A2.3 Viruses

Question 1

what are viruses?

Answer 1

Non cellular agents that infect cells and reproduce inside them.

Question 2

Features of viruses?

Answer 2

1) Small size
2) Fixed size
3)Nucleic Acid as genetic material
4) Capsid made of protein containing genes.
5) No cytoplasm and few/no enzymes when released from host cell.

Question 3

DNA and RNA in viruses can be what types of stranded?

Answer 3

Single or double stranded

Question 4

How is nucleic acid molecule length varied?

Answer 4

May be circular with no ends or linear with two ends.

Question 5

How is there variation in how single stranded viruses replicate and use dna in protein synthesis?

Answer 5

Three uses:
1) + sense RNA Viruses use their own DNA directly as mRNA
2) - Sense RNA transcribe own genes to make mRNA
3) retroviruses make double stranded DNA copies of RNA genes, then transcribe - sense strand of DNA to produce mRNA.

Question 6

What is an enveloped virus?

Answer 6

Virus that is covered in a membrane during lysis.

Question 7

Non enveloped virus?

Answer 7

Virus not covered in membrane during lysis.

Question 8

Where does a virus get its envelope?

Answer 8

Derived from the plasma membrane of host cell.

Question 9

Where do proteins and glycoproteins in the envelope come from?

Answer 9

From the virus itself in order to make contact and infect host cells.

Question 10

How are viruses diverse?

Answer 10

1)Shape and structure
2) Genetic material, double or single stranded nucleic acids.
3) may be circular or linear.
4) variation in replication of genes and use in protein synthesis.

Question 11

What is bacteriophage lambda?

Answer 11

1) non enveloped bacteriophage
2) Has one double stranded RNA molecule with + and - sense strands.
3) has 48,502 base pairs, 32 genes coding for 29 proteins.
4) can undergo lytic or lysogenic cycle
5) e-coli as host.

Question 12

how does bacteriophage lambda enter its host?

Answer 12

1) has proteins at tips of tail, binds to outer surface of host.
2) DNA of virus enters through tubular tail, and converts to circular form.
3) can then undergo lysogenic or lytic cycle.

Question 13

Disadvantage of a virus following lytic cycle?

Answer 13

Destroys the host, can spread to even more hosts, but if they kill entire population they are at risk of death.

Question 14

Disadvantages of a virus being virulent (severe)?

Answer 14

May be detected and destroyed by immune system, or lose host by killing it.

Question 15

What is the lytic cycle?

Answer 15

Virus reproduces and bursts out of host cell by killing it.

Question 16

What is the Lysogenic cycle?

Answer 16

Viral DNA integrates with bacterial DNA molecule, with no new virus being produced.

Question 17

What are the features of Covid-19?

Answer 17

1) Coronavirus, RNA virus with crown-like shape.
2) Enveloped, one single stranded + sense RNA molecule
3) 29,903 bases, 16 genes, code for 29 proteins, 4 structural, 6 enzymes.
4) uses zoonosis, passed down to humans from other species

Question 18

What are the features of HIV?

Answer 18

1) Retrovirus, converts its RNA genome to DNA after infecting,
2) Enveloped, contains enzyme reverse transcriptase, makes double stranded DNA copy. Which is then integrated to host cell chromosome.
3) Hosts are T-helper cells in immune system (human)

Question 19

What does the simpler structure of viruses suggest about the relationship between viruses and cells?

Answer 19

Suggests they evolved first, they have a universal genetic code and use code inherited by LUCA and we are able to deduce viruses evolved from cells from the progressive and regressive hypothesis.

Question 20

What is the progressive hypothesis?

Answer 20

1) states that viruses are built in a series of steps, modifying cell components.
2) This is due to virus like components being present in cells, e.g retrotransposons.
3)these are transcribed to produce RNA, which is translated to make enzymes.
4)enzymes make DNA copies of transposons and inserted to cells chromosomes.
5) Done by reverse transcription.
6) Similar to method of retroviruses integrating their own genes, so must have evolved from them.

Question 21

What are retrotransposons?

Answer 21

Series of nucleotides present in many eukaryotes. Genetic component that copy and paste themselves into different genomic locations

Question 22

What is the regressive hypothesis?

Answer 22

1) made by series of losing cell components.
2) Fits as viruses+bacteria show variation in complexity and self reliance
3) some small+simple, others much larger, more complex
5) larger viruses have enzymes of their own, perform functions so viruses can leave host.
6) bacteria are self reliant, but some parasitic bacteria can replicate inside host cell, losing ability to perform certain metabolic functions.
7) thought to have evolved from bacteria

Question 23

How does chlamydia relate to the regressive hypothesis?

Answer 23

1) Thought to be a virus, but are cells with wall and membrane.
2) Viruses likely to have evolved from intraparasitic bacteria, by loss of life functions

Question 24

What are the steps in the lytic cycle, in the case of bacteriophage lambda?

