VIRUSES WHAT IS A VIRUS? Viruses are not alive A virus in an obligate intracellular parasite Requires host cell to reproduce Can be seen at magnifications
provided by the electron microscope (they are microscopic) CHARACTERISTICS OF VIRUSES 1.) Contains a single type of nucleic acid: either DNA or RNA but not both
2.) Has a protein coat (capsid) surrounding the nucleic acid, some also have a lipid envelope around the capsid 3.) multiply inside living cells by using the synthesizing machinery of the host cell 4.) Cause the synthesis of specialized viral structures that can transfer the viral nucleic acid to other cells
5.) Have a specific host range SIZE OF A VIRUS Usually much smaller than bacteria must be smaller than the cells they infect: 20-14,000nm in length SIZE CONTINUED STRUCTURE OF VIRUSES
Virion = infectious viral particle: completely assembled with a protein coat surrounding the nucleic acid All viruses are made of at least 2 parts Inner core of nucleic acid Enclosed in protein capsid * Some also contain lipoprotein envelope
STRUCTURE OF VIRUSES 1.) Nucleic Acids: Either DNA or RNA, but not both Single or Double Stranded (SS or DS) if RNA, it can be plus sense strand (has codons) or minus/antisense (need to make complement sense strand for translation) If DNA- usually double stranded Linear or circular Genome is SMALL
Only a few genes (most have 6-10 genes) STRUCTURE OF VIRUSES CONTINUED 2. Capsid protein coat (protein shell) Surrounds the nucleic acid protects the virion in the external environment Aids in transfer between host cells Composed of subunits called capsomeres some capsids have protein-carbohydrate pointed projections called pentons
if pentons are present they are used for attachment to the host cell STRUCTURE OF VIRUSES *3. Envelope (not all viruses) Function is to protect the virion some viruses have an envelope around the capsid consisting of lipids, proteins and carbohydrates (cell membrane like) with envelope = enveloped virus
the envelope may be coded for by the virus or taken from the host cell plasma membrane some envelopes have carbohydrate-protein complexes called spikes which are used for attachment to the host cell if a virus does not have an envelope it is called a non- enveloped virus, naked
ENVELOPED VIRUS NON-ENVELOPED VIRUS Capsomere protein MORPHOLOGY/SYMMETRY The capsid can be distinct and sometimes identifies a
particular virus. It is constructed in a highly symmetrical manner Helical Cylindrical capsid, hollow Can be rigid or flexible Made up of a helical structure of capsomeres with the nucleic acid wound up inside Examples: Rabies virus, Ebola virus, tobacco mosaic virus (TMV)
Rabies Virus MORPHOLOGY/SYMMETRY Polyhedral Most are icosahedrons (icosohedral) 20 equilateral triangle faces and made from capsomeres 12 corners made form capsomeres called pentons which contain 5 protomers each Appear spherical
Examples: Adenovirus, Polio virus Polio virus MORPHOLOGY/SYMMETRY Complex Several types of symmetry in one virus Unique shape Examples: Bacteriophage: capsid and accessory structure Pox virus: no clear capsid, just several protein layers around
the nucleic acid Glass sculpture of pox virus REPLICATION Replication must occur in a host cell (multiply only when inside a living cell)
The viral genome codes for viral structural components and a few viral enzymes needed for processing the viral enzymes Everything else is supplied by the host: Ribosomes, tRNA, nucleotides, amino acids, energy etc. The DNA or RNA of the virus takes control of the host cell' metabolic machinery and new viral particles are produced utilizing the raw materials from the host cell.
REPLICATION Replication of viruses is studied in great detail in bacteriophages Bacteriophages are viruses that infect a specific bacteria Two possible types of infection cycles: 1.) Lytic cycle (virulent) Ends with the lysis and death of the host bacterial wall
2.) Lysogenic cycle Host cell remains alive, but carries the virus in its genome LYTIC CYCLE http://sites.fas.harvard.edu/~ biotext/animations/lyticcycle.html 1.) Attachment- phage contacts a bacterium (attachment to host) and uses the tail fibers to attach
to proteins on the bacterial cell wall LYTIC CYCLE 2.) Penetration/Entry- the phage injects its DNA into the bacterium The phage tail releases lysozyme to break down the bacterial cell wall The sheath contracts to drive the tail core through the
weakened cell wall and plasma membrane The DNA is injected into the bacterium through the tail core Uncoating- During or before penetration 3.) Synthesis of new virus particles (Multiplication) Once inside, host protein synthesis is stopped Virus has host make proteins and nucleic acid Virus directs viral nucleic acid replication and transcriptions and translation of viral genes (hosts cell transcription stops)
This results in a pool of viral genomes and capsid parts LYTIC CYCLE 4.) Assembly eclipse period the time of viral entry The bacteriophage DNA and capsid spontaneously assemble into complete virons 5-10 hrs DNA viruses 2-10 hrs RNA viruses
LYTIC CYCLE 5.) Lysis- release of virus and death of host cell A single virus can give rise to up to 1000 new virus particles from on host cell Virions will leave bacteria (host) Lysozyme encoded by viral genes causes the cell wall to break down The bacteria lyses releasing the virions Cycle will then repeat with new phages
THE LYSOGENIC CYCLE http://sites.fas.harvard.edu/~ biotext/animations/lysogeny.html The lysogenic phage infects the cell, but remains inactive in a stage called lysogeny 1.) the phage attaches to the host cell and injects DNA 2.) the phage genome circularizes At this point, the phage could begin a normal lytic cycle or it
can begin the lysogenic cycle/lysogeny THE LYSOGENIC CYCLE Latency- dormant state- unpredictability Viral DNA/RNA integrated into DNA of host = hidden DNA=provirus Can be reactivated in the future Factors that influence: stress, other viral infections, UV light
Example: fever blisters, chicken pox, HIV 2+ years CULTIVATION OF VIRUSES FOR STUDY Embryonated eggs Refer to handout given in class CELL CULTURES
Refer to handout given in class ANIMAL MODELS Refer to handout given in class
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