Transcript Viruses - RMC Science Home

Viruses
What are viruses?
• small, non-living, infectious particles
containing genetic material in the form of
DNA or RNA with a protein capsule, called
the capsid.
• Non-cellular particle
• Over 4000 virus species classified but
scientists believe that there are millions
• Highly specific about what they invade
– ex: plant viruses infect only plant cells
• specialize in the cell types they infect.
Features of Viruses:
• Viruses have no cytoplasm
• Less than 0.1µm in diameter (100s of 1000s
of viruses could fit inside a human cell)
• Cannot grow or reproduce on their own
• Do not produce energy; do not create waste
• Take control of the cell that they infect
Viruses – Living or Non-Living?
• Considered non-living because:
– Do not metabolize energy
– Do not perform cellular respiration
– Cannot grow
– Cannot reproduce on their own (act as
parasites)
Shapes of Viruses
Viral Diseases
• Mild viral diseases such as the common
cold or chicken pox, or more severe viral
diseases such as AIDS or cholera, are all
infectious.
• Epidemic: outbreak confined to a
geographical region
• Pandemic: widespread, global epidemic
Bacteriophages
Vaccination
• preventions against viruses.
• mixtures that contain weakened forms or parts of
dangerous virus.
• they trigger a response by the immune system
without causing an infection.
• creates a form of chemical ‘memory’ allowing
immune system to react quickly if the individual
ever comes in contact with the real virus.
3 ways Viruses enter living cells:
1) enter bacterial cells by punching a hole in the cell
wall and injecting its DNA
2) enter plant cells through tiny rips in the cell
wall
3) enter animal cells by endocytosis
Infectious Cycles
• Viruses must invade a living host cell to
reproduce
• There are two ways to do this
– 1. Lytic Cycle
– 2. Lysogenic Cycle
1.Lytic Cycle
• the virus enters the cell, replicates itself hundreds
of times, and then bursts out of the cell destroying
it
2. Lysogenic Cycle
• the virus enters the cell, viral DNA integrates with
the host DNA and becomes inactive, the host
functions normally
• an environmental change may then cause the virus
to enter the Lytic Cycle
Differences Between
Lytic and Lysogenic Cycles
In the Lytic Cycle:
• Viral DNA destroys Cell DNA, takes over cell functions and destroys
the cell.
• The virus replicates and reproduces.
• There are symptoms of viral infection.
In the Lysogenic Cycle:
• Viral DNA merges with cell DNA and does not destroy the cell.
• The virus does not reproduce.
• There are no symptoms of viral infection.
The Origin of the Virus
• The origin of the modern virus is unclear.
• Two hypotheses exist:
1)They could be runaway stretches of nucleic acid from a
larger organism that detached and became active, therefore
new viruses are forming frequently and many do not have
ancestors
2)Viruses once lived outside of host cells, but over time due
to their parasitic lifestyle, they lost the genes necessary to
live outside the host
Videos
• Flu Virus
• Ebola
• HIV/AIDS
Homework:
• Why are viruses considered to be nonliving?
• What characteristics do viruses share with
all living things?
• Which viral diseases are quite common and
associated with the winter season?
• Explain the relationship between a virus’s
dormant period in a cell and the appearance
of cold sores.