Ch. 21 Viruses
Virus= nucleic acid surrounded by a protein coat
Viruses are not cells!
Viral Genomes, Capsids, and Envelopes
Viral genomes may consist of either
o Double or single-stranded DNA
o Double or single-stranded RNA
A virus can be a DNA virus or RNA virus depending on its nucleic acid
Capsid= protein shell that encloses the viral genome
o Capsids are built from capsomeres
Virus= infectious particle consisting of genes that are packaged into a protein coat
Viruses lead a “borrowed life”
o Exist in a form between a life form and chemicals
There is no evidence as to where or how they came into existence
o This is because there is no fossil record
Viral Envelope
Some viruses can invade their hosts by use of a membranous envelope
o These are called viral envelopes
Viral envelopes are made from the host cells membrane
o They contain both viral and host cell molecules
The molecules that they take from the host include phospholipids, membrane proteins
They also have viral glycoproteins
A host that can be invaded is decided by the proteins on the surface of both the virus and the
host
Plant viruses- most have RNA genome and helical capsid
Bacteriophages- viruses that infect bacteria
o Have the most complex
o Elongated capsid head that holds their DNA
o A protein tail piece attaches the phage to the host and injects the phage DNA inside
Viruses Replicate Only in Host Cells
Viruses are obligate intracellular parasites
o This is because they can only replicate within a host cell
Every virus has a host range
o A host range is a limited number of host cells that a virus can infect
o The host range is determined by the proteins on the surface of the host and the virus
General Features of Viral Replicative Cycles
First, the viral genome enters the host cell and the cell begins to make viral proteins
The virus uses the hosts enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules
Virus= nucleic acid surrounded by a protein coat
Viruses are not cells!
Viral Genomes, Capsids, and Envelopes
Viral genomes may consist of either
o Double or single-stranded DNA
o Double or single-stranded RNA
A virus can be a DNA virus or RNA virus depending on its nucleic acid
Capsid= protein shell that encloses the viral genome
o Capsids are built from capsomeres
Virus= infectious particle consisting of genes that are packaged into a protein coat
Viruses lead a “borrowed life”
o Exist in a form between a life form and chemicals
There is no evidence as to where or how they came into existence
o This is because there is no fossil record
Viral Envelope
Some viruses can invade their hosts by use of a membranous envelope
o These are called viral envelopes
Viral envelopes are made from the host cells membrane
o They contain both viral and host cell molecules
The molecules that they take from the host include phospholipids, membrane proteins
They also have viral glycoproteins
A host that can be invaded is decided by the proteins on the surface of both the virus and the
host
Plant viruses- most have RNA genome and helical capsid
Bacteriophages- viruses that infect bacteria
o Have the most complex
o Elongated capsid head that holds their DNA
o A protein tail piece attaches the phage to the host and injects the phage DNA inside
Viruses Replicate Only in Host Cells
Viruses are obligate intracellular parasites
o This is because they can only replicate within a host cell
Every virus has a host range
o A host range is a limited number of host cells that a virus can infect
o The host range is determined by the proteins on the surface of the host and the virus
General Features of Viral Replicative Cycles
First, the viral genome enters the host cell and the cell begins to make viral proteins
The virus uses the hosts enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules
