9) Viruses Summary_9744_2018

Genetics and Inheritance (9744) Viruses 2018 Prepared by: Mrs Selvamani Nair, Mdm Sharon Cross and Mrs Wong Seok Hui Raffles Institution 1  Are viruses are considered living or non-living? Living as they contain genetic material. However, non-living because they have no cellular organization and only show characteristics of living things when in host cell. Characteristics of living things include1) metabolic activity 2) cellular organization 3) ability to reproduce and grow in numbers 4) ability to respond to stimuli and adapt to environment  Why are viruses obligate parasites? This is because viruses, like obligate parasites, depend on host cells to complete their life cycle. Structure of Viruses Size: 10-300nm Bacteriophages Animal Viruses Enveloped T4 phage Lambda phage Influenza Human Immunodeficiency Virus (HIV) Genome  Nucleic acid that codes for synthesis of viral components and enzymes for viral replication & assembly  Can be either DNA/RNA, single/double-stranded  Double- stranded DNA  (-) strand RNA viral genome is complementary to viral mRNA  8 different segments of single stranded RNA associated with nucleoproteins  Each RNA segment is packed with 3 polymerase proteins which come together to form an RNA-dependent RNA polymerase enzyme complex which replicates and transcribes the viral genome in the host cell  (+) strand RNA viral genome has the same sequence as viral mRNA  2 identical copies of single stranded RNA bound to nucleocapsid proteins Capsid  Protein coat that surrounds and protects viral genome  Comprise subunits called capsomeres  Icosahedral capsid head  Present.  Present, conical shaped Enzymes reverse transcriptase, integrase and protease found in capsid Envelope  Phospholipid bilayer surrounding the nucleocapsid  Derived from host cell membrane  Embedded with viral glycoproteins involved in host cell recognition  Absent  Glycoproteins embedded in envelope: haemagglutinin (80%) & neuraminidase (20%)  Glycoprotein embedded in envelope: gp41 gp120 is attached to gp41 T4 phage Lambda phage Influenza Virus Antigenic Drift and Antigenic Shift Antigenic Drift : When the influenza virus replicates in its host cell, mutations frequently occur due to the poor proofreading mechanism of the viral RNA-dependent RNA polymerase and th