chapter 11: genetics of viruses

Question 1

what is the cell theory?

Answer 1

• cells are the smallest unit of life
• all cells come from pre-exosting cells
• all living organisms are composed of cells

Question 2

why are viruses considered living organisms?

Answer 2

• 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

Question 3

why are viruses considered non-living organisms?

Answer 3

• 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

Question 4

obligate parasites

what is a feature that account for viruses being obligate parasites?
(nucleic acid)

Answer 4

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

Question 5

obligate parasites

what is another feature that account for viruses being obligate parasites?
(no enzymes)

Answer 5

• 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

Question 6

why are viruses parasitic

Answer 6

1. 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
2. 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
3. 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

Question 7

comparing viruses and living microorganisms

compare the nucleic acid type.

Answer 7

viruses: DNA or RNA
living organisms: both DNA and RNA

Question 8

comparing viruses and living microorganisms

compare the outer covering

Answer 8

viruses:
- a protein coat called capsid
- some with envelope

living microorganisms:
- membrane
- cell wall

Question 9

basic structures of viruses

what is the nucleic acid genome?

Answer 9

• 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

2. single stranded or double stranded
3. 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

Question 10

basic structures of viruses

what are capsid proteins?

Answer 10

• 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

Question 11

basic structures of viruses

what are envelopes and how are they formed?

Answer 11

• 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