Chapter 6

Viruses

General Characteristics of Viruses

•         Are they living organisms?

•         Obligate intracellular parasites

•         Contain either RNA or DNA

•         Use the metabolic machinery of host cell to synthesize their own nucleic acids

•         They have no or just few enzymes of their own

•         Can infect: animals, plants, algae, fungi, and bacteria

•         The size of viruses ranges from 20 to 14,000 nm

Viral components

Naked viruses

•         Virus is composed of

–        nucleic acid

–        protein coat - capsid

•         Capsid is composed of protein subunits called capsomeres

Enveloped viruses

•         Some viruses have the capsid covered with an envelope (lipids, proteins, and carbohydrates)

Morphology of viruses

•         Based on capsid structure there are three different morphological types:

–        Helical

–        Polyhedral

–        Complex

Helical viruses

Naked helical viruses

•         Long rods

•         The capsid is cylindrical with helical structure (Tobacco mosaic virus)

Enveloped helical viruses

•         Helical nucleocapsid placed within an envelope (influenza)

Polyhedral viruses

•         The capsid is a regular polyhedron with 20 triangular faces (Poliovirus)

•         Nucleic acid packed in the center

The Viral Envelope

•         Roughly spherical

•         Sometimes the envelope is covered with spikes (carbohydrates)

•         Function of capsid/envelope:

–        Protection

–        Attachment of viruses to the host cells

–        Introduction of nucleic acid into host cell

Complex viruses

•         Bacterial viruses – bacteriophages

–        head - polyhedral

–        tail- helical

–        tail fibers

At the Core of a Virus

•         Either DNA or RNA

–        Double/single stranded DNA

–        Double/single stranded RNA

•         Single stranded RNA

–        Positive-sense (directly translated into proteins)

–        Negative sense (need to be converted into a proper form)

•         Genom is small

–        Hepatitis B virus: 4 genes

–        Human genom: 30,000 genes

•         Besides nucleic acids core may contain enzymes required for replication

Adsorption

•         Viruses use their attachment sites (glycoprotein) to attach themselves to receptor sites on the plasma membrane of animal cell

•         The attachment sites can be:

–        small fibers or spikes on the virus envelope (Influenza)

–        Capsid spike (naked viruses)

•         The receptor for a particular virus is specific – host range

–        Virus can Infect only the specific cells (Hepatitis B – liver cells)

–        Can vary from one person to another

Penetration

Two ways to penetrate the cell

•         1. Endocytosis - it is an active process by which nutrients are brought into a cell

–        Cell membrane folds inwards forming a vesicle.

–        Vesicle is transported inside the cell.

•         2. Fusion with the cell membrane

–        Viral envelope fuses with the host cell membrane

Uncoating

•         The vesicle, viral envelope and capsid are destroyed and the nucleic acid is released into the cytoplasm

–        Enzymes (of the host cell) degrade the proteins of the capsid

Biosynthesis of DNA viruses

•         The viral DNA takes over the genetic expression of the host

•         Viral DNA is synthesized in the host nucleus by viral enzymes

•         Capsid is synthesized in the cytoplasm by using host enzymes and amino acids

•         Assembly of coat proteins and DNA takes place in the nucleus

•         Newly formed virus particle is transported along the endoplasmic reticulum

Maturation and Release

In enveloped viruses

•         Process is called budding or exocytosis

•         The assembled capsid pushes through the plasma membrane

•         A portion of the plasma membrane becomes the viral envelope

•         The release of viral particles is gradual – there is no sudden death of the host cell

In the nonenveloped viruses

•         Host cell plasma membrane raptures

•         Causes death of the host cell

The Biosynthesis of RNA Viruses

•         Different groups of RNA viruses have different mechanism of mRNA formation

•         An example: Retroviridae – HIV

•         Has reverse transcriptase – to produce double stranded DNA

•         Integrated into host DNA – provirus

•         It remains latent or produces new viruses

Viruses and cancer

•         Some viruses can cause cancer

•         Human and animal genomes normally contain oncogenes

–        Activation of these genes causes cancer.

–        They can be activated by mutagenic chemicals, radiation, and viruses

•         When the oncogenic virus infects the cell, its genetic material is integrated into the host cell’s DNA.

•         Such infected cells show different characteristic – uncontrollable growth

Latent viral infection

•         Some viruses can remain in host for long period of time without causing any symptoms.

•         Ex: Herpex simplex virus – infection of skin, lives in nerve cells.

•         Large portion of human population carries this virus, only 10-15% exhibits the disease.

Viruses That Infect Bacteria

•         Bacteriophages

•         Most widely studied bacteriophage is T4

•         They can have two types of life cycle:

–        Lytic

–        Lysogenic

Morphology of T4

–        Capsid

–        Tail

–        Fibers

–        Baseplate

Lytic cycle - (in T4 bacteriophage)

Attachment

•         Tail fibers used as attachment sites,

–        the complementary receptor sites are located on the bacterial cell wall

Penetration

•         The tail sheath contracts and the tail penetrates the cell wall.

–        DNA from the head is injected into bacterial cell. The head remains outside.

Biosynthesis

•         The virus DNA triggers host DNA degradation, stops host protein synthesis

•         The virus uses the host nucleotides and enzymes to:

–        synthesize its own DNA

–        synthesize its own proteins

Maturation

•         Viral DNA and capsid is assembled into a mature viral particle

Release

•         Lysozyme is synthesized within the cell – this causes bacterial cell to break and release the virus particles

Lysogenic cycle (in bacteriophage lambda)

·         After penetration, the viral DNA is integrated into bacterial DNA (prophage)

Lysogenic cycle

•         Action of UV light or some chemicals initiates the lytic cycle

•         The phages that have both of these cycles are called lysogenic phages or temperate phages

•         The bacterial cell containing a lysogenic phage is called lysogenic cell

Characteristics of Lysogenic Cell

•         It is immune to new viral infection

•         Can exhibit new properties – lysogenic conversion (toxicity of Clostridium botulinum)

•         Can transfer genes from one bacterium     to another - transduction

Cultivation of viruses

Viruses must be cultivated within living cells

Cultivation of Bacteriophages

•         Plaque method

–        Melted agar + host bacterial cells + virus

–        A lawn of bacteria is formed

–        A virus infects the bacterial cell and lyses occurs

–        New infection will result in formation of a clear zone of lysed bacteria – plaque

Growing Animal Viruses

Can be grown in:     

•         Living animals (mice, rabbits, guinea pigs)

–        Some human viruses can not grow in animals

•         Bird Embryos

–        Virus injected in the embryonated chicken egg

–        The death or damage of the embryo indicates the presence of viruses

–        Some virus vaccines are produced by this method