•Is the study of viruses.

1.Viruses are a heterogenous intracellular group

of microorganisms.

2.Vary with respect to size, morphology, and

chemical composition that contain either

deoxyribonucleic acid (DNA) or ribonucleic

acid (RNA).

3. Acellular, and metabolically inert outside

the host cell.

4.Obligate intracellular parasites

5.No ATP generating system

6.No Ribosomes or means of Protein

Synthesis

- 1. Nucleic Acid

o DNA or RNA (But never both)

- 2. Capsid (Coat Protein)

- Some Viruses:

o A. Envelope

o B. Enzymes

• Host range is determined by Viruses ability

to interact with its host cell, Binding Sites

should match Receptor Sites

Binding Sites - on viral capsid or envelope

Receptor Sites - on host cell membrane

- Most viruses have a narrow host range:

o Polio virus - nerve cells

oAdenovirus - cells in upper Respiratory Tract

• Some viruses only infect:•plants•invertebrates•protists•fungi•bacteria (Bacteriophages)

The classification is depend

on

1. Nucleic Acid

2. Morphology

3. Strategy for replication

•Viruses are unique life forms different from all other living organisms, either eukaryotes or prokaryotes, for three fundamental reasons:1. The nature of environment in

which they grow and multiply.2. The nature of their genome. 3. The mode of their

multiplication.

Viruses can function and multiply

only inside another living organism,

which may be either a prokaryotic or

eukaryotic cell or any other living

cells depending on the virus.

Whereas all other living forms can use only DNA (and not

RNA) as their genetic material (genome) for information

transmission from parent to progeny, viruses can use either

DNA or RNA as their genome, that is, some viruses can

use only RNA (and not DNA) as their genetic material.

Therefore, these classes of RNA viruses have developed

new sets of enzymes for replicating and transcribing RNA

from an RNA template.

All eukaryotic and prokaryotic cells divide and multiply as a

whole unit, that is, 1 ~ 2 -- 4 ~ 8 and so on. However, viruses

do not multiply as a unit.

In fact, they have developed a much more efficient way to

multiply just as complex machines are made in a modern

factory. Different viral components are made separately from

independent templates, and then these components are

assembled into the whole and infectious units, also called

virus particles (virions).

Is the virus particles which consist of two or

three parts: the genetic material made from

either DNA or RNA, a protein coat that

protects these genes; and in some cases an

envelope of lipids that surrounds the protein

coat when they are outside a cell.

• DNA or RNA (But never both)• The nucleic acid is the genome that contains the information necessary for viral function and multiplication.• The composition of the protein shell called the viral "nucleocapsid" surrounding the genome.

ossDNA - ds DNA - ss RNA - ds RNA

oprotects viral genome from

host endonucleases

oConsists of Capsomeres

which are structural the

units of capsid

oContains binding Sites

oDerived from the membrane of the host

cell.

oContains binding Sites

oThe absence of viral membrane gives the

naked virus.

The size of virions ranges from 20 nm

(parvovirus) to -300 nm (poxvirus) in

diameter, and therefore, too small to be seen

with the light microscope. However, it can be

studied with the electron microscope, but this

microscope is very costly and special training

facilities are required to use it.

• Meaning the capsid is spiral in shape. • It surrounds a spiral shaped core of

nucleic acid.• Helical capsids are usually flexible and

rodlike. • The length of the helical capsid is

usually determined by the length of the nucleic acids, that is, viruses having shorter nucleic acids will have a shorter helical nucleocapsid.

• Helical capsids can package only single-stranded RNA, but not double-stranded DNA or RNA, possibly because of the rigidity of the double-stranded nucleic acids.

• The term cubic is used to describe Icosahedral viruses. •Viruses with icosahedral capsids possess a closed shell enclosing the nucleic acid inside. An icosahedron has 20 triangular faces, 30 edges.

