Biology 11 - Part C Forero
Name: ______________________________ Date: ___________ Blk: _____
NOTES: VIRUSES
I. Viruses: Living or Non-living?
Five characteristics of living things (review!):
Characteristic
Viruses?
(✓/✗)
1. Made up of one or more cells
2. Reproduce
3. Grow and develop (and die)
4. Obtain and use energy (metabolism)
5. Respond to the environment
II. History of the virus
The name “virus” means poison in the Latin language.
- In 1892, Dmitri Ivanovsky describes a non-bacterial pathogen infecting
tobacco plants  filter experiment
- 1931: electron microscope allows for first imaging
- 1983: Pasteur Institute isolates HIV retrovirus
Timeline:
- Viruses are an important natural means of transferring genes between
different species, which increases genetic diversity and drives evolution.
- It is thought that viruses played a central role in early evolution, before the
diversification of bacteria, archaea and eukaryotes, at the time of the last
universal common ancestor of life on Earth.
Biology 11 - Part C Forero
III. Viral Structure
- Viruses are complicated assemblies of molecules, including proteins,
nucleic acids, lipids and carbohydrates.
- Virion: Protein coat wrapped around a nucleic acid core (DNA or RNA)
- Labeled sketch (choose one):
- Some shapes of viruses:
- helical
- polyhedral
- spherical
- complex
- Too small to be seen by light microscopy
- Most smaller than bacteria: 20 to 400 nanometers
IV. Classification
- 1962: Linnaean hierarchical system
- 2013: Baltimore Classification system
- based on the mechanism of mRNA production
Biology 11 - Part C Forero
V. Viral Reproduction
- Able to multiply only within the living cells of a host
- Viruses can infect all types of life forms, from animals and plants to
microorganisms, including bacteria and archaea.
Lytic Cycle:
1. Attachment:
- to host cell
2. Penetration:
- enter host (virus or just genetic material) pore or endocytosis
3. Replication and protein synthesis:
- direct host to make viral genetic material and protein
4. Assembly:
- of viral nucleic acids and proteins
5. Release:
- of new viral particles (virions) through lysis or exocytosis
Lysogenic Cycle:
- Steps 1 & 2
2a. Viral DNA inserted into DNA of host cell (prophage)
- may remain in host DNA for many generations
- activates to direct synthesis of new virions (Lytic Cycle steps 3-5)
- Factors that activate the virus:
- change in temperature
- UV light
- availability of nutrients
- chemicals
- Retroviruses: group of RNA viruses that insert a DNA copy of their
genome into the host cell in order to replicate (e.g. HIV).
- Viral Specificity: can only attack particular cells of a particular organism
- Bacteriophage: virus that infects and replicates within a bacterium
Biology 11 - Part C Forero
VI. Basic Lines of Defense
1. Primary:
- skin
- mucous membrane with cilia
- tears
2. Secondary:
- phagocytic white blood cells engulf viruses
3. Tertiary:
- white blood cells called lymphocytes produce antibodies
Antiviral drugs:
- nucleoside analogues (fake DNA building-blocks)
- viruses mistakenly incorporate them into their genomes
- the life-cycle of the virus is halted because the newly synthesized
DNA is inactive.
VII. Reducing Spread of Viral Infectious Disease
1. Cover your nose and mouth when you sneeze
2. Keep your hands clean
3. Do not share cutlery or beverages
4. Stay home if you are sick
5. Get the appropriate antivirus immunization  E.g. Hep A, B, C
*Stay healthy: get plenty of sleep, be physically active, manage your stress,
drink plenty of fluids, and eat nutritious food. The healthier you are, the
better your immune system will be at defending your body against a virus.
*GO BACK TO FIRST PAGE: check off right column
Biology 11 - Part C Forero
VIII. CURRENT VIRUS DEFINITION
- An infective agent that typically consists of a nucleic acid molecule in a
protein coat, is too small to be seen by light microscopy, and is able to
multiply only within the living cells of a host.
Evidence for living:
- carry genetic material
- reproduce  multiply
- evolve by natural selection
Evidence for non-living:
- not cellular  only made up of genetic material and protein coat
- do not grow or develop
- do not do anything in nutrient agar (grow nor die)
- if undisturbed, will never age or die
- do not obtain or use energy  host cell is does this for the virus
- completely at the mercy of their environment  do not respond to it!
* When a virus encounters a cell, a series of chemical reactions occur that
lead to the production of new viruses.
* These steps are completely passive: predefined by the nature of the
molecules that comprise the virus particle.
SO… WHAT DO YOU THINK?
Evaluate the evidence above and state your choice (circle one):
VIRUSES:
LIVING
OR
NON-LIVING