Micro test III, Ch. 13

#8 Entance through Endocytosis (Naked Viruses or Enveloped viruses)

1.) Attachment to receptors trigger endocytosis 2.) Endocytosis: Cytoplasmic membrane surrounds the virion, forming an endocytic vesicle. 3.) Release from vesicle. If enveloped, the viral envelope fuses with the endosomal membrane. 4.) Uncoating: Nucleic acid separates from capsid.

#8 Entrance through Membrane Fusion (Enveloped Viruses only)

1.) Attachment: spikes of virion attach to specific host cell receptors. 2.) Membrane fusion: Envelope of virion fuses with cytoplasmic membrane. 3.) Nucleocapsid released into cytoplasm (viral envelope remains part of cytoplasmic membrane). 4.) Uncoating: Nucleic acid separates from capsid.

#10 Viral assembly

Involves bringing together newly formed viral nucleic acid with capsid proteins and packaging them to for the nucleocpasid. Non-envleoped viruses mature fully in the host cell cytoplasm. *Capside proteins - arranged around copies of the viral genome*

#1 Complex

More complex, usually present in phages. Many have icosahedral nucleocapsid, referred to as the head, with a long helical protein component, the tail.

#9 Which virus (DNA or RNA) replicates its genome and transcribes its genes in the host cell's *nucleus*?

Most DNA viruses replicate in the nucleus of the host cell and use the host cell machine for DNA synthesis as well as gene expression. They often encode their own DNA polymerase; allowing them to replicate even if the host cell is not actively replicating its own chromosome.

#21 Explain how antigenic drift (e.g. HIV) can make vaccine ineffective in preventing infection.

1) The virus that causes Acquired Immune Deficiency Syndrome (AIDS) is a highly genetically variable virus, for several reasons. First, it reproduces much more rapidly than most other entities. It can produce billions of copies of itself each day. As it makes rapid-fire copies of itself, it commonly makes errors, which translate into mutations in its genetic code. The more beneficial the mutations are to the virus's survival, the more likely that mutated virus will be to reproduce itself. 2) Another cause of the variability in HIV results from the virus's ability to recombine and form new variants within an individual. This happens when a host cell is infected with two different variations of HIV. Elements of the two viruses may combine to result in a new virus that is a unique combination of the two parents. 3) The rapid rate of HIV evolution has important consequences. HIV can quickly develop resistance to anti-HIV drugs. Additionally, targeting a vaccine to a rapidly changing virus is challenging. To date, researchers have developed several candidate HIV vaccines, but none has performed well enough in clinical trials to warrant licensure.

#12 Explain what it means to say that the outcome of exposure to a virus depends in large part on host defenses

A host's immune system (innate & adaptive) can prevent viruses from infecting cells & destroy infected cells. When exposed to a virus, the host's immune system recognizes the viral surface components (eg, antigens). If the immune response is fast enough (eg, adaptive immunity, conferred by previous exposure), viral multiplication & transmission can be prevented, & the host will be protected from infection & disease. When an individual who lacks immunity is exposed to a virus, it can infect cells, multiply & transmit to others. In an infected host, the virus may cause no symptoms, mild disease, or severe even life-threatening disease.

#9 Which virus (DNA or RNA) replicates its genome and transcribes its genes in the host cell's *cytoplasm*?

A majority of RNA viruses are single-stranded and replicate in the cytoplasm.

#1 Helical Virus

Appears cylindrical, capsomeres are arranged in a helix, somewhat similar to a spiral staircase. Some are short and rigid, whereas others are long and filamentous.

#1 Icosahedral Virus

Appears spherical, but it actually 20 flat triangles arranged in a manner similar to a soccer ball.

#7 HIV

Binds to CD4 receptor found on helper T cells and macrophages. Coreceptors CCR5 and CXCR4 interact with HIV-CD4 complex to allow penetration of cell membrane.

#14 Persistant viral infection

Can continue with or without symptoms for years or even a lifetime.

