Chapter 5 (Viral Structure and Multiplication) - EG

Common causes of acute infections

• colds

• hepatitis

• chicken pox

• influenza

• herpes

• warts

Prominent viral infections worldwide

• dengue fever

• yellow fever

Infections w/ high mortality rates

• rabies

• AIDS

• covid-19

Infections that cause long-term disability

• neonatal rubella

Connection to chronic infections

• type 1 diabetes

• MS

• various cancers

• alzheimers

How are viruses described

described as either active or inactive

• NOT described as dead or alive

Louis Pasteur

hypothesized that rabies was caused by a “living thing” smaller than bacteria

• proposed the term “virus”, which is latin for “poison”

What are viruses in nature?

ubiquitous in nature

• they’ve had major impacts on development of biological life

Size of viruses

ultramicroscopic

• ranges from 20 nm to 1,000 nm (diameter)

Are viruses cells?

NO

• structure is very compact and economical

Basic structure of viruses

consists of a protein shell (capsid) surrounding a nucleic acid core

Can nucleic acid be DNA and RNA?

it can be EITHER DNA or RNA —> not both

Possible strands of nucleic acid

• double-stranded DNA

• single-stranded DNA

• single-stranded RNA

• double-stranded RNA

What gives viruses high specificity?

molecules on virus surfaces give them high specificity for attachment to host cell

How do viruses multiply?

by taking control of host cell’s genetic material and regulating the synthesis and assembly of new viruses

What do viruses lack?

• enzymes for most metabolic processes

• machinery for synthesizing proteins

How are viruses classified and named

• hosts and diseases they cause

• structure

• chemical composition

• similarities in genetic makeup

Smallest and largest virus sizes

• smallest —> parvoviruses around 20 nm in diameter

• largest —> herpes simplex virus around 150 nm in length

Viral components

• external coating

• core containing nucleic acid (DNA or RNA)

• sometimes one or two enzymes

Capsid

protein shell constructed from identical protein subunits called capsomeres

Nucleocapsid

the capsid together w/ the nucleic acid

Naked viruses

consist only of a nucleocapsid

Envelope

external covering of a capsid, usually a modified piece of the host’s cell membrane

What can be found on naked or enveloped viruses?

spikes

• project from the nucleocapsid or envelope

• allow viruses to dock w/ host cells

Virion

a fully formed virus that is able to establish an infection in a host cell

Enzymes for specific operations within their host cell

• polymerases that synthesize DNA and RNA

• replicases that copy RNA

• reverse transcriptase synthesizes DNA from RNA

Phases of the animal viral replication cycle

• adsorption

• penetration

• uncoating

• synthesis

• assembly

• release

How does the length of replication vary?

varies from 8 hours in polioviruses to 36 hours in herpesviruses

How do viruses cause disease?

• they multiply within living cells

• cells destruction during viral release by lysis or budding off

• infected cells may produce toxins

• inflammation

• cancer

Cancers that viruses cause

• hepatitis viruses —> liver cancer

• HPV —> cervical cancer

Cytopathic effects to the host cell

virus-induced damage to the cell that alters its microscopic appearance

Syncytia damage to the host cell

fusion of multiple damaged host cells into single large cells containing multiple nuclei (giant cells)

What kills most host cells?

accumulated damage from a virus infection

DNA viruses replication

enter the host cells nucleus and are replicated and assembled there

RNA viruses replication

replicated and assembled in the cytoplasm

Retroviruses replication

turn their RNA genomes into DNA

Viral infections of the upper respiratory tract

common cold (acute viral rhinitis)

• most common viral infections

• many viruses cause colds

• other cold-causing viruses include adenoviruses

• transmission occurs via respiratory secretions

Viral infections of lower respiratory tract

• influenza, flu —> transmission is via infected humans

• SARS (severe acute respiratory syndrome) —> kills quickly, transmission: as above

• covid-19 —> kills slowly, transmission: as above

Paramyxovirus

• german measles (rubella) —> rubella virus, an RNA virus

• measles (hard measles, rubeola) —> measles virus, an RNA virus

Symptoms of measles and rubella

• rash all over body

• highly contagious

• spreads through coughing and sneezing

• rubella is more mild

Parvovirus

• fifth disease/erythema infectiosum/slapped cheek syndrome

• distinctive red rash on face

• common in kids ages 5-15

Poxvirus

• chickpox —> itchy red blisters, rash all over

• monkeypox —> rash that forms blisters and crusts

Mononucleosis (mono)

• caused by epstein-barr virus (EBV)

• known as human herpesvirus 4

• DNA virus

• transmission —> person-to-person saliva

Mumps

• caused by mumps virus (an RNA virus)

• transmission —> person-to-person saliva

HIV AIDS

ssRNA virus

Viral infections of the eyes

conjunctivitis and keratoconjunctivitis

• caused by various types of adenoviruses

• kerato. may also be caused by herpes simplex

Viral infections of the oral region

cold sores (fever blisters, herpes labialis)

• usually caused by herpes simplex virus type 1

• dna viruses in the family herpesviridae

• genital herpes infections are commonly caused by hsv-2

Type A hepatitis

HAV infection, infectious hepatitis, epidemic hepatitis

• HAV - a linear ssRNA virus

• fecal-oral transmission

Type B hepatitis

HBV infection, serum hepatitis

• HBV - an enveloped, circular dsDNA virus

• sexual transmission or household contact w/ an infected person ; injected drug use ; tattooing

Type C hepatitis

HCV infection, non-A non-B hepatitis

• HCV - an enveloped, linear ssRNA virus

• sexual transmission or household contact

Oncoviruses

viruses capable of initiating tumors

• papillomaviruses

• herpesviruses

• hepatitis b virus

• HTLV-I (human t-cell leukemia)

What percentage of cancers are caused by viruses?

13%

Transformation

the effect of oncogenic or cancer-causing viruses

Treatment of viral infections

• vaccinations work best

• antiviral drugs (limited efficiency)

What treatment doesn’t work toward viral infections?

antibiotics

Covid-19 antivirals

• remdesivir

• paxlovid

• molnupiravir

• over the counter medications —> may help treating symptoms such as aches, coughs and sore throat