AP Biology: Pathogens Review

Concept 19.1: A virus consists of a nucleic acid surrounded by a protein coat.

Structure of Viruses:

• They are not cells, but particles that contain genetic material enclosed by a protein coat.

  • can have double-strand DNA, singh-stranded DNA or RNA

  • multiple nucleic acids.

• capsid: protein shell that encloses the genetic material.

  • made from subunits called capsomeres 

• viral envelopes: accessory to infect better

  • made from host cell's membrane: phospholipid and proteins

  • made from virus origin: proteins + glycoproteins (proteins w/ carbs covalently attached.)

    • glycoproteins lock with host’s receptor proteins on cell surface —> allow entry of virus into host’s cell

bacteriophages: infect bacteria

Concept 19.2: Viruses replicate only in host cells.

• Viruses don't have any mechanism or equipments to make proteins: other than genes. 

→ ie. enzymes; metabolic equipment; ribosomes; etc..

• Can only replicate in a host all = parasites intracellular.

• host range: species the vins can infect

  • specific cells/tissues

  • developed by a recognition system: "lock and key fit" using receptor proteins on host’s cell surface

  • Enters host cell via endocytosis

Viral Replicative Cycles

1. enveloped viruses fuse with the host's plasma membrane.

2. virus proteins reprogram cell to copy and manufacture viral materials

3. host provides mechanisms to manufacture more viruses:

   1. Ie: nucleotides; enzymes; ribosomes; ARNAs, amine acid; ATP.

   2. DNA vines use host's DNA polymerase

   3. RNA viruses use their own RNA polymerase.

4. New viruses: come from nucleic acid & capsomeres

→ exit the cell and destroy the cell. → infect new cells.

Replicative Cycles of phages:

• The Lytic Cycle: results in dead host cell, → bacterium lyses break the cell and release.

• virulent phage: replicate only w/ lytic cycle.

• The Lysogenic Cycle: coexisting with host → replicate without destroying host 

  • lysogenic: active phages that lyse (break and destroy) their host cells

  • temperate phage: can do both Lytic and Lysogenic.

  • Prophage: joins DNA with host's - silent genome.

    • → host cell replication = replicates virus’ genome too.

Bacterial Defenses against Phages

• lysogeny allows bacteria and viruses to coexist

• Natural selection: bacteria mutants w/ surface proteins that aren't receptive to phages.

• Restriction enzymes: identify phage DNA as foreign and destroy it.

  • bacterium DNA is methylated to distinguish itself.

• CRISPR - Cas system:

• palindrome sequences: read the same forward & backwards.

• Spacer sequence: cell’s memory = corresponds with a phage that infected the cell 

• Cas: CRISPR associate nucleases that cut phage's DNA thru recognition w/ spacer sequences.

1. When a virus infects a cell their DNA is integrated into the spacer of CRISPR.

2. The cell uses spacer sequences to remember the vines' DNA for future immunity.

3. Host transcribes CRISPR region into RNA

4. RNA cut into pieces a bound by cas proteins

5. Cas uses that piece to identify phage's DNA and cut it.