BS1040 TOPIC 8: INTRODUCTION TO VIROLOGY (LECTURE 1)

what is a virus

it is an obligate (must be) intracellular (inside a cell) parsite and so has to reproduce inside cells

"a chemical outside the cell, a living entity inside the cell"

what do viruses look like?

-rod shaped or spherical

-typically 20-40nM so theyre sub-microscopic

-they assemble from pre-formed componenets (think about tryingf to build an ikea wardrobe, the parts are the pre-formed componenets)

-virus paritcles dont grow or divide because they -lack geentic information and appartus for the generation of metabolic energy/ protein synthesis

what are the common viral components?

-protein shell (packaging which keeps everything inside)

-nucleic acids (type of nucleic acid is specific to the species of virus)

-sometimes lipids (envelope)

how many viruses are there?

About 4000 species of viruses have been classified but scientists believe there may be millions...

how do we classify viruses?

classification: hierarchical

it is based on the viral genomes sequence. families have suffix viridae. genus have suffix virus. species is important definition

order, family, genus, strain/type, RNA virus quasi species

explain viral replication

-attachement and penetration and uncoating

-replication of viral genome

-protein synthesis

-assembly and exit

explain what nfection may be?

- productive: entry into permissive cells followed by virion formation

- abortive: entry into a non permissive cell which doesnt support virion formation and could be due to a variety of factors

- restrignent or restrictive: cell is transiently permissive and a few virus are produced

multiplicty of infection (MOI)

-key concept in virology

-number of virus particles divided by number of bacterial cells

-for a MOI of 1, then if you have 10^7 colony forming units

-however if particle to infectivity ratio is low, sya 1 in 100, you need 10^9 pfu for an effective MOI of 1

what is the eclipse phase?

Period from virus entry until new infectious virions released. low amounts of parental infectious material present. genome replication has been initiated

whats the maturation phase?

viral material accumulates in cell or surrounding medium

cells infected wit lytic viruses become metabolically disordered and die, viral production ceases. titres slowly drop

cells infected with non lytic viruses can continue to produce viral particles indefinelty

What is the reproductive cycle duration for many bacteriophage?

Less than an hour

How long is the reproductive cycle for picornaviridae?

6-8 hours

What is the reproductive cycle duration for herpesviridae?

More than 40 hours

How many copies of virus can cells infected with polio virus release?

More than 100,000 copies per infected cell

How many copies of virus can phage lambda and E. coli release?

~50 copies

What is the function of the virion attachment protein?

It binds to a cell surface receptor

What is the anti-receptor of the influenza virus?

Haemagglutinin (e.g. H5N1)

What is the anti-receptor of HIV?

gp120 envelope glycoprotein

What largely determines the tropism of most viruses?

The expression (or absence) of receptors on the surface of cells

Attachment

Energy independent.

Uptake

Endocytosis of the entire particle resulting in accumulation of virus particles inside a cytoplasmic vesicle, an endosome, e.g. influenza.

Fusion

Fusion of the cellular membrane with the virion envelope and direct release of the capsid into the cytoplasm e.g. HIV.

Uncoating

General term applied to events after penetration that allows the virus to express its genome.

Gene Expression

Dependent on the genome of the virus - will focus on in the next session.

Assembly of Filamentous Virus

Filamentous virus assembly e.g. Tobacco Mosaic Virus.

Icosahedral Assembly

Twelve pentamers form the particle e.g. polio.

What happens when some viruses cause cell lysis?

They lead to the destruction of the host cell.

How do some viruses acquire an envelope?

They bud through a cell membrane, such as the plasma membrane.

Which virus buds into the Golgi and retains its envelope?

Pox virus.

How do herpes viruses acquire their envelope?

They bud through the inner and outer nuclear membrane and then into the Golgi.

What type of envelope do herpes viruses retain after budding?

Golgi envelope.