#14 Acute viral infection

Characterized by the sudden onset of symptoms of a relatively short duration. -Usually eliminated after a short time (days or weeks) -Eliminated by the immune system usually -Or host cell dies -Host symptoms go away once virus is eliminated

#5 Enteric Transmittance

Fecal-oral route, ingested into GI tract

#4 Retroviruses

Groups of viruses that have a single-stranded genome; their enzyme reverse transcriptase synthesizes a single-stranded DNA copy from an RNA template that is then integrated into the host cell chromosome (eg. HIV)

#3 Enveloped Viruses (structures)

Have a lipid bilayer outside of the capsid, obtained from the host cell. Sandwiched between the nucleocapsid and envelope is the matrix protein, which is unique to enveloped viruses. In general enveloped viruses are more susceptible to disinfectants because these chemicals damage the envelope, making the virus non-infectious, because its spikes are removed, therefore it cannot attach to the host cell.

#2 Capsomers

Identical protein subunits, arranged in a precise manner to form the capsid

#22 Explain the process of genetic reassortment in influenza viruses (type A), which have segmented RNA genomes. Explain how genetic reassortment can result in antigenic shift, and what this means. Explain how antigenic shift can enable flu viruses to infect new hosts. Explain why antigenic shift can lead to epidemic flu.

In very serious cases of viral mutations, something known as antigenic shift occurs. This is a sudden and major change in the surface antigens of a virus. Antigenic shift occurs when two different strains of influenza virus simultaneously infect the same cell in your body and undergo a process called genetic reassortment. This is a process whereby two viruses mix and match parts of their genome. Since the genome is radically changed, so are the HA and NA proteins on the surface of the viruses that are coded for by this new genome.The change is so drastic that it can cause pandemics, or outbreaks of a disease, over large areas of land, such as multiple continents or even all over the world. That's because the virus is so new, barely anyone has any prior immunity to it, and a lot of people, therefore, end up getting sick.

#21 Explain how viral mutations can result in antigenic drift, and what this means. Explain how antigenic drift can enable a virus to cause more infections in a population.

Influenza viruses can evolve in a gradual way through mutations in the genes that relate to the viral surface proteins hemagglutinin and neuraminidase (HA and NA in shorthand). These mutations may cause the virus's outer surface to appear different to a host previously infected with the ancestor strain of the virus. In such a case, antibodies produced by previous infection with the ancestor strain cannot effectively fight the mutated virus, and disease results. (Hemagglutinin and neuraminidase lend their first initials to flu subtypes. For example, the 2009 influenza pandemic was caused by an influenza A H1N1 virus.) As mutations accumulate in future generations of the virus, the virus "drifts" away from its ancestor strain. Antigenic drift is one reason that new flu vaccines often need to be created for each flu season. Scientists try to predict which changes are likely to occur to currently circulating flu viruses. They create a vaccine designed to fight the predicted virus. Sometimes the prediction is accurate, and the flu vaccine is effective. Other times the prediction misses the mark, and the vaccine won't prevent disease.

#5 Zoonotic Transmittance

Multiply in animals/insects and transmitted to humans through direct contact, insect bites, ingestion, inhaled droppings...

#19 Explain how viruses undergo mutations. Which types of viruses mutate more frequently, DNA or RNA viruses?

Mutation is referred to as certain heritable changes in the genetic material (genome). Viruses infect the host cell. However, after some time the host cell gains resistance towards the viruses. Hence, in order to overcome this and to multiply viruses mutate. Mutation is caused as a result of faulty coding of genes. Example: In RNA viruses such as Influenza, the virus keeps on mutating. Hence, individuals are often prone to 'Common Cold' caused by the Influenza virus. Since, the virus mutates and changes its form every time there is no definite vaccine or drug available against common cold. Same is the case with the HIV virus. In general, viral RNA genomes are much more mutation-prone than those based on DNA. This distinction is important because RNA-based viruses have repeatedly evolved resistance to drugs.

