© 2014 Pearson Education, Inc.

© 2014 Pearson Education, Inc.

© 2014 Pearson Education, Inc.

Overview: A Borrowed Life

§  A virus is an infectious particle consisting of little more than genes packaged into a protein coat

§  Viruses lead “a kind of borrowed life,” existing in a shady area between life-forms and chemicals

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A virus consists of a nucleic acid surrounded by a protein coat

§  Even the largest known virus is barely visible under the light microscope

§  Some viruses can be crystalized

§  Viruses are not cells but are a nucleic acid enclosed in a protein coat

§  Some viruses have membranous envelopes that help them infect hosts

§  These viral envelopes are derived from the host cell’s membrane and contain a combination of viral and host cell molecules

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Viral Genomes

§  Viral genomes may consist of either § Double- or single-stranded DNA, or

§ Double- or single-stranded RNA

§  Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus

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Figure 17.2

RNA

Capsomere of capsid

Glycoproteins

Capsomere Membranous envelope RNA

Capsid Head DNA

DNA

Tail sheath

Tail fiber

Glycoprotein 80 × 225 nm 80–200 nm (diameter) 70–90 nm (diameter) 18 × 250 nm

(a) Tobacco mosaic virus

20 nm (b) Adenoviruses

50 nm (c) Influenza viruses

50 nm (d) Bacteriophage T4

50 nm

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Figure 17.3 VIRUS

Replication

Entry and uncoating

DNA

Capsid Transcription and manufacture of capsid proteins

HOST CELL

Viral DNA

Viral DNA

mRNA

Capsid proteins

1

2

3

4 Self-assembly of new virus particles and their exit from the cell

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Figure 17.6

Capsid

RNA

Envelope (with glycoproteins)

HOST CELL

(RNA)

New virus

Copy of genome

Viral genome (RNA) Template

mRNA

ER

Glycoproteins

Capsid proteins

© 2014 Pearson Education, Inc.

© 2014 Pearson Education, Inc.

Epstein-Barr virus (EBV) is the most common cause of infectious mononucleosis, but other viruses can also cause this disease. It is common among teenagers and young adults, especially college students. At least 25% of teenagers and young adults who get infected with EBV will develop infectious mononucleosis. EBV is a dsDNA virus.

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Viral Infection = Chronic Fatigue (CFS)? Various types of infections have been studied to determine if they might cause or trigger CFS:

☣  Epstein-Barr virus infection, also known as mononucleosis

☣  Human herpesvirus 6 infection, a virus that can cause problems for people with impaired immune systems, such as AIDS patients or organ transplant recipients taking immune-suppressant drugs

☣  Enterovirus infection, a type of virus that enters through the gastrointestinal track and can have no symptoms, mild flu-like symptoms, or rarely severe and even deadly symptoms

☣  Rubella, a viral infection also known as German measles

☣  Candida albicans, a fungus that causes yeast infections

☣  Bornaviruses, which cause borna disease, an infectious neurological syndrome

☣  Mycoplasma, a cause of atypical pneumonia ☣  Ross River virus, which causes Ross River Fever, a mosquito-borne tropical disease

☣  Coxiella burnetti, the agent that causes Q fever

☣  Human retrovirus infection, such as HIV, the virus that causes AIDS, or xenotropic murine leukemia virus-related virus (XMRV), a gammaretrovirus

© 2014 Pearson Education, Inc.

© 2014 Pearson Education, Inc.

http://www.sumanasinc.com/webcontent/animations/content/herpessimplex.html

Go to the site below to view an animation of how Herpes, a DNA virus, infects. You may need to watch on a desktop computer, as the animation requires Flash.

© 2014 Pearson Education, Inc.

Concept 17.2: Viruses replicate only in host cells

§  Viruses are obligate intracellular parasites, which means they can replicate only within a host cell

§  Each virus has a host range, a limited number of host cells that it can infect

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General Features of Viral Replicative Cycles

§ Once a viral genome has entered a cell, the cell begins to manufacture viral proteins

§  The virus makes use of host enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules

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

§  These exit from the host cell, usually damaging or destroying it

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Replicative Cycles of Animal Viruses

§  There are two key variables used to classify viruses that infect animals §  The nature of the viral genome (single- or double-

stranded DNA or RNA) §  The presence or absence of an envelope

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Viral Envelopes

§  An animal virus with an envelope uses it to enter the host cell

§  The envelope is derived from the plasma membrane of a host cell, although some of the molecules on the envelope are specified by the genome of the virus

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RNA as Viral Genetic Material

§  The broadest variety of RNA genomes is found in viruses that infect animals

§  Retroviruses use reverse transcriptase to copy their RNA genome into DNA

§  HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome)

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§  Viral DNA that is integrated into the host genome is called a provirus

§  Unlike a prophage, a provirus is a permanent resident of the host cell

§  The host’s RNA polymerase transcribes the proviral DNA into RNA molecules

§  The RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new viruses released from the cell

© 2014 Pearson Education, Inc.

© 2014 Pearson Education, Inc.

© 2014 Pearson Education, Inc.

Youth aged 13 to 24 accounted for an estimated 26% of all new HIV infections in the United States in 2010. Over 50% of youth with HIV in the United States do not know they are infected.

© 2014 Pearson Education, Inc.

© 2014 Pearson Education, Inc.

© 2014 Pearson Education, Inc.

© 2014 Pearson Education, Inc.

Figure 17.7a

Reverse transcriptase

HIV

Glycoprotein Viral envelope

Capsid

RNA (two identical strands)

HOST CELL

Reverse transcriptase Viral RNA

RNA-DNA hybrid

DNA

NUCLEUS

Chromosomal DNA

RNA genome for the next viral generation mRNA

Provirus

New virus

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Figure 17.7aa

Reverse transcriptase

HIV

Glycoprotein Viral envelope

Capsid

RNA (two identical strands)

HOST CELL

Reverse transcriptase Viral RNA

RNA-DNA hybrid

DNA

© 2014 Pearson Education, Inc.

Figure 17.7ab

NUCLEUS

Chromosomal DNA

RNA genome for the next viral generation mRNA

Provirus

New virus