4. Viruses And Viral Replicaiton

Question 1

What is a virus? 6

Answer 1

1. Very small, infectious, obligate, intracellular parasites
2. Virus’ particles, virions, are non living and a free living virus is yet to be discovered
3. They are an assemblage of chemicals, mostly proteins, that can’t replicate without a host
4. No known virus contains ribosomes, mrna is translated by host ribosomes, therefore they can’t produce their own proteins
5. Can’t generate ATP
6. Unusual genomes. Some are rna and there are some with single stranded genomes that can be dna or rna. This suggests viruses were some of the first biological entities

Question 2

Describe the sizes of viruses and virions. 6

Answer 2

1. Viruses are smaller than human rbcs
2. Virions range from 20-300nm
3. An electron microscope is needed to view most, but a few can be viewed with a light microscope
4. Megavirus is 680nm, has 1200000 base pairs and is dsDNA. Fairly big, freshwater, affect amoebae.
5. Poliovirus is 30nm, 7kb (ssRNA). Can paralyse from neck down
6. Smallpox is 200nm, 186 000 bp (dsDNA). Pandora virus is very large

Question 3

Describe a newly discovered, large virus. 3

Answer 3

1. Pithovirus is bigger than pandora virus but has a smaller genome
2. Found in a 30000yr old Siberian perma frost
3. Virus identification is difficult because viruses can’t be grown in culture

Question 4

Describe the types of virus ultra structure. 6

Answer 4

1. Symmetrical shapes and coats made of protein subunits
2. Helical symmetry is found in rods and spheres, TMV was the first to be identified
3. It is a helical polymer of protein with rna core in a rod
4. Those which are roundish but not spherical have icosahedrons symmetry eg. Adenovirus
5. Membrane bound viruses have projections of viral proteins from membrane, membrane from host cell is studded with viral proteins eg. Flu
6. Bacteriophages show both symmetry types and are a protein she’ll containing DNA genome and enzymes

Question 5

Describe viral genomes. 6

Answer 5

1. Dna and RNA can be single stranded or double stranded, positive or negative
2. Use both dna and RNA, dependent on strange in the replication cycle
3. In retroviruses, rna genome replicates via a dna intermediate and uses reverse transcriptase to translate rna to dna in host cell eh. Hiv
4. Hepadnavirus has a dna genome which replicates via an rna intermediate eg. Hepatitis b
5. Retroposons can Move around in the genome
6. No matter what the genome is like, mrna needs to be formed, they don’t learn to there is a lot of diversity

Question 6

What are the general properties of viruses? 6

Answer 6

1. Has an extracellular state, the virion
2. Contains nucleic acid
3. Surrounded by proteins
4. Has more or less of other components eg host membrane
5. Metabolically inert, role is to get from host to host
6. Stable structures that don’t fall apart until replication

Question 7

Describe what the influenza virus does during infection. 3

Answer 7

1. Host cell completely taken over during synthesis
2. Binds to host with Haemoglutin
3. Leaves with neurominidase

Question 8

What are the general properties of viruses during their intracellular state? 6

Answer 8

1. Virus replication occurs dependent on host cell metabolism
2. Virus redirects pre existing host machinery eg. Polio prevents host cell from producing proteins
3. It takes over the er and uses vesicles to assemble proteins
4. Virions are produces late in the infection
5. Some viruses need to get into nucleus because there is no DNA polymerase in rna viruses
6. Bacteriophage replication takes 20mins, fastest eukaryotic replication is 24 hours

Question 9

What is the structure of a virion?

Answer 9

1. Nucleic acid surrounded by the capsid (protein coat or shell)
2. Capsid composed of structure protein subunits called capsomers
3. Can be one type of protein in a capsomer or several proteins assembled into capsomers
4. Can be naked or enveloped

Question 10

Describe naked viruses. 6

Answer 10

1. Have icosahedrons symmetry
2. Are host adaptive
3. Usually/often can be a symptomatic
4. Eg. TMV an ssrna virus causes yellowing, chlorosis, curling, mottling mosaic, distortion a Nd blisterin of leaves
5. Eg poliovirus
6. Eg adenovirus has four proteins on surface which allow binding. Other proteins allow entry into cells by endocytosis

Question 11

Describe enveloped viruses. 6

Answer 11

1. Envelope is a host membrane with or without virus specific proteins
2. Eg rabies, external proteins allow virus to enter host cell
3. Rna viruses like rabies bring rna dependent rna polymerase with them, host don’t have them as they don’t need them
4. Eg herpes is icosahedral, dsDNA
5. Paramyxovirus is helical, ssrna, causes measles and mumps
6. Flu aka orthomyxovirus ssrna with haemagglutinin (H) and neurominidase (N) spikes eh. H5n1

Question 12

Describe the different forms of viral symmetry. 3

Answer 12

1. Seeing the same form again upon rotation around axis
2. Rod viruses have helical symmetry
3. Spherical viruses have icosahedral symmetry, same from all angles

Question 13

Describe helical symmetry as is displayed in viruses such as TMV. 5

Answer 13

1. +ssrna genome of 6.4kb
2. 18x300n
3. Length determined by rna length
4. Width determined by size and packing of subunits
5. Rna and proteins can assemble themselves correctly

Question 14

Describe icosahedral symmetry as shown in viruses such as parvovirus. 6

Answer 14

1. One protein polymer which causes slap cheek disease
2. Twenty faces, most efficient packing in a closed she’ll
3. 12 pentons, triangles arranged in a pentagon
4. 5-3-2 rotational symmetry. 5 pentons, 3 triangles, 2 halves
5. Adenovirus has capsomer a and fibre projections, dsDNA and causes respiratory infections
6. Casing made of two proteins and projections of adhesins

Question 15

Describe complex viruses eg. Bacteriophages. 6

Answer 15

1. Made of parts, each with own symmetry
2. Icosahedral head
3. Helical tail sheath
4. Helical tail fibres
5. Infect bacteria only
6. Carry lysozyme for cell entry

Question 16

How do bacteriophages function? 4

Answer 16

1. Most abundant viruses
2. Carry lysozyme which breaks down peptidoglycan
3. Allows injection of genome into host cell
4. Head does not enter host cell

Question 17

Describe the virus replication cycle. 6

Answer 17

1. The virus induces the host cell to synthesis all virion components
2. Attachment/absorption to receptors on host cell
3. Penetration/entry by endocytosis in animal cells
4. Uncoating, exposure of nucleic acids. Coat falls apart
5. Synthesising of nucleic acid and protein and assembly of structural subunits and packaging of genome
6. Release of mature virions from cell, exocytosis for membrane bound and cell lysis for naked

Question 18

Describe the one step growth curve method. 5

Answer 18

1. Taught us how to study viruses
2. 10 viruses simultaneously added for every one E. coli
3. Unbound virions removed
4. Viruses were undetectable right away
5. Process halted in various stages, taught us how viruses replicate

Question 19

Explain the one step growth curve. 6

Answer 19

1. Eclipse (latent period) - coat and nucleic acids separated. We don’t see anything but genome replication and protein synthesis occurs. Particles assemble after this
2. Latent period - nucleic acid and protein replication
3. Maturation - titre of active virion rises
4. Assembly and release
5. Number of virions -> burst size, 1000n+
6. 20-60 min for bacteriophage, 8-40hours for animal viruses

Question 20

What is the significance of viruses? 5

Answer 20

1. Significant part of the ecosystem
2. Found everywhere including air and surge faces. Most survive in freshwater and seawater
3. All water on earth contains 10^30 viruses, more biomass than all elephants
4. Everything can be affected, even bacteria and other viruses
5. Many won’t make you sick