S.M Lecture 1 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
- Viruses and cellular organisms evolved together…both originating from self replicating molecules present in pre-cellular world
- Viruses were once cells that lost all cell functions, retaining only the ability to replicate using a host cell
- 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.
Where is DNA found within bacteriophages?
- DNA in polyhedral head: helical tail.
- E.g. T4 bacteriophage
Process of Viral Replication
- Adsorption: the attachment of viruses to host cells. Phage is adsorbed onto bacterial cell.
- Penetration: entry of virions into host cells. Phage penetrates bacterial cell wall and cell membrane. Phage DNA is injected.
- Synthesis: new nucleic acids, capsid proteins, and other viral components
- Maturation: assembly of newly synthesized viral components into complete virions
- Release: departure of new virions from host cells.
Replication of virulent T4 bacteriophage
A virulent phage undergoes a lytic cycle to produce new phage particles within a bacterial cell. Cell lysis releases new phage particles than can infect more bacteria.
All viruses must..
- Replicate viral genome within host cell
- Synthesize viral proteins
- Assemble viable progeny
- Disseminate to new cells and hosts
nucleocapsid
- Nucleocapsid is the term used to describe the complex of nucleic acid and capsid protein that makes up the viral core of a virus.
- It is the combination of the nucleic acid genome, either DNA or RNA, and the capsid that encases it.
- The nucleocapsid is the basic structural unit of most viruses and is responsible for protecting the viral genome from degradation and facilitating viral replication and transmission.
- Not every virus has a nucleocapsid, but it is a common feature of most viruses.
