Bio II Viruses CH 21

Virus vs. Living Cells

• Viruses are acellular ( not cells) and composed of nucleic acids from vaious sources acquired through evolution

• scientist cannoy agree on their status as “living”

• they have no metabolism and cannot grow

• they depend on host cells for replication

  • They depend on host cells using its machinery

• They can infect nearly all types of organisms and are not only species specific but also tissue specific.

virology

study of viruses

epidemiology

study of health conditions and how diseases spread in populations

Epidemic

widespread outbreak of a disease in a region ( more controlled)

pandemic

outbreak and spread of a disease across a very wide region (globally) usually more out of control

Virion

a single virus particle- the complete, extracellular and infectious form

• composed of nucleic acid, protein capsid, and some may contain envelope.

nucleic acid

may be dna, or rna, double stranded or single stranded

protein capsid

protein shell that surrounds and protects nuclear material

• nucleic acid core surrounded by a protein capsule

envelope

similar to a cell membrane

Classifications of viruses

• enveloped and non-enveloped

• RNA or DNA

• single or double stranded

• circular or linear

• in one piece or in multiple segments

• positive, negative, or ambisense RNA

Unique capsid shapes

a. Helical

b. Polyhedrons (icosahedral 20-sided)

c. complex ( bacteriophages and poxviruses)

reverse transcription

• Discovered by David Baltimore and colleagues in 1970 in a retrovirus. Received a Nobel prize.

• challenged the idea of central dogma

• provided important information in the understanding of viruses and immune response.

• Baltimore also developed a system for classifying viruses

Host specific strains

• Viruses are obligate intracellular parasites

• some viruses can remain dormant or latent for years

  • more kinds of viruses than organisms?

Host range

types of organisms infected

tissue tropism

types of cells infected

Viral replication

• viruses can only reproduce inside cells

  • outside, they are metabolically inert virions

• Virus hijacks the cell’s transcription and translation machineries for the assembly and release more viruses

HIV retrovirus

• Causes AIDS ( acquired immunodeficiency syndrome)

  • was first reported in the US in 1981

  • some people are resistant to HIV infection due to a mutation in the CCR5 gene

    • CCR5 encodes a receptor for HIV

    • also provides resistance for HIV

• is enveloped

• can reverse transcribe its RNA genome into DNA form (retro- going back)

• targets CD4⁺ cells, mainly helper T cells (thymocytes)

  • without these cells, the body cannot mount an effective immune response

  • Host may die from a variety of opportunistic infections

• Tests for HIV detect for anti-HIV antibodies

  • not circulating viruses

Cycle of a Viral Infection

Viral replication

A. Lytic cycle

B. Lysogenic cycle

• Many viruses can switch from lysogenic to lytic, referred to as induction.

Lytic Cycle

• virus kills the host cell

1. Destroys the host cell

   • Attachment (adsorption)

   • Penetration

   • Replication (synthesis)

   • Assembly

   • Release

2. Replicative growth

3. Cells machinery is hijacked

Virulent

lytic phages

Lysogenic cycle

viruses incorporates into the cell’s genome

1. Usually does not kill the host

2. no new virions

3. viral genome replicated along with host DNA

   • Attachment

   • Penetration

   • Intergration

   • Replication (propagation)

Temperature

lysogenic phages

Roles of Viruses in Environment

• Causes damage and death to cells and organisms (virulent, emerging diseases)

  • lysis ( the cell ruptures or bursts open)

  • apoptosis (programmed self destruction)

  • widespread symptoms due to the body’s immune response

    • in some cases, replicated viruses can leave the cell by “budding”, which does not immediately kill the cell.

• Pandemics and Epidemics- world wide concern

• shape evolution of organisms- 5-8% of the human genome consists if remnants of viral genomes from past infections (ex. lateral gene transfer and function of human placenta)

Factors affecting viral infection

• Enter cells by directly injecting genetic material or by binding to specific membrane molecules (fusion)

• lysozomes degrade bacteria cell walls

• protein made ribosomes on ER or by ribosomes in cytoplasm

• DNA/ RNA synthesis

  • viral DNA polymerase

  • RNA replicase

  • reverse transcriptase (retrovirus)

• Vaccines

Vaccines to prevent viral infection

• trigger immune protection

• Prepared using

  • attenuated “live” virus

  • “killed” virus

  • molecular subunits

  • complementary DNA made by reverse transcriptase

    • RNA → ss cDNA → ds cDNA

• very small risk of infection ( polio epidemic in 2007 due to back mutations)

Origins of Viruses

3 Hypotheses

• regressive - “deveoped” from free-living cells or intracellular prokaryotic parasites

• progressive - RNA or DNA ( self-replicating molecules) escaped from host cell

• virus first- Early RNA based life forms, first self-replicating entities

Potential uses of viruses in medicine

Lines of research

• gene therapy ( adenorivus inserts missing gene)

• Oncolytic viruses ( adenovirus attack and kill cancer cells

• phage therapy and antibiotic resistance

Prions

• proteinaceous infectious particles

• cause tranmissible spongiform ecephalopathies (TSEs)

  • mad cow disease ( Bovine spongiform ecephalopathy)

  • Creuzfeldt- jacob disease in humans ( comes from eating infected cows)

  • Kuru ( ritualistic canabilism) causes uncontrollable laughter

• Animals have normal prion proteins- misfolded proteins cause disease

viroids

• tiny naked molecules of cicular RNA

• Only known to infect plants

  • i.e coconuts and potatoes

• can replicate within cells

• do not manfacture any proteins

• can cause crop failures

• it is unclear how they cause disease

Rectovirus

• a type of RNA virus.

• It carries its genetic material as RNA, not DNA.

• Once it infects a host cell, it uses an enzyme called reverse transcriptase to make a DNA copy of its RNA.

• That DNA is then integrated into the host’s genome, becoming a provirus.

• From there, the host cell’s machinery makes new viral RNA and proteins, which assemble into new viruses.

Bacteriophage

• (often just called a phage) is a type of virus that infects bacteria.

• The word comes from “bacteria” + “phagein” (to eat) → meaning “bacteria eater.”

• They are among the most abundant and diverse viruses on Earth.