Viruses

Question 1

How many viruses are there on the planet?

Answer 1

10^31

Question 2

What’s an obligate parasite?

Answer 2

Needs a host to replicate and survive

Question 3

What are the most abundant viruses on the planet?

Answer 3

Bacteriophages - viruses that specifically infect bacteria

Question 4

What are bacteriophages role in the ocean?

Answer 4

It is estimated that bacteriophages kill and lyse between 15% and 40% of the ocean’s bacteria every single day

Question 5

What is a viral shunt?

Answer 5

Bacteriophages recycle energy by lysing bacteria

Energy is not passed up the food chain but instead moved back down in the form of dissolved organic material when infected cells are lysed

Question 6

What is viral transduction? How is it beneficial? (Phage therapy)

Answer 6

Transfer of genes (allows beneficial genes to be shared)

8% of human DNA come from viruses our ancestors were infected with

Phage therapy – infecting bacteria with viruses (could be useful for antibiotic resistance)

Question 7

How big are viruses in comparison to bacteria?

Answer 7

0.02-0.3 um or 20-300 nm

1-2 um

Question 8

What components make up a virus?

Answer 8

Nucleic acid.

Protein coat (capsid) with surface receptors.

Lipid membranes (some).

Enzymes (some).

Question 9

What don’t viruses have?

Answer 9

Viruses don’t generate ATP

Don’t possess machinery for translation

Don’t have ribosomes and can’t independently form proteins from mRNA.

Steal lipid membrane from host cell when they leave the cell

Question 10

What is the capsid made up of?

Answer 10

Repeating subunits called capsomeres - viruses have symmetry because of how they’re assembled.

When new parts added they fold which protect nucleic acid inside.

Question 11

What are enveloped viruses?

Answer 11

Enveloped viruses take the lipid membrane from their host as they exit the cell.

Relatively few enveloped plant or bacterial viruses because of cell walls surrounding cell membrane.

Most enveloped viruses are animal viruses.

Envelope proteins attach to and infect animal host cell.

Question 12

What are surface proteins?

Answer 12

Attach to specific receptors on host cells.

Tricks cell into taking in the virus.

Corona virus S-Protein – ACE receptor

Influenza for example has Hemagglutinin (H) and Neuraminidase. The H binds to sialic acid on the host cell.

Question 13

What are the enzymes and their roles?

Answer 13

Neuraminidases - Cleaves glycoproteins and allows exit of virus from cell.

Lysozyme - Makes hole in cell wall to allow nucleic acid entry and exit.

Nucleic acid polymerases:
RNA-dependent RNA polymerases.

Reverse transcriptase (RNA-dependent DNA polymerase) in retroviruses - (converts RNA to DNA so it can be used for cellular machinery. )

Question 14

Viral replication - attachment and penetration

Answer 14

Find host – viruses specific in particular hosts and cell types determined by surface proteins

Surface proteins attach receptors to cell surface and trick cells into thinking virus is useful

Cell lets virus in – either genetic material moves in or whole virus.

Nucleic acid replicated by enzymes in the host cell

Question 15

Viral replication - synthesis

Answer 15

Synthesis of viral nucleic and protein

Viral replication depends on nucleic acid

Production of mRNA depends on starting point of virus – some viruses can produce it quicker

Aim to create rRNA to make viral proteins in host cell

Question 16

Viral replication - assembly and release

Answer 16

Assembly and packaging of new viruses

Cells lysis and release of new virions

Question 17

What infection particles are smaller than viruses?

Answer 17

Prion – proteins no genetic material, don’t reproduce just infold

Viroids – smaller then viruses

Question 18

What are viroids?

Answer 18

(RNA) 10-20 nm, smallest known just 220 bases
(Virus 20-300nm)

A few hundred nucleotides of RNA.

Does not code for any genes.

No protein coat.

Folded shape gives stability.

Can be replicated.

Mostly found in plants.

Causes stunted growth.

Question 19

DNA/RNA formation of virus

Answer 19

Virus = Small DNA/RNA molecule surrounded by a
protein coat. Codes for genes. Can be replicated. Can
be transferred from one organism to another.

Question 20

RNA of viroid

Answer 20

Viroid = Circle of RNA. A few hundred nucleotides,
Does not code for any genes. Can be replicated.

Question 21

DNA of plasmid

Answer 21

Plasmid = Small circular DNA molecule. Separate from
main genome. Codes for genes. Can be replicated. Can
be transferred from one organism to another.

Question 22

DNA of transposon

Answer 22

Transposon = DNA segments that can move from one
location to another in a cells genome.

48% of our DNA are jumping genes – transposon, can move to other cells

Question 23

What are the problems when investigating the origin of viruses?

Answer 23

No fossil record – too small

No ‘LUCA’ equivalent – No same gene present in all viruses.

High mutation rates -
Genome size vs mutation rate.

Gene transfer between host and virus.

Question 24

What are the main theories for the origin of viruses?

Answer 24

Virus first - RNA-world viruses were first lifeform, later left vents to infect cells.

Reduction – A virus was a small cell, that became an intracellular parasite, and then lost all other cellular components.

Escape hypothesis – Transposons or other ‘jumping genes’ ‘escaped’ from a cell and later developed into the viruses we know today.

Co-evolution – like virus first theory above, except some viruses became cells with the addition of membranes while others stayed as viruses (nutrients concentrated in vesicles as you move away from vent).

Chimeric-origins hypothesis – A mixture of virus-first and escape hypotheses, this theory says that the original virus pick up genes from hosts.