BioWilliamology: Viruses

VIRUSES

All viruses are microscopic parasites.

# Generally, they are smaller than bacteria.

# They range in size from about 20-300 nm (about 50 times smaller on average than bacteria).

Viruses have a simple non-cellular structure.

# Viruses consist of two parts:

↦ the genetic material (either DNA or RNA), and

↦ a protein coat (capsid), which surrounds and protects the genetic material.

# In some cases, an envelope of lipids that surrounds the protein coat.

Structures of a Virus

The DNA or RNA found in the core of the virus can be single stranded or double stranded.

# Viral genomes are generally small, coding only for essential proteins such as capsid proteins, enzymes, and proteins necessary for replication within a host cell.

Capsids protect viral nucleic acids from being hydrolysed and destroyed by host cells’ enzymes.

Some viruses have a second protective layer known as the envelope.

Viruses are parasite that cannot reproduce by itself.

# Viruses do not have the complicated chemical machinery needed to thrive and reproduce outside of a host body.

# They, thus seek out host cells in which they can multiply.

Life Cycle of a DNA Virus

Life Cycle of a DNA Virus

During adsorption, viral coat protein interact with specific receptor proteins on the exterior of the host cell. The virus enters into the cells by receptor mediated endocytosis. The virus travels to the appropriate location in the cell and expose its genome.

Next, host-cell enzymes transcribe viral “early” genes into mRNAs and subsequently translate these into viral “early” proteins and enzymes, which replicate the viral DNA and induce expression of viral “late” proteins by host-cell enzymes.

The viral late proteins include capsid, assembly proteins and enzymes that degrade the host-cell DNA, supplying nucleotides for synthesis of viral DNA. Progeny virions are assembled in the host cell and released when the cell is lysed by viral proteins.

Newly liberated viruses initiate another cycle of infection in other host cells.