SM L1 Flashcards
What is a virus?
-Microscopic particle that can infect the cells of a biological organism
-Cannot reproduce on their own. Can replicate themselves only by infecting a host cell - obligate intracellular.
-Consist of nucleic acids contained within a protective protein coat called a capsid.
-Metabolically inert
-multi-component agent with a nucleic acid genome. Contain DNA or RNA but never both
What is present within every virus?
-Genome: DNA or DNA
-Capsid: A protein shell enclosing the geneic material
-Some viruses have an envelope ( a lipid bilayer (host) decorated with viral proteins
Viral Specificity
-Most viruses limited to 1 host
-Determined by the ability of the virus to attach to the hose cell
-Depends whether host has appropriate enzymes and other proteins which the virus can use/hijack
-Whether replicated virus can be released
Origin of Virus
No definitive answer, but 3 theories
1.Viruses and cellular organisms evolved together…both originating from self replicating molecules present in pre-cellular world
2.Viruses were once cells that lost all cell functions, retaining only the ability to replicate using a host cell
3.Viruses evolved within the cell possibly from plasmids
What proves that viruses are not alive?
-Do not carry out metabolic processes.
-Cannot generate ATP
-Do not possess the necessary machinery for translation
-Do not possess ribosomes and cannot independently form proteins from molecules of messenger RNA
What are the two classification systems for viruses?
Two systems: Classical and Baltimore
Based on:
1. Nucleic Acid (DNA v RNA)
2. Capsid symmetry
3. Enveloped or naked
4. Size
Who developed the classical classification? What was it based on?
-The internation committee on taxonomy of viruses (ICTV)
-Based on nucleic acid type - wheter the nucleic acid is single- or double- stranded, with/without an envelope and capsid symmetry.
-Characteristics such as capsid shape, immunological properties and type of disease it causes are considered after.
Who developed baltimore classification? What is it based on?
-Definied by Nobel Prize winner, David Baltimore
-Based on classification of viruses into one of seven groups depending on a combination of their nucleic acid (DNA or RNA), standedness, and method of replication, relation to messenger sense RNA (mRNA or +RNA)
-Strandedness - single stranded/ double stranded.
What are the seven groups for baltimore classification?
Group 1: double-stranded DNA viruses
Group II: single-stranded DNA viruses
Group III: double-stranded RNA viruses
Group IV: positive-sense single-stranded RNA viruses
Group V: negative sense single stranded RNA viruses
Group VI: reverse transcribing Diploid single-stranded RNa viruses
Group VII: reverse transcribing circular double-stranded DNA viruses.
What are capsids?
Capsids - composed of protein subunits called capsomeres
May be single type or may be composed of different proteins, depending on the virus type
What are the two types of capsid symmetry?
Helical symmetry
Icosahedral symmetry or sometimes complex forms
Helical symmetry
Stacking of simple repeated symmetrical components (capsid protein that forms a ribbon-like structure/spiral around the nucleic acid)
Icosahedral symmetry
In viruses with icosahedral symmetry, the capsomeres are arranged in a highly symmetric structure that resembles a 20-sided polygon, known as an icosahedron.
20 equilateral triangles as faces.
Envelope
-Bilayer membrane outside the capsid consisting of lipids, proteins and carbohydrates
-Viruses acquire envelope as they bud/move out of a host membrane
-Envelopme may contain spikes, generally glycoproteins
-Some viruses may modify or mask their envelope proteins to make them less recognizable to the immune system.
-Extreme pH (>8, <6) may damage envelope as will increased temperature, freezethaw, disinfectants
Bacteriophages
Viruses that infect and replicate within bacteria.
