Chapter 19 viruses
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Chapter 19: Viruses. (a). 2009 pandemic H1N1 influenza A virus. Viruses. In 2009, a general outbreak ( epidemic ) of a flu-like illness first appeared in Mexico and the United States Caused by an influenza virus H1N1 Flu epidemics= caused by new strains of influenza virus

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Chapter 19: Viruses

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Chapter 19 viruses

Chapter 19: Viruses



2009 pandemic H1N1influenza A virus


  • In 2009, a general outbreak (epidemic) of a flu-like illness first appeared in Mexico and the United States

    • Caused by an influenza virus H1N1

  • Flu epidemics= caused by new strains of influenza virus

    • People have little immunity to new strain and more susceptible to infection

1 m



  • New viruses arise from mutations of original virus into different form

    • Viral diseases in a small isolated population can emerge and become global

    • New viral diseases can emerge when viruses spread from animals to humans

      • Often requires a mutation that changes host range

    • Viral strains that jump species can exchange genetic information with other viruses

      • Humans may have no immunity, increasing severity of disease



  • H1N1 continued to spread and infected people around the world

    • Large-scale spread of these strains can cause pandemics (global epidemics)

  • 2009 flu pandemic= likely passed to humans from pigs

    • “Swine flu”

What are viruses

What are viruses?

  • Viruses are not cells

    • Small infectious particle consisting of nucleic acid enclosed in a protein coat

    • Obligate intracellular parasites

Bacteriophages attacking E. coli cell

0.5 mm

What are viruses1

What are viruses?

  • Viruses lead a “borrowed life” between life-forms and chemicals

    • Viruses can not reproduce without using a host cell

    • Viruses do not have any metabolic activity

  • Viruses do contain genetic material

    • DNA virus= Double- or single-stranded DNA

    • RNA virus= Double- or single-stranded RNA

What are viruses2

What are viruses?

  • A capsid is the protein shell that encloses the viral genome

    • Built from protein subunits called capsomeres

  • Viral envelopes= membranous envelopes found in some viruses

    • Help infect hosts

    • Found in influenza viruses and many other animal viruses

    • Derived from the host cell’s membrane

      • Contain a combination of viral and host cell molecules

Viruses and hosts

Viruses and Hosts

  • Each virus has a host range

    • Limited number of host cells that it can infect

  • Recognition systems for host cells

    • Surface proteins of virus recognize specific receptor molecules on outside of cells

Some viruses have broad host range

Some viruses have broad host range

Zoonosis= infectious disease transmitted between humans and other animals

Rabies Virus

Some viruses have limited host range

Some viruses have limited host range


Some viruses have limited host range1

Some viruses have limited host range

HIV only infects certain types of human white blood cells



  • Viruses “take over” the cell machinery to replicate

    • Once a viral genome has entered a cell, the cell begins to manufacture viral proteins, DNA, RNA

    • Uses host enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules

    • Viral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses



  • Bacteriophages, also called phages, are viruses that infect bacteria

  • Most complex capsids

    • Elongated capsid head that encloses their DNA

    • Protein “tail” attaches the phage to the host and injects the phage DNA inside

50 nm

(d) Bacteriophage T4



  • Two reproductive mechanisms: the lytic cycle and the lysogenic cycle

  • Lytic cycle

    • Causes death of host cell

  • New phages produced in cell

  • Cell lyses (breaks open) to release new viruses

    • Releases large amount of viruses at one time

  • Virulent phage= reproduces only by the lytic cycle is called a

  • Bacteria have defenses against phages

    • Restriction enzymes that recognize and cut up certain phage DNA



  • Two reproductive mechanisms: the lytic cycle and the lysogenic cycle

    2. Lysogenic cycle

    • Does not result in death of host cell

  • Viral DNA incorporated into the host cell’s chromosome

    • Known as a prophage

  • Every time the host divides, it copies the phage DNA and passes the copies to daughter cells

    • Environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode

  • Temperate Phages= use both the lytic and lysogenic cycles

Chapter 19 viruses

Lysis of host cell causes release of progeny phages

Genome integrates into bacterialchromosome as prophage, which(1) is replicated and passed on todaughter cells and(2) can be induced to leave the chromo-some and initiate a lytic cycle

The phage attaches to ahost cell and injects its DNA.




Lysogenic cycle

Lytic cycle

• Virulent or temperate phage

• Temperate phage only

• Destruction of host DNA

• Production of new phages

Animal viruses

Animal Viruses

  • Viruses can cause disease in hosts

    • May damage or kill infected cells

    • Cause infected cells to produce toxins that lead to disease symptoms

    • Molecular components, like envelope proteins, that are toxic

Animal viruses1

Animal Viruses

  • Antiviral drugs help to treat, but not cure, viral infections

  • Viral infections cannot be treated by antibiotics

  • Vaccines can prevent certain viral illnesses

    • Harmless derivatives of pathogenic microbes

    • Stimulates immune system to mount defenses against pathogen

  • Animal viruses2

    Animal Viruses

    • Classified by

    • DNA or RNA

    • Single-stranded or double-stranded



    Single-strand RNA

    Double-strand DNA

    Herpes virus

    Herpes Virus

    • Double-stranded DNA genome

    • Reproduce within host cell nucleus

      • Copies of viral DNA remain in nuclei of nerve cells

      • Remains dormant in host system until stress triggers new round of virus production

      • Infection of other cells by virus results in blisters characteristic of herpes virus



    • HIV (human immunodeficiency virus) is type of retrovirus

      • Single-strand RNA virus

      • RNA acts as template for DNA synthesis

        • Uses reverse transcriptase

        • Changes flow of genetic information RNA to DNA

      • Viral DNA incorporated into DNA of host chromosome

        • Provirus- integrated viral DNA

        • Never leaves host’s genome

    Plant viruses

    Plant Viruses

    • Over 2,000 known types of viral diseases of plants cause

      • Discoloration of leaves and fruits

      • Stunted growth

      • Damaged flowers, roots

    • Most plant viruses havean RNA genome

    Plant viruses1

    Plant Viruses

    • Plants are sessile, limiting modes of transmission of virus

    • Spread disease in two major modes:

    • Horizontal transmission= infection from external source

      • Entering through damaged cell walls

      • Vectors transmit virus

        • insects, worms, bacteria

      • Once inside of cell, viruses can spread to adjacentcells through plasmodesmata

    Plant viruses2

    Plant Viruses

    • Plants are sessile, limiting modes of transmission of virus

    • Spread disease in two major modes:

      2. Vertical transmission= inheriting the virus from a parent

      • Asexual reproduction- infected cells present in clone or fragment

      • Sexual reproduction- infected seeds

    Plant viroids

    Plant Viroids

    • Viroids= smaller than viruses

      • Circular single-stranded RNA molecules

        • No capsid

      • Infect plants

    • Cause errors in regulatory system of plant growth

      • Abnormal development

      • Stunted growth

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