Viruses Flashcards
What is a virus ?
A small infectious, obligate intracellular parasite
Properties of life
- Cellular respiration
- Reproduction
- Metabolism
- Heredity
- Responsiveness
- Growth and development
What do host cells provide viruses with ?
- Nucleotides for nucleic acid production
- Amino acids & ribosomes - translation of proteins
- ATP for energy
- Golgi & endoplasmic reticulum - protein processing
How are viruses classified ?
- Nucleic acid - double/single stranded (DNA/RNA)
- Capsid symmetry - helical, icosahedral, complex
- Absence of envelope - naked, enveloped
- Genome characteristics
What does a virus contain ?
- Nucleic acid (DNA/RNA)
- Capsid - protein coat surrounding nucleic acid
- An envelope sometimes
- Capsule - sometimes
Virus nucleocapsid
- Highly symmetrical
- Composed of one protein - capsomers (capsid protein)
- Way in which capsomers (capsid proteins) are arranged
- Capsid is formed by self-assembly of the capsomers
Examples of viruses from each host
- Bacteria
- Plants
- Animals
Bacteriophages
- Infect and replicate within bacteria
- Highly virulent for bacteria
- Complex structure
- Classified by shape and nucleic acid
Structure of bacteriophages
- Head
- Collar
- Tail
- Tail pins
- Tail fibers
Life cycle of bacteriophages
- Attachment
- Penetration/entry (injection)
- Synthesis of nucleic acid and protein
- Assembly and packaging
- Release (lysis)
Life cycle of bacteriophages - Attachment
- Attachment proteins on the outside of the virus attaches to specific receptors on the host cell surface
- Virus receptors are the normal proteins found on the outside of a host cell
Lytic pathways
Infected bacteria re production factory for lambda bacteriophage and are lysed to release new bacteriophages
Lysogenic cycle
Bacteriophage genome is incorporated into a bacterial host chromosome by genetic recombination that occurs at a specific site on chromosome
Prophage
The name for bacteriophage DNA when it is inserted in bacterial host chromosome
Life cycle of viruses - Assembly and packaging
Viral components are assembled into virions
Life cycle of viruses - Release (lysis)
Host cell lyses and new virions are released
Example of plant viruses - Tobacco mosaic virus
- Helical symmetry
- 2130 identical capsomers
- Helical nucleocapsid
- Viral RNA
Example of non-enveloped viruses - Picornaviruses
It is a icosaheral virus …
Enveloped viruses
- The envelope is derived from cell, Golgi or nuclear membrane
- Enveloped viruses have to bud or burst from cell
- Examples are measles virus and influenza
Virus glycoproteins
- Matrix protein links nucleocapsid to membrane
- Virus glycoproteins are inserted in membrane
- Form spikes on outside of virus
- They are the main antigens for the immune system
The enveloped virus life cycle
- Envelope is a lipid bilayer with glycoprotein spikes
- Virus envelope fuses with cell membrane
- Synthesis of nucleic acid genome and viral proteins
- Endoplasmic reticulum of cell makes viral glycoproteins
- Site of virus assembly varies - Golgi complex/nuclear or cell membrane
- Progeny viruses bud from the cell
Cell membrane doesn’t kill host cell and remains intact
Cell cultures
- Primary - obtained form live animal tissue
- Diploid cell lines - homogenous cells from embryos , divide 100x and then die
- Continuous cell lines - immortal cell line, from cancerous cells
