Class notes Biology microscopic organisms Biology 11, ISBN: 9780070915800

Microscopic Organisms
Chapter 2

Viruses
= Neither prokaryotic or eukaryotic
= Non-cellular structure
- No cytoplasm, membrane-bound organelles, cell membranes
= Cannot live independently outside of cells
- Outside a cell, they’re dormant
= Made of protein covered in genetic material (DNA or RNA)
= Must invade cells and alters the host’s genetic makeup to survive and reproduce
- Causes disease
= Doesn’t metabolize energy (NOT living)
= Each virus can only infect certain cell types
- Bacteriophage - Viruses that infect bacteria

Classifying Viruses
= Size + Shape of the capsid
- Ex: Spherical, rod shaped, hexagonal, phage, amorphous (shapeless)
Capsid - Outer protein layer that surrounds the genetic material

= Types of diseases they cause
- Ex: Viruses that infect humans are classified into 21 groups (genomes, sets of
genes, replication method etc.)
- Virus’s name is based on what it infects
- Ex: Hepatitis - Infects liver (hepa = liver)

Virus Reproduction
= Viruses aren’t cellular so they don’t engage in cell division
= They replicate within host cell
- Replication is when genetic material is copied before the cell produces
- Ex: Lytic cycle, Lysogenic cycle
- Occurs in cytoplasm of host cell

,Lytic Cycle
= Immediate disease symptoms
= Some viruses can be lytic at a reduce rate
- Ex: Flu virus was released on at a time
= RNA viruses only reproduce via lytic cycle
- Cannot join with host DNA
- RNA acts like mRNA (messenger RNA) to cause protein production

= Attachment
- Proteins on the surface of the virus bind onto the protein receptors on the
surface of the host’s cell membrane
= Entry
- Virus injects the genetic material into the host cell
= Replication
- Host cell makes more more viruses (viral DNA/RNA or proteins)
= Assembly
- New viral particles are assembled
= Lysis & Release
- Host cell breaks open and releases the new virus




Lysogenic Cycle
= Genetic material enters the nucleus of the host cell
- Becomes part of the host cell’s chromosomes
- Infect cell permanently has viral genes

, = Viral DNA that has become part of the host chromosome is a provirus (prophages
for bacterial viruses)
- Can invade a cell (not kill it)

= Attachment
= Entry (injection)
= Provirus Formation
- Viral DNA becomes part of the host cell’s chromosome
- If the host cell does cell division, the provirus replicates along with host's
chromosome
- Once the genes are activated, the provirus will leave the host’s chromosome
and engage in lytic cycle
- May be triggered by stress, environmental factors etc.
= Replication
= Assembly
= Lysis & Release




Viruses & Disease
= During lysis and release, the viruses burst out of the host cell and (usually) kills it