PowerPoint® Lecture Presentations prepared by Bradley W. Christian, McLennan Community College

C H A P T E R

© 2016 Pearson Education, Inc.

Viruses, Viroids, and Prions

13

© 2016 Pearson Education, Inc. Hepatitis B pox virus

© 2016 Pearson Education, Inc.

General Characteristics of Viruses

•  Obligatory intracellular parasites •  Require living host cells to multiply

•  Contain DNA or RNA •  Contain a protein coat •  No ribosomes (no protein synthesis) •  No ATP-generating mechanism (no metabolism)

© 2016 Pearson Education, Inc.

Host Range

•  The spectrum of host cells a virus can infect •  Most viruses infect only specific types of cells

in one host •  Determined by specific host attachment sites and

cellular factors •  Bacteriophages—viruses that infect bacteria •  Range from 20 nm to 1000 nm in length

© 2016 Pearson Education, Inc.

Figure 13.1 Virus sizes.

Bacteriophages f2, MS2

Poliovirus

Rhinovirus

Adenovirus

Rabies virus

Prion

Bacteriophage T4

Tobacco mosaic virus

Viroid

Vaccinia virus

24 nm

30 nm

30 nm

90 nm

170 × 70 nm

200 × 20 nm

225 nm

250 × 18 nm

300 × 10 nm

300 × 200 × 100 nm

Bacteriophage M13

Ebola virus

300 nm

E. coli bacterium 3000 × 1000 nm

Plasma membrane of red blood cell 10 nm thick

Human red blood cell 10,000 nm in diameter

Chlamydia bacterium elementary body

800 × 10 nm

970 nm

© 2016 Pearson Education, Inc.

Viral Structure

•  Virion—complete, fully developed viral particle •  Nucleic acid—DNA or RNA can be single- or double-

stranded; linear or circular •  Capsid—protein coat made of capsomeres (subunits) •  Envelope—lipid, protein, and carbohydrate coating on

some viruses •  Spikes—projections from outer surface

© 2016 Pearson Education, Inc.

General Morphology

•  Helical viruses—hollow, cylindrical capsid •  Polyhedral viruses—many-sided •  Enveloped viruses •  Complex viruses—complicated structures

© 2016 Pearson Education, Inc.

Figure 13.4 Morphology of a helical virus.

Nucleic acid

Capsomere Capsid

Ebola virus

© 2016 Pearson Education, Inc.

Figure 13.2 Morphology of a nonenveloped polyhedral virus.

Nucleic acid

Capsomere Capsid

© 2016 Pearson Education, Inc.

Figure 13.3 Morphology of an enveloped helical virus.

Nucleic acid

Capsomere

Spikes Envelope

Influenza A2

© 2016 Pearson Education, Inc.

Figure 13.5 Morphology of complex viruses. 65 nm

Capsid (head)

DNA

Sheath

Tail fiber

Pin Baseplate

A T-even bacteriophage

Orthopoxvirus

Variola virus = smallpox

© 2016 Pearson Education, Inc.

Growing Bacteriophages in the Laboratory

•  Viruses must be grown in living cells •  Bacteriophages are grown in bacteria

•  Bacteriophages form plaques, which are clearings on a lawn of bacteria on the surface of agar •  Each plaque corresponds to a single virus; can be

expressed as plaque-forming units (PFU)

© 2016 Pearson Education, Inc.

Figure 13.6 Viral plaques formed by bacteriophages.

Plaques

© 2016 Pearson Education, Inc.

Growing Animal Viruses in the Laboratory

•  In living animals •  In embryonated eggs

•  Virus injected into the egg •  Viral growth is signaled by changes or death of the

embryo

© 2016 Pearson Education, Inc.

Figure 13.7 Inoculation of an embryonated egg.

Air sac

Shell Amniotic cavity

Chorioallantoic membrane Chorioallantoic

membrane inoculation

Amniotic inoculation

Allantoic inoculation

Yolk sac inoculation Allantoic

cavity Albumin

Shell membrane

Yolk sac

© 2016 Pearson Education, Inc.

Multiplication of Bacteriophages

•  Lytic cycle •  Phage causes lysis and death of the host cell

•  Lysogenic cycle •  Phage DNA is incorporated in the host DNA •  Phage conversion •  Specialized transduction

© 2016 Pearson Education, Inc.

