Download Notes: Viruses

Notes: Viruses
SB3d. Compare and contrast
viruses with living organisms.
Characteristics of Living Things:
Made of cells
Grow & develop
Use energy
Respond to stimuli
Can reproduce
If something lacks any of these traits then it is considered
NONLIVING.
Viruses are considered Nonliving because:
Viruses are not made of cells
Viruses do not use energy
Viruses do not grow & develop
Viruses can only reproduce when they are inside of a living
host cell
Viruses can form crystals and remain viable for a long time
Virus Structure
Viruses are infectious particles that
contain nucleic acid inside a protein coat.
Virus Structure
The nucleic acid can be DNA or RNA
• (they don’t have both like living cells)
The protein coat is called a capsid.
• The shape of a virus is determined by the proteins in
its capsid
May also have a viral envelope formed
from the host cell membrane.
• It may be covered with spikes used to recognize and
attach to cells it infects.
Virus
Structure
Look in your
book at page
575
Reproduction in Viruses
Viral Replication- the process by which
viruses reproduce.
Reproduction in Viruses
Lysogenic Cycle:
1. virus attaches to a host cell
2. virus injects its DNA or RNA into the host cell
3. viral DNA is added to the host cell’s DNA
4. viral DNA is copied along with the cells DNA
5. viral DNA is passed on to new daughter cells
At some point the DNA activates itself and enters
the lytic cycle.
Reproduction in Viruses
Lytic Cycle:
Virus attaches
Virus injects DNA
host cell begins making new viral parts from viral mRNA
New viruses are formed
Cell is destroyed and new viruses are released from the cell
These viruses then go and infects new cells
Many viruses cause disease
Difficult to cure because they replicate very
quickly
Undergo frequent mutations making it hard to
find a way to destroy the virus
Medications often attack & harm healthy cells
The best defense we have are vaccines- a
piece of the virus is injected into you and
your body produces antibodies against it.