•Unlike helical nucleocapsids that package only single-stranded nucleic acid, icosahedral capsids can be used to package either single- or double-stranded RNA and DNA molecules.•An icosahedral virus can be either naked or enveloped; but, unlike the helical enveloped viruses, the icosahedron capsid structure is rather rigid.

Means the capsid symmetry is

neither icosahedral nor helical.

Only the poxviruses among the

medically important viruses show

complex symmetry.

The overall shape of poxviruses is

usually described as brick-shaped.

•Enveloped Helical•Enveloped Polyhedral

• Many of the helical

viruses and a few of

Icosahedral viruses are

surrounded by an

envelope.

Because viruses possess neither cellular structure nor

organelles, they are unable to make their own proteins

and essential enzymes. They are therefore completely

dependent on their host cells for energy and

multiplication.

Outside of living cells, viruses are metabolically inactive.

The information required for a virus to replicate is

contained in its nucleic acid. This information is used by

the host cell to produce new viruses.

• In animal host the viruses can cause:

• The virus leave the host cell by breaking the

cytoplasmic membrane and is released out as

viroins to attack other cells (enveloped viruses

come out of the cells like budding without

rupturing the cell.)

• As the virus comes out of the cell the host cell will

rupture i.e lyses (killed) and die.

A small group of viruses are able to change, or

transform their host cells from normal cells into

abnormal ones with properties of cancerous cells,

here the cell is not dead.

Immune system of host cell produces

antibodies to fight the virus .Antibodies will

react with virus (antigen), and Antigen-

Antibody complex is produced that deposit

in different places of body e.g. Hepatitis B

deposit in glomeruli of kidney

Some viruses after infecting cells do not

replicate, or they become active for a time and

then become inactive (latent). In response to

certain stimuli, latent viruses can be

reactivated and become active replicating

particles, for example herpesviruses.

The internationally agreed system of virus

classification is based on the structure and

composition of the virus particle (virion).

In some cases, the mode of replication is also

important in classification.

Viruses are classified into various families on

this basis.

Primary characteristics used in classification:

•Viruses are classified according to the nature of

their genome and their structure

 

VIRAL CLASSIFICATION

Nucleic acid

 

RNA or DNA

single-stranded or double-stranded

non-segmented or segmented

linear or circular

If genome is single stranded RNA, can it

function as mRNA?

whether genome is diploid (such as in

retroviruses)

Virion

structure

symmetry (icosahedral, helical, complex)

enveloped or not enveloped

number of capsomers

Adsorption

The first step in infection of a cell is attachment to the cell surface. The viral attachment protein recognizes specific receptors, which may be protein, carbohydrate or lipid, on the outside of the cell. Cells without the appropriate receptors are not susceptible to the virus.

Penetration occurs almost rapidly after attachment and is a next step for gaining entry into the cytoplasm by crossing the plasma membrane.

Thus, penetration allows the viruses to deliver their genome into the host cells to initiate replication.

The penetration process differs for different viruses:

a) Penetration by the process of endocytosis is common to both enveloped and non-enveloped viruses.

b) While some virus species can directly penetrate the plasma membrane and inject their genetic material into the cytoplasm, the majority of viruses enter cells via endocytosis.

c) Direct penetration is observed only in non-enveloped viruses, membrane fusion is observed only in enveloped viruses. Some of the non-enveloped viruses like picornaviruses and phages are capable of directly injecting their genome into the host cell.

3-Uncoating:Release of the viral genome

from its protective capsid to enable the viral nucleic acid to replicate.•  4-Transcription:Synthesis of m-RNA• 5-Translation:The viral m-RNA is translated on

cell ribosomes into structural and non-structural proteins.

• 6-Replication of the viral genome

• 7-Assembly:

New virus genomes and proteins are assembled to form new virus particles.

• 8-Release:

-Enveloped viruses are released by budding

-Unenveloped viruses are released by rupture and lysis of the infected cells.