The Viruses

Viruses

• Viruses may be defined as acellular organisms whose genomes consist of nucleic acid,

• obligately replicate inside host cells using host metabolic machinery and ribosomes to form a pool of components

• which assemble into particles called VIRIONS, which serve to protect the genome and to transfer it to other cells.

• They are distinct from other so-called VIRUS-LIKE AGENTS such as VIROIDS and PLASMIDS andPRIONS

•

VIRUS STRUCTURE

• range in size

• All viruses contain

– a nucleic acid genome (RNA or DNA) and

– a protective protein coat (called the capsid).

• may or may not have an envelope

Viral Shapes

• Three basic shapes

– Helical– Icosahedral or Polyhedral

– Complex

HelicalComposed of capsomeres that bond together in

a spiral fashion to form a tube around the nucleic acid; tobacco mosaic virus

Helical symmetry

Icosahedral or PolyhedralRoughly spherical, with the shape similar

to a geodesic dome; common cold

VIRION

The icosahedral shape of a soccer ball.

penton subunits (black) and

hexon subunits (white)

ComplexHave capsids of many different shapes that do not fit

into the two other categories; small pox virus with several covering layers and bacteriophage T4, with

a icosahedral head and tail

ComplexMany bacteriophages contain the icosahedral - many

sided, three dimensional, hexagonal shape made up of many small triangles – head, containing the

genome, attached to helical tails with tail fibers

Virus Facts

• viruses do not respire,

• nor do they display irritability;

• they do not move

• and nor do they grow,

Virus Facts 2

• Cause many infections of humans, animals,

plants, and bacteria

• Cannot carry out any metabolic pathway

• Do not respond to the environment

• Cannot reproduce independently

• Obligate intracellular parasites

Characteristics of Viruses

• Cause most diseases that plague industrialized world; common cold, influenza, herpes, AIDS

• Virus – Miniscule, acellular, infectious agent having one or

several pieces of either DNA or RNA (genome); never both

• No cytoplasmic membrane, cytosol, organelles

• Have extracellular and intracellular state

Characteristics of Viruses 2

• Extracellular state – outside of the cell– Called virion

• Protein coat (capsid) surrounding nucleic acid

– Nucleic acid and capsid also called nucleocapsid– Some have phospholipid membrane called an

envelope– Outermost layer provides protection and recognition

sites for host cells

• Intracellular state– Capsid removed– Virus exists as nucleic acid

How viruses are distinguished

• Type of genetic material they contain• Kinds of cells they attack• Size of virus• Nature of capsid coat• Shape of virus• Presence or absence of envelope

Viral Genetic Material

• Show more variety in nature of their genomes than do cells

• May be DNA or RNA; never both• Primary way scientists categorize and

classify viruses• Can be dsDNA, ssDNA, dsRNA, ssRNA• May be linear and composed of several

segments or single and circular• Much smaller than genomes of cells

Viral Hosts

• Most only infect particular kinds of host’s cells– Due to affinity of viral surface proteins or

glycoproteins for complementary proteins or glycoproteins on host cell surface

• May only infect particular kind of cell in host – HIV – T lymphocytes

• Generalists – infect many kinds of cells in many different hosts– Rabies – humans to bats

Viral Hosts 2

• All types of organisms are susceptible to viral attack

• A bacteriophage is a virus that infects bacteria

• Most studies have focused on bacterial and animal virus

• Some studies on viruses that infect crops

• Fungal virus studies are limited, – It is known that they have no extracellular stage

Tobacco leaf infected with tobacco mosaic virus;

bacteriophage attacking bacteria

Viral Size

Capsid Morphology

• Capsids – protein coats that provide protection for viral nucleic acid and means of attachment to host’s cells

• Capsid composed of proteinaceoussubunits called capsomeres

• Some capsids composed of single type of capsomere (protein); others composed of multiple types

The Viral Envelope

• Some viruses, particularly animal viruses, have a membrane similar in composition to a cytoplasmic membrane surrounding their capsids

• Such a membrane is called an envelope, and thus the virus called an enveloped virus

• A virus without an envelope is called a nonenveloped or naked virus

Enveloped Viruses

• Enveloped viruses acquire their envelope from the host cell during viral replication or release

• Envelope is a portion of the membrane of the host cell

• Composed of phospholipid bilayer and proteins coded for by host DNA

• Some of the proteins are virally coded glycoproteins, which appear as spikes protruding outward from the envelope’s surface

Envelope

• The envelope’s proteins and glycoproteins often play a role in the recognition of host cells

• The envelope does not perform other physiologic functions of cell membranes

Enveloped VirusCoronavirus with helical - spiral in form- capsid

Enveloped VirusTogavirus with icosahedral capsid

Viral Replication• Dependent on host’s organelles and

enzymes to produce new virions• Replication cycle usually results in

death and lysis of host cell → lytic replication

• Stages of lytic replication cycle– Attachment– Entry– Synthesis– Assembly– Release

Classification of Viruses

• Viruses are categorized by their type of nucleic acid, presence of an envelope, shape and size

