Download Viruses*

Viruses*
 Nonliving: no cells, don’t use energy, reproduce on its own
 Nucleic acid wrapped in protein
 Obligate endoparasites: only reproduces inside host cells
 Called bacteriophages or Phage* if it infects bacteria
 Structure
o Genome: DNA or RNA; double or single stranded, contains
info to make a virus
o Capsid: protein shell, surrounds genome
o Envelope: membranous cover over capsid; derived from host,
contains viral proteins and glycoproteins (proteins+carbs),
Protects against the host defenses
o Shapes: Rods, Polyhedrons (sphere–like) & Complex (head,
tail, tail fibers)
 Reproduction
o Lytic cycle:* results in death of host cell, virulent viruses (disease causing)
1. Phage (virus) attaches; capsid binds at receptor site
2. Nucleic acid is injected

DNA: proteins synthesized

RNA: retroviruses (backward), HIV
▫ Reverse Transcriptase:* copies RNA into DNA
3. DNA is used to make parts for virus

Enzymes to kill cell & make genome

Protein parts (capsomeres) for capsid
4. Phage assembled, capsid around nucleic acid
5. Viruses emerge destroying cell (lysis)
▫ Host cell may defend itself with restriction enzymes that chop up virus genome
o Lysogenic cycle:* does not result in the killing of the cell – temperate viruses
1. Virus binds the cell; capsid binds a receptor site
2. Nucleic acid is injected
3. DNA (or RNADNA) molecule incorporates itself into a specific site of host chromosome =
prophage*

Will be copied every time the host cell reproduces, can remain in your DNA
4. At some time virus comes out and goes through lytic cycle
 Diseases
o Polio, AIDS, chicken pox, scabies, colds, flu, hepatitis, herpes, mononucleosis
o Vaccine:* inactivated form of a virus (protein only), triggers immune response without illness. Body
is prepared when virus attacks, kills before disease sets in
o Several drugs now available to slow virus reproduction
o Cancer: uncontrolled growth and division of cells

Virus inserts & excises during lysogenic cycle, can cause damage to host genes that can be
passed on

Oncogenes: genes involved in triggering
 Origins: Evolved after cells, probably from a host cell genomes that could move