#11 How naked viruses are released from host cells and their effects

Non-envleoped (naked) viruses are released when the host cell dies. *cell lysis releases the viruses*

#23 Define prions, give an example of a prion disease, and describe general symptoms.

Prions are composed solely of protein, and cause a number of transmissible spongiform encephalopathies. These agents are linked to a number of slow, always fatal, human diseases including Creutzfeldt-Jakob disease and Kuru as well as mad cow disease. The symptoms In all these diseases, is a chronic wasting. They attack the brain.

#2 Capsid

Protein coat which protects the nucleic acid from enzymes and toxic chemicals in the environment

#2 Spikes (2 examples)

Protein structures that stick out from either the lipid bilayer of enveloped viruses or the capsid of non-enveloped viruses, allowing viron to attach to specific receptor sites on host cells. Approximately 80% of the spikes are haemagglutinin (HA) and the remaining 20% are neuraminidase (NA). The HA and NA surface proteins are involved in viral attachment and entry to host cells. They are also the main part of the virus recognized by our immune system as foreign, and most of the antibodies we make after infection are against these antigens.

#9 Which virus requires viral enzymes for genome replication?

RNA virus replication always requires a virally encoded RNA polymerase (i.e. replicase). (This is a retrovirus)

#4 DNA Viruses

Replicate in nucleus of host cell and use the host cell's machinery for DNA synthesis as well as gene expression. Can be single or double stranded

#4 RNA Viruses

Replicate in the cytoplasm, vast majority are single-stranded.

#5 Respiratory Transmittance

Respiratory or salivary route (coughing and inhalation)

#17 What is reverse transcriptase?

Reverse transcriptase is an enzyme that copies RNA back to DNA. This is unusual because DNA is always transcribed to RNA in cells. Reverse transcriptase enzymes can be found in viruses, which use reverse transcriptase to make more viruses. Reverse transcriptase in viruses can cause mistakes in their genome, which changes the new virus particles. This makes it hard to design drugs for HIV and leads to drug resistance. Reverse transcriptase enzymes are also found in eukaryotes and prokaryotes. These cells use reverse transcriptase to repair their genomes and create genetic diversity.

#5 Sexually Transmitted

Sexual contact through genitals

#1 How Viruses Multiply

Since viruses are inert particles, incapable of metabolism, replication, or motility, a viral genome finds needs to find its way into a host cell to hijack the cell's replication machinery, inducing the cell to produce more viral particles.

#6 Attachment (adsorption)

Spikes on viron surface bind to glycoproteins on the host cell's cytoplasmic membrane, and often more than one receptor is required for effective attachment (ex. is HIV). The normal function of these receptors is completely unrelated to their role in virus attachment. Because a virion must bind to specific receptors, a particular virus may be able to infect only a single or limited number of cell types and tissues, and most viruses can infect only a single species.

#2 Nucleocapsid

The capsid together with the nucleic acid it encloses

#20 Explain how viral mutations can enable a virus to infect more hosts and/or cause more severe disease symptoms. Explain how viral mutations can enable a virus to become resistant to an antiviral drug (and what can be done to avoid this problem).

The influenza virus uses two methods to mutate and infect us every single winter: A gradual minor point mutation in the genes responsible for encoding HA and NA proteins on the surface of the influenza virus, called antigenic drift, may occur. Antigenic shift may occur as well. This is a sudden and major change in the surface antigens of a virus. Antigenic shift occurs using a process called genetic reassortment. This is a process whereby two viruses mix and match parts of their genome. In either case, changes to proteins on the surface of the influenza virus called hemagglutinin, or HA, and neuraminidase, or NA for short, will occur. These changes will cause us to fall ill with a new strain of the flu virus.

#4 Why do viral genomes only encode for a few proteins?

The only proteins present in viruses are their capsid, spikes and some enzymes. No need for structures involving replication, metabolism or motility.

#16 Latent or chronic viral infection (examples)

These categories can overlap in an actual infection, but with different cells experiencing different type of infection Chronic infections are characterized by the continuous production of low levels of viral particles. In some cases, the infected cell lyses, but only a small proportion of cells is infected at any given time, resulting in a low number of viral particles being continuously released. -*Virus continues to multiply, host remains infectious (able to transmit to other hosts) -Host may have symptoms or may not (asymptomatic host w/ infectious virus)* Ex.) Hepatitis B and C virus, Herpes simplex virus and HIV. HIV is a retrovirus

#11 How enveloped viruses are released from host cells and their effects

They are released by budding, a process whereby the virus acquires its envelope. Before this, virally encoded protein spikes insert into specific regions of the host cell's membrane.

#2 Virion

Viral particle, consisting of nucleic acid surrounded by a protein coat

#4 Segmented Viruses

Virus that has a genome consisting of multiple different nucleic acid fragments. For example influenza A has a segmented genome which can undergo reassortment if two different strains infect the same cell, so that new viral particles are made that have combinations of genome segments from the initial infecting strains. Immune system will not recognize these mutated versions of influenza; making them more able to replicate and cause illness.

#17 What is a provirus?

Viruses can use two cycles: lysogenic and lytic. All lysogenic cycles must eventually become a lytic cycle, where the host cell is destroyed. A provirus is part of the lysogenic cycle and is when the virus incorporates its genetic material into the host's DNA The lysogenic cycle (and provirus) can cause cancer because they damage tumor suppressor genes in the host.

#17 Describe characteristics of retroviruses (e.g. HIV) Explain how retroviruses reproduce in host cells.

Viruses invade host cells and turn them into virus-making machinery. Usually, the host cell is killed in this process. Retroviruses do not kill the host cell at first because they can insert their genome into the host genome. This process is called reverse transcription and is done by the viral protein reverse transcriptase. In the case of HIV, viral protein integrase then inserts the HIV DNA into host DNA. Host proteins make copies of the viral DNA. However, there are many HIV proteins linked together. This is a problem because these proteins are not functional until they are cut apart by protease. Reverse transcriptase, integrase, and protease are packaged with the HIV RNA genome into a virion. The HIV virion exits the host by budding, taking a bit of the host membrane with it. The host survives however, and continues to make new HIV.

#3 Naked Viruses (structures)

Viruses that do not have an envelope. Nearly all phages are non-enveloped.

#13 Is it possible for a viral infection to exist in a host without disease symptoms?

Whether or not an infected host has symptoms, as long as the virus is multiplying, viral particles (virions) can be transmitted to other hosts (ie, infectious).

#18 Give examples of viruses that can cause cancer, and explain a few mechanisms of how viral infections can lead to cancer.

While viruses can cause cancer in a variety of ways, the actual mechanisms fall into two broad categories. In the direct method, oncoviruses (cancer-causing viruses) infect normal cells and slip some of their genes into the cells' DNA, causing the cells to produce a few abnormal proteins. If the cells acquire additional gene mutations, or if the individual has a weakened immune system, the cells can begin to behave cancerously. In the other, indirect, method, viral infection can cause tissue to become inflamed as the immune system tries to quell the infection. Such inflammation, persisting for years or even decades, increases the chances that tissue will become cancerous. HPV, Hepatitis B, Epstein Barr, Hepatitis C.

#15 Can viruses that persist in host cells produce new viral particles?

Yes, if genes are expressed and producing new virions. Lytic or virulent phages exit the host at the end of the infection by lysing the cell. These viral infections result in the formation of new viral particles.

#15 How can viruses remain in host cells for long periods of time?

Yes, it can remain latent. They infect and then stop producing particles, however, the viral genome is still within the host cell. The result is that the virus can reactivate and can then multiple (viral progeny) without the host being infected by a new outside virus. Continue to use cell as the factory (lysogenic). Lytic is explodes the cell *Virus is not multiplying during latent infection (not transmissible to others). Remains in infected host cells. Virus can start multiplication and host may or may not have symptoms*