genetics of viruses Flashcards
what is the cell theory?
- cells are the smallest unit of life
- all cells come from pre-exosting cells
- all living organisms are composed of cells
why are viruses considered living organisms?
- all viruses have a common heriditary molecule (DNA or RNA) that can result in the production of polypeptides
- viruses are able to replicate to pass on genes from one viral generation to the next
- some viruses contain viral enzymes which may be used in their reproductive cycles
- viruses are also able to evolve
why are viruses considered non-living organisms?
- viruses are unable to replicate indepedently outside the host cell
- it is metabolically inactive when outside its host cell
- viruses are acellular and challenge the cell theory
obligate parasites
what is a feature that account for viruses being obligate parasites?
(nucleic acid)
they only contain one type of nucleic acid as genome
- either DNA or RNA but not both
- However, most viruses need bothe RNA and DNA like prokaryotes and eukaryotes
- DNA is needed for the replication of genome
- and RNA is needed for translation for protein synthesis
- hence, viruses use their host’s cell machinery to synthesise both types of nucleic acids
obligate parasites
what is another feature that account for viruses being obligate parasites?
(no enzymes)
- viruses are unable to synthesise essentaial molecules like ATP
- so they use host cell’s raw materials or metabolic machinery to synthesise them:
- ATP
- amino acids, nucleotides
- enzymes involved in protein synthesis ( aminoacyl tRNA synthesase, DNA polymeras, RNA polymerase and ribosomes
why are viruses parasitic
- they take over the host cell’s metabolic machinery to synthesise and assemble new viral componenet to produce progeny viruses
- this disrupts the host cell aciticities upon infecting host cell
- may cause death of host cell upon release of progeny virus after infection
- therefore some viruses cause diseases - exhibit specific host range
- each type of virus infects only a limited variety of hosts, though they can increase host range via antigenic drift and shift
- antigenic drift: small mutations in a virus’s surface proteins can help it evade immune responses, potentially allowing virus to infect new hosts, expanding its host range
- antigenic shift: when two different strains of a virus combine, the resulting new subtype may have the ability to infet a broader range of hosts, significantly increasing the virus’s host range - exhibit viral specificity
- each type of virus infects a limited variety of cells within the multicellular host as a consequence of virus binding to specific cell surface receptors molecules found on only certain type of cells
comparing viruses and living microorganisms
compare the nucleic acid type.
viruses: DNA or RNA
living organisms: both DNA and RNA
comparing viruses and living microorganisms
compare the outer covering
viruses:
- a protein coat called capsid
- some with envelope
living microorganisms:
- membrane
- cell wall
basic structures of viruses
what is the nucleic acid genome?
- it is either DNA or RNA
- determines whether viruses are divided into RNA or DNA viruses
nucleic acid can be:
1. linear, circular or segmented
- single stranded or double stranded
- for ss RNA genomes, they can be positive sense or negative sense
- positive sense ss RNA can act directly as mRNA for protein translation
- negative sense ssRNA is complemetary to positive sense RNA
- negative sense cannot be translated directly, it needs to be transcribed into a positive sense
basic structures of viruses
what are capsid proteins?
- they are composed of individual protien subunits called capsomeres
- it is arranged in a precise and repetive pattern around the nucleic acid genome
function:
- the capsid protein determines the specific shape of the virus, together with the envelope in enveloped viruses
- provides the only protection for the nucleic acid genome in non-enveloped viruses
- for some viruses, the capsid has glycoprotein for attachment to host cell’s receptors for entry
basic structures of viruses
what are envelopes and how are they formed?
- a typical phospholipid bilayer membrane outside capsids in enveloped viruses
- the envelope is connected to the capsid by a layer of matrix proteins
how it is formed:
- as the virus buds, the host cell surface membrane surrounds the nucleocapsid and other proteins encoded by the virus forming the envelope
- as the envelope is acquired from the host cell’s cell surface membrane
- the envelope consists of phospholipids, cholesterol and glycoproteins from the host cell
- the envelope also contains glycoproteins encoded by virus genome
- which are syntheised and embedded on the host cell surface membrane
- these glycoproteins are important for the attachemnt of virus to specific host cells