T-Even Bacteriophages: The Lytic Cycle

•  Attachment: phage attaches by the tail fibers to the host cell

•  Penetration: phage lysozyme opens the cell wall; tail sheath contracts to force the tail core and DNA into the cell

•  Biosynthesis: production of phage DNA and proteins

•  Maturation: assembly of phage particles •  Release: phage lysozyme breaks the cell wall

© 2016 Pearson Education, Inc.

Figure 13.11 The lytic cycle of a T-even bacteriophage.

Attachment: Phage attaches to host cell.

Penetration: Phage penetrates host cell and injects its DNA.

Biosynthesis: Phage DNA directs synthesis of viral components by the host cell.

Maturation: Viral components are assembled into virions.

Release: Host cell lyses, and new virions are released.

Bacterial cell wall

Bacterial chromosome

Capsid DNA

Capsid (head) Sheath

Tail fiber

Baseplate

Pin

Cell wall

Plasma membrane

Tail

Sheath contracted

Tail core

Tail DNA

Capsid

Tail fibers

© 2016 Pearson Education, Inc.

Bacteriophage Lambda (λ): The Lysogenic Cycle

•  Lysogeny: phage remains latent •  Phage DNA incorporates into host cell DNA

•  Inserted phage DNA is known as a prophage •  When the host cell replicates its chromosome, it also

replicates prophage DNA •  Results in phage conversion—the host cell exhibits

new properties •  Ex. Cornyebacterium diptheriae – tthe prophage carries

the toxin.

© 2016 Pearson Education, Inc.

Figure 13.12 The lysogenic cycle of bacteriophage λ in E. coli.

Phage DNA (double-stranded)

Phage attaches to host cell and injects DNA.

Bacterial chromosome

Occasionally, the prophage may excise from the bacterial chromosome by another recombination event, initiating a lytic cycle.

Many cell divisions

Lysogenic bacterium reproduces normally.

Phage DNA integrates within the bacterial chromosome by recombination, becoming a prophage.

Prophage

Phage DNA circularizes and enters lytic cycle or lysogenic cycle.

New phage DNA and proteins are synthesized and assembled into virions.

Cell lyses, releasing phage virions.

OR

Lysogenic cycle

Lytic cycle

© 2016 Pearson Education, Inc.

Multiplication of Animal Viruses

•  Attachment: viruses attach to the cell membrane •  Entry by receptor-mediated endocytosis or fusion •  Uncoating by viral or host enzymes •  Biosynthesis: production of nucleic acid and

proteins •  Maturation: nucleic acid and capsid proteins

assemble •  Release by budding (enveloped viruses) or

rupture

© 2016 Pearson Education, Inc.

Figure 13.14 The entry of viruses into host cells.

Host plasma membrane proteins at site of receptor-mediated endocytosis

Fusion of viral envelope and plasma membrane

Entry of pig retrovirus by receptor-mediated endocytosis.

Entry of herpesvirus by fusion.

© 2016 Pearson Education, Inc.

Figure 13.20 Budding of an enveloped virus.

Viral capsid

Host cell plasma membrane

Viral protein

Bud

Bud

Envelope

Release by budding

Lentivirus

© 2016 Pearson Education, Inc.

The Biosynthesis of DNA Viruses

•  DNA viruses replicate their DNA in the nucleus of the host using viral enzymes

•  Synthesize capsid in the cytoplasm using host cell enzymes

© 2016 Pearson Education, Inc.

Figure 13.15 Replication of a DNA-Containing Animal Virus.

Papovavirus

DNA

Capsid

Nucleus

Cytoplasm

Host cell

Capsid proteins

MATURATION Virions mature.

RELEASE Virions are released.

BIOSYNTHESIS Viral DNA is replicated, and some viral proteins are made.

mRNA

Viral DNA

Capsid proteins

ENTRY and UNCOATING Virion enters cell, and its DNA is uncoated.

A papovavirus is a typical DNA-containing virus that attacks animal cells.

ATTACHMENT Virion attaches to host cell.

Late translation; capsid proteins are synthesized.

Viral replication in animals generally follows these steps: attachment, entry, uncoating, biosynthesis of nucleic acids and proteins, maturation, and release. Knowledge of viral replication phases is important for drug development strategies, and for understanding disease pathology.

