VirusesPacket #47

Chapter #18

Introduction

A virus, or virion, is a tiny particle consisting of DNA or RNA surrounded by a protein coat called a capsid.

Viruses cannot metabolize on their own and biologists consider them as non-living particles.

“Evolutionary” Connection

According to the theory of evolution, viruses may be bits of nucleic acid that “originally” escaped from animal, plant or bacterial cells.

Some biologists hypothesize that viruses evolved before the three domains diverged.Thought is that it is unlikely that viruses, that infect

Archaea and Eubacteria, evolved twice.

Bacteriophages

Bacteriophages are viruses that infect bacteria and more than 2000 have been identified.

There are two possible reproductive cycles for viruses: - Lytic Lysogenic.

Lytic Reproductive Cycle

Introduction

When the virus infects a susceptible host cell, it forces the host to use its metabolic machinery to replicate viral particles.

Afterwards, the virus lyses/destroys the host cell.

Lytic Reproductive Cycle

Attachment (or absorption) Virus attaches to

receptors on the host cell wall.

Lytic Reproductive Cycle

Penetration Virus injects its nucleic

acid into the cytoplasm of the host cell. Capsid of a phage

remains on the outside

Lytic Reproductive Cycle

Replication and Synthesis Once inside host, virus

degrades the host cell nucleic acid

Phage DNA is replicated and phage proteins are synthesized.

Lytic Reproductive Cycle

Assembly Newly synthesized viral

components are assembled (genome and proteins) into new viruses.

Lytic Reproductive Cycle

Release Assembled viruses are

released from the cell. Lytic enzymes produced

late in the replication process destroys the host cell membrane. Animal viruses

normally are released slowly and bud off the plasma membrane.

Phages released are used to infect other cells.

Lysogenic Cycle

Introduction

Viruses, that replicate via the lysogenic cycle, are called temperate viruses.

Composed of a sleeper stage and the lytic cycle. During the “sleeper”

stage, the host does not die immediately and carries the viral genome.

When the virus is ready to multiply and be released, it utilizes the lytic cycle to complete the reproductive process—destroying the host.

Lysogenic Cycle II

During the “sleeper” stage and, more importantly, once the viral genome becomes integrated into the host, the bacteriophage is referred to as a prophage. These prophages are

also known as lysogenic cells.

Lysogenic Cycle III

Once conditions are right, the temperate virus reverts back to the lytic cycle, producing new viruses and killing the host cell.

Viruses & Animal Cells

Viruses Infecting Animal Cells

Viruses enter animal cells via membrane fusion or by endocytosis.

Membrane fusion Viral capsid and nucleic acid are both released into the animal

cell while the viral envelope remains fused with the plasma membrane

Endocytosis Plasma membrane of animal cell invaginates to form a

membrane bound vesicle that contains the virus Including the viral envelope.

Viral nucleic acid is replicated within the host cell

Proteins are synthesized

New viruses are assembled and released from the cell

Classes of Animal Viruses

Viruses may contain DNA or RNA Double stranded DNA

(dsDNA) Single stranded DNA

(ssDNA) Double stranded RNA

(dsRNA) Single stranded RNA

(ssRNA) Three classes

Retrovirus

RNA viruses that have a DNA polymerase named reverse transcriptase.

Enzyme is used to transcribe the RNA genome into a DNA intermediate

DNA becomes integrated into the host DNA

Viral RNA is synthesized by the host RNA polymerases HIV (Human

Immunodeficiency Virus) behaves in this fashion.

Viruses & Plants (Plant Cells)

Viruses Infecting Plants

Plant viruses can be spread by insect vectors

Once viruses are in the plant, they spread via the plasmodesmata

The genome of most plant viruses contain RNA

Viruses Infecting Plants

Symptoms Reduced plant size Spots Streaks Mottled patterns in leaves,

flowers or fruits

Infected crops produce lower yields

Cures are not known for most viral diseases of plants

Infected plants are normally burned

Scientists today are trying to develop virus-resistant strains of important crops

Viroids & Prions

Viroids & Prions

Smaller than viruses.

Viroid Consists of a short

strand of RNA with no protein coat

Prion Consists only of protein. Cause transmissible

spongiform encephalopathies (TSEs)

Review

Review

Viruses

Bacteriophages

Viruses & Animal Cells

Viruses & Plant Cells (Plants)

Viroids & Prions