Virus Flashcards
What are the features of viruses?
- simplest biological system
- very small 20-30nm
- non-cellular particles lacking cytoplasm or organelles
- are obligate intracellular parasites that can affect both euk and prok
- made up of nucleic acids and proteins
Why are viruses obligate intracellular parasites?
They lack the cellular machinery for self-reproduction
They can reproduce only by invading host cells and hijacking their cellular machinery
What are enveloped and naked viruses?
Enveloped viruses : have an additional membrane (lipid membrane) outside the protein coat
Naked viruses : does not have the additional membrane
What is the capsid / capsid coat?
Protein coat enclosing viral genome
Made up of protein subunits - capsomeres
What is the nucleocapsid?
Capsid coat + enclosed nucleic acid
What does the central core of nucleic acid of the virus contain?
Viral genes coding for the synthesis of different viral proteins
- viral enz involved in viral reproductive cycles
- components of protein capsid coat
- (for enveloped virus) glycoproteins inserted into viral lipid envelop
- other proteins that help stabilise the viral genome (some)
Features of nucleic acid
DNA or RNA
Linear or circular
Single or double stranded
What are the functions of capsid coat?
- protect the nucleic acid from damage or digestion by enz
- attach the virus to receptors on specific host cells via special sites or structures on capsid coat (nakes virus)
- enable the virus to penetrate the cell membrane of the host cell or (in some cases) inject viral nucleic acid into the cytoplasm of the host cell cytoplasm
- some contain viral enz molecules
What is the lipid/viral envelope composed of?
How is it formed (briefly)
Composed of phospholipids and cholesterols derived from the host cell membrane by budding when newly formed viruses leave the cell
What are glycoprotein spikes?
Incorporated viral proteins in the lipid envelope, by the virus
What is the function of glycoprotein spikes?
Attach the virus to receptors on specific host cells
What are the characteristics that define a living organism? (7)
- Display order - organised structures
- Exhibit homeostasis - maintaining constant internal environment
- Respond to stimuli from env
- Display heredity - controls growth and development
- Metabolism - ability to acquire, process and utilise energy
- Reproduce
- Evolve and adapt
Are viruses living or non-living?
They are on the borderline of being living and non-living
- do not appear alive until it affects host cell
What are the non-living characteristics of viruses ? (4)
Cannot reproduce on their own
- they lack the machinery to synthesis nucleic acids or proteins (strict intracellular parasites)
Do not carry out metabolism on their own
- unable to generate usable forms of energy like ATP
Do not have any sensory components or response mechanisms
Do not have internal mechanism to maintain homeostasis, lack cytoplasm = lack internal env to be maintained
What are the living characteristics of viruses?
Have organised structure and shape
- capsid coat
Are able to reproduce at a fast rate
- within host cells, can direct host cell machinery to synthesis its viral components
Contains genes and shows inheritance
Evolution occurs
- undergoes spontaneous mutation
Respond to certain external signals to trigger reproduction in host cell
What are the three principles of cell theory?
- all living organisms are composed of one or more cells
- The cell is the smallest and most basic unit of life
- Cells grow from pre-existing cells
How viruses challenge cell theory ?
- Viruses are non-cellular
- all viruses have the basic structure made up of a core of nucleic acid surrounded by protein and this structure lacks cytoplasm and organelles - Virus are smaller and more basic than a cell but are able to display some characteristics of life
- Viruses do not undergo cell division from pre-existing viruses
- they infect their respective host cells and make use of host cell machinery and raw materials to synthesis new viral components for assembly into many new viruses
What are the 5 stages of the viral reproductive cycle?
- Attachment / Absorption
- Penetration / Entry
- Replication
- Assembly and maturation
- Exit / Release
What are bacteriophages (phages) ?
Viruses that infect bacteria
They multiply inside bacteria by making use of some or all of host cell machinery
What are the features of bacteriophages ?
- 24-200nm in length
- all contain a head structure made up of capsid coat
- linear, double-stranded DNA
- many have attached tail
- may have base plate at the end of the tail with attached tail fibres
What is the function of tail in bacteriophage?
Tail consists of a hollow tube through which the viral DNA enters host bacterium
During infection, contractile sheath surrounding the tail contracts to inject viral DNA into host
What is the function of the base plate at the end of the tail of a bacteriophage with attached tail fibres?
Involved in the attachment of the phage to the bacteria cell
What are the two alternative mechanisms viruses reproduce by?
(What are the two cycles?)
Lytic cycle
Lysogenic cycle
What does the lytic cycle lead to ?
Death of the host cell