chapter 17 - virsues

Viruses consist of

• Nucleic acid genome of DNA or RNA

• Single stranded or double stranded

• Single molecule either linear or circular

• Encodes 3-1000 genes

Capsid

• protein coat that encloses viral genome

• Consist of a small number of different proteins arranged in a specific pattern

• Some contain enzymes

Capsomere

Protein subunits that make up the capsid

Viral envelope

• surrounds the capsid

• Derived from the host cell membrane

How are viruses Obligate intracellular parasite

• they can only replicate within the host cell

Host range

• viruses ability to only infect a limited number of species

• Viruses that infect multicellular organisms only infect specific tissues

Basic replication cycle of virus

1. Virus attatches to host cell as viral proteins on capsid or envelope bind to receptor molecules on the host cell

2. Enveloped viruses enter through fusion with the host cell membrane (same molecules) if there’s no envelope viruses enter through injection or specific proteins/enzyme that aid in fusion

3. Viral genome is then replicated by host cell enzyme producing multiple copies

4. Viral proteins are expressing using host machinery for transcription and translation

5. Viral genome and capsid proteins self assemble to produce new viruses

6. Viral particles exit host cell

Bacteriophages

Viruses which only infect bacteria

2 mechanisms for phage replication

1. Lytic cycle - death of host cell

2. lysogenic cycle - coexistence of the phage and its host

Lytic cycle steps

1. Phage attaches to host and injects its genome through its tail

2. Phage gene encodes a hydrolytic enzyme that destroys host genome and is protected by a having a modified form of cytosine

3. Phage DNA directs replication of genome and phage protein synthesis using host machinery

4. Phage particles self assemble as components are produced

5. Phage enzyme damages host cell wall resulting in lysis of cell (bursts)

6. Phage particles exit/release

Why does Lytic cycle occur

Due to stress factors that cause it to kill its host and leave

How many phage particles get released by the Lytic cycle and how long does this take in favourable conditions

100-200 phage particles, taking 20-30 mins at 37 degrees Celsius

Lysogenic cycle steps

1. Phage genome injects DNA and circulizes

2. Genome intergrates into host genome and is replicated with each cell division of host

3. Viral DNA is now called pro phage

4. Maintence of hysogenic cycle requires production of viral proteins self assemble to prevent transcription of prophage

Lysogenic effects

• may alter phenotype of host cell

• Induce bacteria to produce toxins which can harm their human hosts

Temperate phages

• phage wil use lysogenic as long as environmental conditions are favorable for bacterial growth and replcaiation

• Environmental stress such as chemicals or high energy radiation will induce prophage exit from the bacterial genome and enter Lytic cycle

Human immunodeficieny virus

HIV is the retrovirus which causes aids

• HIV is enveloped virus containing a double copied ssRNA gene one and 2 molecules of reverse transcriptase

Retrovirus

Transcribes its RNA into DNA then incorporates into the host genome

HIV replication

• DNA enter nucleus and integrates into host genome becoming a provirus

• Provirus becomes permanent resident of the cell within the host genome

• Provirus is transcribed to produce new viral genomes and mRNA for the production of viral proteins

• Viral particles self assemble into capsids containing 2 copies of the viral RNA genome and 2 molecules of reverse transcriptase, which all buds up with envelope to produce new virus

• Viral glycoproteins are transported to the cell surface and viruses exit the cell

• Provirus remains within host cell