Summary Study Unit 2 Viruses Part 1

GENETICS 344: ADVANCED TOPICS IN GENETICS

STUDY UNIT 2: VIRUSES

LECTURE 1

1. WHAT IS A VIRUS?
• Lie between:
o Large molecular complexes
o Very simple biological entities
• Contain some of structures & exhibit some of activities that are common to organic life
o Missing many other:
▪ Don’t respire
▪ Don’t display irritability (respond to stimulus)
▪ Don’t move
▪ Don’t grow
• Composed of copy of 1 type of nucleic acid encased within protein capsule
• Lack most of internal structure & machinery which characterize ‘life’
o Biosynthetic machinery necessary for reproduction
• In order for virus to replicate  Must infect suitable host cell
• Continued survival  Spread between individuals of its host
• Definition: (rybicki.blog)
o Acellular organisms
o Genomes consist of nucleic acid
o Obligately replicate inside host cells
o Using host metabolic machinery & ribosomes to form pool of components
o Assemble into particles = VIRIONS
▪ Serve to protect genome
▪ To transfer it to other cells

2. VIRUS STRUCTURE
• Virus particle = VIRIONS
o Inert carriers of genome
o Assembled(replicated) inside cells from virus-specified components
▪ Don’t grow & Don’t form by division
o Can be regarded as extracellular phase of virus
▪ Take viral genomes from cell to cell
▪ Protect genome in inhospitable environments where virus can’t replicate
o Virion = Entire virus particle consisting of:
▪ Outer protein shell = capsid
▪ Inner core of nucleic acid (DNA / RNA)

,2. VIRUS STRUCTURE
• Difference between Virion & Virus?
o Viruses can be found intracellular / extracellular
o If found extracellular  Virus = Virion
o Virion contains protein coating = capsid
▪ Surrounds core of virus containing nucleic acid
• Basics:
o Viruses consist of:
▪ Capsid = Composed of many capsomeres
• Protective protein shell
• Provides specificity to virus
▪ Nucleic acid = RNA or DNA
• Genome
• Infectivity
▪ Sometimes envelope = Lipid membrane
• Virus can be inactivated by exposure to fat solvents
• Sometimes with additional glycoproteins = spikes
o Viruses = DIVERSE wrt:
▪ Morphological characteristics
▪ Genome compositions
▪ Replication strategies
▪ Gene expression
o Many virions are spheroidal (icosahedral)
▪ Regularly arranged units = capsomeres
▪ Nucleic acid is densely coiled within
o Other virions have capsid consisting of irregular number of surface spikes
▪ Nucleic acid is loosely coiled within
o Virions of most plant viruses
▪ Rod-shaped
▪ Capsid = naked cylinder – lacking fat membrane
▪ Straight / helical rod of nucleic acid within
• Compositions
o Simplest viruses  Non-enveloped viruses = Capsid & Nucleic acid
o Enveloped viruses  Envelop usually acquired by budding
▪ Animal viruses are frequently enveloped
• Nucleic Acid

TYPE OF NUCLEIC ACID
VIRUSES PROKARYOTES EUKARYOTES
• RNA • Single component circular / • Multicomponent linear dsDNA
• ssDNA linear dsDNA • Semi-conservative replication

,2. VIRUS STRUCTURE
• Nucleic Acid
o Inside virion (virus particle)
o DNA  Circular / Linear
o RNA  Linear
▪ Only viroids are circular




GENOME CAN BE:
MONOPARTITE Single molecule of nucleic acid Soybean mosaic virus
BIPARTITE 2 molecules of nucleic acid Tobacco rattle virus
MULTIPARTITE • AKA segmented • Influenza virus
• 2 or more separate but • Bunyaviridae
incomplete particles
o Representation of genome organisation of monopartite viruses




o Representation of genome organisation of multipartite viruses

, 2. VIRUS STRUCTURE
• Nucleic Acid

RNA CAN BE:
POSITIVE • Can serve directly as mRNA
SENSE Translation Virus structural proteins
• Positive sense RNA (+) →
Enzymes
NEGATIVE • Can’t serve directly as mRNA
SENSE Transcription with virus
RNA-dependent RNA polymerase
• Negative sense RNA (-)→ Positive sense RNA (+)
Translation Virus structural proteins
o Positive sense RNA (+)→
Enzymes