Answer 24

1) Attachment
2) DNA Entry
3) DNA replication
4) DNA transcription
5) protein synthesis
6) lysis
7) spread

Question 25

what happens during attachment phase? (lytic)

Answer 25

Proteins in tip of tail bind to maltoporin in the outer membrane of e coli.

Question 26

What is maltoporin responsible for?

Answer 26

Absorption of carbs.

Question 27

What happens during the DNA entry phase? (lytic)

Answer 27

Viral DNA enters host by pore in maltoporin, and another pore in inner membrane.

Question 28

What happens during the DNA replication phase? (lytic)

Answer 28

Ends of the linear viral DNA bind and become circular, and is replicated around 100 times.

Question 29

What happens during the DNA transcription phase? (lytic)

Answer 29

mRNA copies of viral genes are made, and then translated to make viral proteins.

Question 30

What happens during the protein synthesis phase? (lytic)

Answer 30

1)Proteins synthesised by host ribosome, are made for DNA replication and other functions.
2) Then large amounts of ‘head’ and tail proteins made to form a capsid, by self assembly. With one copy of viral dna molecule in each.

Question 31

What happens during the lysis phase? (lytic)

Answer 31

Viral proteins make holes through cell wall and membrane.

Question 32

What happens during the ‘spread’ phase? (lytic)

Answer 32

Cell contents then burst with about 100 viruses, spread to infect other host cells.

Question 33

What are the stages in the lysogenic cycle?

Answer 33

1) Attachment
2) DNA entry
3) integration
4) Cell division

Question 34

What happens during the ‘attachment’ phase? (lysogenic)

Answer 34

Proteins in tip of tail bind to maltoporin

Question 35

What happens during the ‘DNA entry’ phase? (lysogenic)

Answer 35

DNA enters via pore in maltoporin and another in inner membrane

Question 36

What happens during the ‘Integration’ phase? (lysogenic)

Answer 36

Viral DNA becomes circular, inserted into specific position of bacterial DNA by integrase enzyme. After, virus becomes a prophage.

Question 37

What is a prophage?

Answer 37

the genetic material of a bacteriophage, incorporated into the genome of a bacterium and able to produce phages if specifically activated. Single length of DNA

Question 38

What happens during the ‘Cell division’ phase? (lysogenic)

Answer 38

When host replicates DNA prior to division, also replicates prophage, Both daughter cells contain prophage.

Question 39

What does the word ‘temperate’ mean?

Answer 39

when a virus remains in cycle and causes minimal harm.
remains undetectable, prophage is inherited but cannot spread. However can switch to the lytic cycle.

Question 40

Why does a virus undergoing the lysogenic cycle switch to the lytic cycle?

Answer 40

To happen, genes must be activated due to stimuli in/out of cell.

Question 41

How can a temperate virus benefit the host cell?

Answer 41

DNA may include genes transferred from a previous host. Genes become integrated into bacterial DNA along with viral genes, increasing genetic diversity of host.

Question 42

What are the reasons for why a virus can undergo heritable changes, evolving?

Answer 42

1) Evolutionary change only happens between one gen and the next, can be less than an hour for viruses.
2) Evolution depends on genetic variation, mutation rates are high in viruses.
3) Evolution, result of natural selection acting on variation in a population, intensity is high, as host has mechanisms to destroy virus.

Question 43

Give an example of natural selection acting on variation on viruses.

Answer 43

Viruses with a new variant protein in the capsid or in enveloping membrane can evade immune system and multiply.

Question 44

What is influenza?

Answer 44

1) Enveloped virus, - sense single stranded RNA.
2) Replicates genetic material using RNA replicase, doesn’t proof read and correct issues, so high mutation rate.
3) Genome has 8 seperate RNA molecules
4) Transmitted between species, mainly birds and humans.

Question 45

How does influenza have rapid evolution?

Answer 45

1) RNA replicase doesn’t proof read, so has high mutation rate allowing for diversity.
2) New strains can appear if two different strains with their RNA molecules combined.
3) hence why an individual can contract flu quickly, and why vaccine contains several strains.

Question 46

What are the two proteins in the enveloping membrane of influenza act as?

Answer 46

1) Haemagglutinin, binds to host cell.
2) Neuraminidase, helps release with host cell.
3) Both act as antigens, and can act together in combinations

Question 47

How does HIV have a rapid evolution?

Answer 47

1) uses reverse transcriptase, converting single stranded RNA genome to DNA, does not proofread/correct errors
2) High mutation rate, also caused by cytidine deaminase, made by host. Leading to more strains
3) if host invaded by multiple, viral genes can combine leading to more diversity.

Question 48

How does rapid strain generation help virus evade?

Answer 48

1) Most infections become chronic (persist)
2) mutations in binding proteins on surface in env gene of HIV allows to access more cell types as hosts.
3) Can evolve to become resistant to antiretroviral drugs.