Download VIRUSES - Biology

VIRUSES
Viruses Are Not Cells!
There are several structural and functional
differences between cells and viruses
 The structural differences include:

– No nucleus
– No cell membrane
– No organelles

The functional differences include:
–
–
–
–
Do
Do
Do
Do
not
not
not
not
grow
respire
develop
reproduce
Viral Structure

Viruses are made up of 2 parts:
– Capsid—a protein coat which enables the
virus to bind to a cell
– Nucleic acid—either DNA or RNA

Some viruses have an additional protective
coating called an envelope
Replication vs. Reproduction
Viruses do not reproduce
 Viruses replicate
 Reproduction requires cell division and
replication does not
 A virus requires a host for it to replicate
 A host is a living cell that provides all the
materials a virus needs to replicate

Classification of Viruses

Shape: allows viruses to attach
to only a few kinds of cells; like
two pieces of a jigsaw puzzle
– Filovirus—has no distinct shape
– Polyhedral virus—multifaceted
geometric shape
– Binal virus—has a polyhedral
capsid and a helical tail
– Helical virus—tightly coiled DNA
or RNA inside a long, narrow
capsid
Classification of Viruses (cont.)

Host:
– Plant virus
– Animal virus
– Bacterial virus—these are also called
bacteriophages
Lytic Cycle
Bacteriophage
Bacterial chromosome
DNA or RNA
Entry
Bacterial host cell
Attachment
Lysis and
Release
Replication
Assembly
Lysogenic Cycle
Entry:
Lysis &
Release:
Assembly:
Not pictured
Prophage
Replication:
Retrovirus
Contains an RNA core that is replicated by
1st transcribing its RNA into DNA
 Ex: HIV—the virus that infects white blood

cells and causes AIDS
1. RNA & reverse
transcriptase
enter the host
cell
4.Viral DNA
produces
new viruses
2. Enzymes
copy the viral
RNA into DNA
3. Viral DNA
attaches to
cell DNA
Nonviral Particles

Viroid:
– No capsid
– Contains RNA
– Cause plant diseases

Prion:
– Protein molecule with no RNA or DNA
– Cause animal diseases (i.e. mad cow disease)