KEY CONCEPTS

A portion of viral DNA is transcribed, producing mRNA that encodes "early" viral proteins.

© 2016 Pearson Education, Inc.

Basis of Classification

Nucleic acid type

Morphology

Presence or absence of envelope

© 2016 Pearson Education, Inc.

DNA Viruses

Adeno- respiratory diseases, some animal tumors

Papova- papilloma, polyoma, vacuolating

Warts and cancer

Parvo- mostly animal infections

Fetal deaths and GI infections in humans

© 2016 Pearson Education, Inc.

DNA Viruses

Herpes- cold sores and genital infections chickenpox, shingles, mononucleosis

Hepadna- hepatitis B, liver tumors has a reverse transcriptase

Pox- brick shaped. Skin lesions

Double-stranded DNA, enveloped

© 2016 Pearson Education, Inc.

RNA Viruses

Picornavirus- rhinovirus causes the common cold; enterovirus causes polio, hepatitis A and other enteric diseases

Togavirus- arthropod borne equine encephalitis; rubella (German measles)

Flavivirus- hepatitis C, yellow fever, dengue, and West Nile encephalitis

© 2016 Pearson Education, Inc.

RNA Viruses

Coronavirus- common cold and SARS

Rhabdovirus- rabies and other animal diseases

Filovirus- hemorrhagic fevers (Ebola, Marburg)

Orthomyxovirus- influenza

© 2016 Pearson Education, Inc.

RNA Viruses

Paramyxovirus- measles, mumps

Retrovirus- has reverse transcriptase to aid in integration to host genome (provirus). Tumors, animal leukemias, AIDS

Reovirus- respiratory infections

© 2016 Pearson Education, Inc.

Biosynthesis of RNA Viruses That Use DNA

•  Single-stranded RNA, produce DNA •  Use reverse transcriptase to produce DNA from the

viral genome •  Viral DNA integrates into the host chromosome as a

provirus •  Retroviridae

•  Lentivirus (HIV) •  Oncoviruses

© 2016 Pearson Education, Inc.

Figure 13.19 Multiplication and inheritance processes of the Retroviridae.

Reverse transcriptase

Capsid Envelope

Virus

Host cell

Two identical + strands of RNA

Retrovirus enters by fusion between attachment spikes and the host cell receptors.

Uncoating releases the two viral RNA strands and the viral enzymes reverse transcriptase, integrase, and protease.

DNA of one of the host cell's chromosomes

Mature retrovirus leaves the host cell, acquiring an envelope and attachment spikes as it buds out. Viral

enzymes

Viral RNA

Viral DNA

Reverse transcriptase copies viral RNA to produce double- stranded DNA.

Viral proteins are processed by viral protease; some of the viral proteins are moved to the host plasma membrane.

Identical strands of RNA Viral

proteins

RNA

Provirus

Transcription of the provirus may also occur, producing RNA for new retrovirus genomes and RNA that encodes the retrovirus capsid, enzymes, and envelope proteins.

The new viral DNA is transported into the host cell's nucleus, where it's integrated into a host cell chromosome as a provirus by viral integrase. The provirus may be replicated when the host cell replicates.

© 2016 Pearson Education, Inc.

Table 13.2 Families of Viruses That Affect Humans (1 of 4)

© 2016 Pearson Education, Inc.

Table 13.2 Families of Viruses That Affect Humans (2 of 4)

© 2016 Pearson Education, Inc.

Table 13.2 Families of Viruses That Affect Humans (3 of 4)

© 2016 Pearson Education, Inc.

Table 13.2 Families of Viruses That Affect Humans (4 of 4)

© 2016 Pearson Education, Inc.

Viruses and Cancer

•  Several types of cancer are caused by viruses •  May develop long after a viral infection •  Cancers caused by viruses are not contagious

© 2016 Pearson Education, Inc.

The Transformation of Normal Cells into Tumor Cells

•  Oncogenes transform normal cells into cancerous cells

•  Oncogenic viruses become integrated into the host cell's DNA and induce tumors

© 2016 Pearson Education, Inc.

Prion

Proteinaceous infectious agent

No nucleic acid Abnormal form of a normal protein acts

as a template to convert normal to abnormal

© 2016 Pearson Education, Inc.