BIOLOGY 2EE3 Lecture Notes - Lecture 5: Henrietta Lacks, Cas9, Tissue Culture
23 Jun 2018
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Viruses
Small particles
Replicate only in host cells, take over transcription machinery of host cell
Obligate intracellular parasites
Diameter between 10-100 nm
5000-20000 nt in length
Viral Genome
Single or double stranded RNA or DNA
Most linear, some circular, some segmented
DNA and RNA are enclosed in a protective shell called a capsid
Nucleic acid and capsid are called nucleocapsids, which can be icosahedral or helical in shape
Enclosed viruses have a lipid bilayer around capsid
Non-enveloped viruses – don’t have a lipid bilayer around it, thus are naked.
HA Haemagglutinin cause red blood cells to agglutinate
NA Neuraminidases are enzymes that cleave sialic acid in glycoproteins
Influenza Virus
The influenza virus genome consists of eight RNA segments, each of which is enclosed with capsid protein.
The viral polymerase is bound to the end of each nucleocapsid. These nucleocapsids are surrounded by an
envelope. Embedded in the envelope are two viral proteins, hemagglutinin (HA) and neuraminidase (NA
Viral Replication
Recognition and attachment to host
Penetration of the host cells’ membrane, capside disassembly
Expression of viral genes to make proteins (synthesis)
Replication of viral genome
Assembly of new virus particles and exit from the cell
Replication Cycle - entry
Mechanism depends on host cell
Animal viruses - don’t have to pass through a cell wall
oRhinoviruses - common viral infective agents in humans, cause of common cold. The virus attaches to
cell receptor, endocytosis is initiated, endosome forms with the virus inside and the nucleocapsid
escapes to cytoplasm and uncoats to release genome
oHIV - lentivrius (slowly replicating retrovirus) causes acquired is AIDS. The virus attaches to cell
receptor, a conformation change in the attachment protein and bound protein initiates a membrane
fusion. The viral envelope fuses with plasma membrane and the nucleocapsid enters the cytoplasm and
uncoats to release genome
oRetrovirus - Retroviridae is a family of SS-RNA enveloped viruses that replicate in host cell through
process of reverse transcription. The virus attaches to cell receptor, endocytosis is initiated, endosome
forms with virus instead, the low pH of endosome initiates fusion of the viral envelope with the
endosome membrane, the nucleocapsids are released.
Plant, fungal and bacterial virus - have to pass through cell wall
oIn bacteria, tail fibers attach to receptors. Conformational changes in tail fibers bring base of the tail in
contact with host cell surface. Rearrangement of tail proteins allows inner core tube proteins to extend
down in cell wall. Contact with plasma membrane initiates a transfer of DNA through a pore formed
lipid bilayer
Virus Purification
Filtration to remove large cells and cellular debris
Purification of virus via differential centrifuge or gradient centrifuge
oDifferential centrifuge - take a sample of cell and virus suspension, centrifuge at low speed. Pellet of
whole and broken cells will be seen at bottom, transfer to a medium speed centrifuge, after, a pellet of
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Document Summary
Replicate only in host cells, take over transcription machinery of host cell. Single or double stranded rna or dna. Dna and rna are enclosed in a protective shell called a capsid. Nucleic acid and capsid are called nucleocapsids, which can be icosahedral or helical in shape. Enclosed viruses have a lipid bilayer around capsid. Non-enveloped viruses don"t have a lipid bilayer around it, thus are naked. Ha haemagglutinin cause red blood cells to agglutinate. Na neuraminidases are enzymes that cleave sialic acid in glycoproteins. The influenza virus genome consists of eight rna segments, each of which is enclosed with capsid protein. The viral polymerase is bound to the end of each nucleocapsid. Embedded in the envelope are two viral proteins, hemagglutinin (ha) and neuraminidase (na. Penetration of the host cells" membrane, capside disassembly. Expression of viral genes to make proteins (synthesis) Assembly of new virus particles and exit from the cell.
