A2.3 Viruses

Question 1

Why are viruses obligate parasites

Answer 1

they require living cells to reproduce ( host cell)

Question 2

What are the features common to all viruses

Answer 2

extremely small size (20 - 400nm)
nucleic acid as their genetic material
a capid made of proteins surrounding nucleic acid
no cytoplasm
few or no enzymes

Question 3

what is advantage of small genome viruses

Answer 3

highly adapted for virus replication

Question 4

What can size of viral genome depend on

Answer 4

the type of host cell
Prokaryotic host cells tend to replicate rapidly due to high rate of binary fission

Question 5

Viruses with an RNA genome can possess:

Answer 5

positive sense RNA
Negative sense RNA
retroviruses

Question 6

What is positive sense RNA

Answer 6

they have viral mRNA, which can be directly into virus proteins

Question 7

what is negative sense RNA

Answer 7

They have RNA that must be transcribed into mRNA before protein production

Question 8

what is retroviruses

Answer 8

they have RNA that is reverse transcribed to make DNA which is then transcribed and translated into virus proteins

Question 9

what is the capsid

Answer 9

the protein coat enclosing the viral genome.
The protein subunits are known as capsomeres

Question 10

What shapes are capsids

Answer 10

they are usually helical, icosahedral but sometimes they have complex architecture

Question 11

why are some viruses classified as enveloped and non enveloped

Answer 11

enveloped viruses go through budding from host membranes and the host cell membrane contains viral glycoproteins

Question 12

what is the effect of non enveloping viruses on the host cell and itself

Answer 12

they usually rupture the membrane of the cell, causing cell death and damage to the organism
viruses can be more resistant to extreme pH, heat, dryness and disinfectants

Question 13

what is the advantage of enveloped viruses

Answer 13

they protect the virus from enzymes and other chemicals.
the glycoproteins on envelopes can also help viruses enter cells through receptor binding

Question 14

what are bacteriophages / phages

Answer 14

they are viruses that infect very specific bacteria
they go through lytic or lysogenic life cycle

Question 15

describe lytic life cycle

Answer 15

tail fiber of phage binds to receptors, injecting genome into cell ( penetration)
Viral DNA remains separate and replicates separately using host ribosomes
bacterial DNA broken down while viral DNA replicates and forms new particles
Components assembled to form phage
release of phage into environment involves lysis, resulting in cell death

Question 16

viruses that undergo lytic life cycle are referred to as?

Answer 16

virulent, because they actively infect and damage host cells

Question 17

describe lysogenic life cycle

Answer 17

phage infects bacteria by injecting DNA into cell
viral DNA incorporated into host cell’s DNA to form prophage
through incorporation, when the cell divides, the phages DNA is also replicated and passed on ( doesn’t produce new phages)

Question 18

what is a lysogen

Answer 18

bacterial host with prophage

Question 19

What happens to phage DNA under stressful conditions in the lysogenic life cycle

Answer 19

the phage DNA is removed from the bacterial chromosome and enters the lytic life cycle, leading to lysis, releasing phages into the environment

Question 20

what does it mean that viruses are considered polyphyletic

Answer 20

hypothesized that viruses probably arose numerous times in the past by one or more mechanisms

Question 21

What is the virus first hypothesis

Answer 21

suggests that viruses evolved before or co evolved with thir current host cells.
Viruses may have come from bits of RNA that had self complementary sequences which allowed it to fold like proteins
Gradually, they became more and more complex and could self replicate

Question 22

What is the escaped genes / progressive hypothesis

Answer 22

suggests viruses came from genetic elements that gained the ability to move between cells
A cell could’ve led to two mini cells which were a result of unequal cell division
One of them may have survived because of a protein coat, leading to replication and infection of organisms

Question 23

what is proof behind progressive hypothesis for viruses

Answer 23

they are DNA sequences that can move between chromosomes known as transposons

Question 24

What is the regressive/ reduction hypothesis

Answer 24

suggests viruses are remnants of cellular organisms
single cell organism forms mutual relationship with another cell
small organism lost its protein building structures
smaller organism able to replicate itself and infect new organisms, becoming a virus

Question 25

What are NCLDV’s

Answer 25

nucleocytoplasmic large DNA viruses which are argued to be the precursors of a group of viruses and were involved in the emergence of eukaryotic cells

Question 26

evidence behind NCLDV

Answer 26

newly discovered viruses which have large genome and presence of genes involved in many functions
nucleus in eukaryotic cells may have arisen from endosymbiosis where viral DNA became incorporated in ancestral eukaryotic cells
irreversible succession of gene losses may have been the dominant force in viral evolution

Question 27

why is virus evolution regarded as convergent evolution

Answer 27

because possible no single hypothesis is correct and it may have been a combination of all

Question 28

Why do RNA viruses have a higher rate of mutation than DNA viruses?

Answer 28

because errors are not corrected by proofreading

Question 29

features of influenza

Answer 29

enveloped virus with a genome made of negative sense, single stranded, segmented RNA
they infect epithelial cells of the respiratory tract
they recognize target cells by complementary binding to haemagglutinin in viral envelope and receptors on plasma membrane

Question 30

What is antigenic drifts

Answer 30

mutations of genes of viruses that can lead to changes surface proteins of the virus ( haemagglutinin)
the changes happen continually over time, leading to surface antigens that are no longer recognized by immune system

Question 31

what is antigenic shift

Answer 31

a sudden major change in viruses, resulting in new haemagglutinin and neuraminidase proteins, resulting in new subtypes
Process results in recombination, DNA from different virus strains are recombined to produce combinations
Possible because two different viruses can infect the same cell

Question 32

What type of virus is HIV

Answer 32

retrovirus
Virus RNA is converted into DNA under Reverse Transcriptase which is then integrated into the hosts cell nucleus to form a provirus

Question 33

features of HIV

Answer 33

high mutation rate in its genome
Patients often carry many different genetic variants, hard to combat different strains

Question 34

How can viruses with high mutation rates be combatted

Answer 34

combination of drugs that target different virus proteins are able to inhibit and prevent replication