• Taxa and classification system still under development as viruses are not yet fully understood

Viral Replication• Dependent on host’s organelles and

enzymes to produce new virions• Replication cycle usually results in

death and lysis of host cell → lytic replication

• Stages of lytic replication cycle– Attachment– Entry– Synthesis– Assembly– Release

Lytic Replication of Bacteriophages

Attachment

• T4 bacteriophage of Escherichia coli

• Attachment dependent on random collisions

• Complementary fit of viral tail proteins and receptor proteins on host’s cell wall– Ensures virus will only

attach to correct host cell

Entry

• Releases lysozyme, protein carried in capsid, to weaken peptidoglycan of cell wall

• Tail contracts forcing internal hollow tube through cell wall and membrane

• Genome moves into bacterium

Synthesis

• Viral enzymes degrade the host’s DNA into nucleotides

• Viral genome starts transcription and translation of its own proteins, using host’s ribosomes

• Production of protein components for new virions made

Assembly

• Capsomeres accumulate in cell and spontaneously attach to form new capsids within the cell

• Tail assembles and attaches to head

• Genome inserted after assembly

Release

• Newly assembled virions are released as lysozyme completes its work on the cell wall

• Bacterial cell disintegrates

Lysogeny

• Some bacteriophages have a modified replication cycle in which the infected host cells grow and reproduce for many generations before they lyse

• Additional portion of replication cycle is called lysogeny (lysogenic replication cycle)

• Phages are called lysogenic phages or temperate phages

Lysogeny

1. Attachment

2. Entry3. Lysogeny

a. Prophage incorporation

b. Replication

4. Induction

5. Synthesis6. Assembly

7. Release

Lysogeny

Attachment, Entry and Prophage

• Similar to the lytic replication cycle except the bacterial genome is not destroyed

• Viral genome remains dormant and is called a prophage

• Prophage incorporates into bacterial DNA

Replication (Lysogony)

• Cell reproduces reproducing prophage in every daughter cell

• All daughter cells are thus infected with the quiescent virus

• Can continue for many generations

Induction, Assembly and Release

• Some induction agent causes the prophage to proceed forward with assembly and lysis

• Induction agents are generally the same ones that damage DNA; UV light, carcinogenic chemicals

Nonviral infectious agents

• Prions– PIECE OF PROTEIN– CAUSE OF MAD-COW

DISEASE – CAN INFECT ANIMALS –

INCLUDING HUMANS

• VIROIDS– Single strand of RNA– Causes plant diseases

Viroids and Virusoids

• Infectious agents, • Differ from viruses in several ways.

– they have a single-stranded circular, RNA genome.

– Their genomes are very small and do not code for proteins.

– Viroids replicate autonomously inside a cell, but virusoids cannot.

Viroids, the Smallest Infectious Units

• An infectious particle, • similar to but smaller than a virus, • Lack a capsid• consists solely of a strand of RNA • is capable of causing disease in plants.

Viroids

• Very small, covalently closed, circular RNA molecules capable of autonomous replication and induction of disease

• Sizes range from 250-450 nucleotides

• No coding capacity - do not program their own polymerase

• Use host-encoded polymerase for replication

• Mechanically transmitted; often seed transmitted• More than 40 viroid species and many variants have

been characterized• “Classical” viroids have been found only in plants

ViroidsLeft is T7 genome with small potato spindle tuber

viroids in between; right is potatoes stunted by PSTV

Characteristics of Prions• Prions = Proteinaceous infectious particles• Composed of single protein called PrP• All mammals contain gene that codes for

primary sequence of amino acids in PrP• Two stable tertiary structures of PrP

– Normal functional structure with α-helices called cellular PrP

– Disease-causing form with β-sheets called prion PrP

• Prion PrP converts cellular PrP into prion PrP by inducing conformational change

Characteristics of Prions

• Normally, nearby proteins and polysaccharides force PrP into cellular shape

• Excess PrP production or mutations in PrP gene result in initial formation of prion PrP

• When prions present, they cause newly synthesized cellular PrP to refold into prion PrP

Prion Diseases• All involve fatal neurological degeneration,

deposition of fibrils in brain, and loss of brain matter

• Large vacuoles form in brain; characteristic spongy appearance

• Spongiform encephalopathies – bovine spongiform encephalitis (“mad cow” disease) in cows, scrapie in sheep Creutzfeldt-Jakob disease in humans

Prion Diseases

• Transmission through ingestion of infected tissue, transplantation of infected tissue, or mucous membrane contact

• Only destroyed by incineration; not cooking or sterilization

• There is no known treatment

Prion Diseases

Scrapie in sheep; “mad cow” disease; CJ disease

Other Autonomous or Semi-Autonomously Replicating Genomes

• Retrons• Bacterial and fungal plasmids• Satellite nucleic acids• Satellite viruses• Viroids• Prions

Prions (Diseases)

• "small proteinaceous infectious

particles which resist inactivation by

procedures that modify nucleic acids".

• spongiform encephalopathies

Retroid Elements and Retroviruses