by the HIV retrovirus (when you have less than 200 T4 cells per microliter of blood) Medications that treat virus work at different parts of the replication process – “cocktails”

HIV Treatments

Virus Evolution Developed after cells – because they infect every type of cell Mobile pieces of genetic material like plasmids and transposons Viruses change in response to host cell Viruses often become drug resistant and can turn into “super viruses” (HIV)

Viruses in Animals Viral infections cause symptoms through a variety of ways May destroy cells May produce toxins that lead to disease symptoms Some of the molecular components are toxic How much damage happens depends on the infected cells ability to regenerate itself Ex: cold doesn’t permanently damage cells because lung cells can be repaired, but polio kills nerve cells which do not regenerate easily if at all Other symptoms are a result of bodies defenses like fever, and body aches (immune response)

Vaccinations Major medical technology for preventing viral infections Vaccine – harmless derivative of pathogen that builds immune systems defenses against specific virus Successful inoculation can eradicate a virus completely (ex: smallpox, polio, etc.) The anti-vaccination movement has brought back many viruses once thought under control (whooping cough) Viruses are nearly impossible to treat once infection has occurred – antibiotics do not work on viruses!!!

Emerging Viruses Viruses that suddenly become apparent (come out of nowhere) HIV, West Nile Virus, H1N1, Bird flu, etc. EBOLA – first recognized in Central Africa in 1976 One of several emerging viruses causing hemorrhagic fever 2014 epidemic (general outbreak) is now largest in history Pandemic is a global epidemic or outbreak Ex: H1N1 – by November of 09, 4 months after initial outbreak, 207 countries had cases of the disease

How Viruses Emerge 1.Mutation of existing viruses 2.Dissemination of viral disease from a small, isolated human population 3.Spread of existing viruses from other animals 1.About ¾ of new diseases start this way Example influenza

Plant Viruses More than 2,000 types of plant viruses known Kill tons of crops = loss of money Part of what genetic engineering of plants aims to create is virus resistant plants 2 ways viruses spread in plants Horizontal transmission – from outside source Vertical transmission – inherits viral infection from parents

Viroids and Prions Viroids – circular RNA molecules, only a few hundred nucleotides long, that infect plants Do not encode proteins A single molecule can be an infectious agent that spreads a disease Prions – infectious proteins that cause degenerative brain diseases Mad cow disease/Creutzfeldt-Jakob disease Act slowly (incubate for at least 10 years) Virtually indestructible Misfolded form of protein normally found in brain cells

Summary!! Aka THE END A virus consists of a nucleic acid surrounded by a protein coat… generally considered non-living Viruses replicate only in host cells – need to infect to spread Viruses, viroids, and prions are formidable pathogens in animals and plants, as well as humans GET VACCINATED!!!!