The Genetics of Viruses

Question 1

What are viruses?

Answer 1

Viruses are microscopic obligate intracellular parasites that can infect eukaryotic and prokaryotic cells by hijacking living host cell machinery.

Obligate: Capable of surviving only in a particular condition.
Intracellular: Located within a cell.
Parasite: Organism that lives on or in a host organism and gets its food from or at the expense of its host.

Question 2

State the general features of viruses.

Answer 2

1. Central core of nucleic acid
2. Capsid coat
3. Lipid envelope

Question 3

What are the non-living characteristics of viruses?

Answer 3

• Strict obligate intracellular parasites, cannot reproduce on their own: lack machinery to synthesise building blocks (eg. a.a.)
• Do not carry out metabolism on their own: unable to generate ATP
• Do not have sensory components or response mechanisms on their own.
• Do not have internal mechanism to maintain homeostasis and lack of internal environment to maintain

Question 4

What are the living characteristics of viruses?

Answer 4

• Capsid coat that maintains organised structure and shape.
• Within host cells, able to reproduce at a fast rate
• Contain genes and show inheritance. Able to undergo spontaneous mutation, allowing evolution to occur.
• Respond to certain external signals to trigger reproduction in host cell.

Question 5

How do virsues challenge the cell theory?

Answer 5

• Viruses are non cellular, lack cytoplasm and organelles
• Viruses are smaller and more basic than a cell.
• Viruses do not undergo cell division from pre-existing viruses.

Recall the cell theory: All living organisms are made up of cells, a cell is the most basic unit of life, cells arise from pre-existing cells

Question 6

State the structure of a bacteriophage.

Answer 6

• Head structure: contains linear, dds DNA and is made of capsid coat of capsomeres
• Hollow tube where viral DNA enters
• Contractile sheath contracts during penetration
• Tail fibres and base plate involved in binding of phage to bacterium

Question 7

How does a bacteriophage undergo Attachment to the host bacteria?

Answer 7

Tail fibres and base plate of the T4 phage recognise and bind to specific receptors on cell surface of the host bacterium.

Question 8

How does a bacteriophage undergo Penetration into the host bacteria?

Answer 8

T4 phage penetrates bacterium cell wall by contracting its contractile sheath of the tail which drives the hollow tube of the tail into the host bacterium –> facilitates entry of viral DNA into bacterium.

Question 9

How does a bacteriophage undergo Replication in the host bacteria?

Answer 9

One of the phage genes expxressed codes for a phage enzyme that degrades the host cell DNA, shutting down bacteria’s protein, DNA and RNA synthesis. The phage then replicates its genome and synthesising phage enzymes and structural components using bacterial protein synthesis machinery.

Question 10

How does a bacteriophage undergo Assembly and Maturation in the host bacteria?

Answer 10

T4 phage enzymes and other components assemble around genome to form mature phage particles.

Question 11

How does a bacteriophage undergo Exit/Release in the host bacteria?

Answer 11

Phage-coded lysosyme breaks down bacterial peptidoglycan cell wall, causing cell lysis and release of bacteriophages.

Question 12

How does an influenza virus undergo Attachment to the host cell?

Answer 12

Haemagglutinin on the viral envelope bind to sialic acid on specific receptors found on surfaces of epithelial cells of respiratory tract.

Question 13

How does an influenza virus undergo Penetration into the host cell?

Answer 13

Via endocytosis, host cell surface invaginates and pinches off, placing virus in endosome. Low pH in endosome causes conformational change in haemagglutinin, causing viral envelope and endosome membrane to fuse, releasing nucleocapsid into cytoplasm.

Question 14

How does an influenza virus undergo Replication in the host cell?

Answer 14

RNA dependent RNA polymerase enters nucleus and catalyses synthesis of ss +ve sense RNA using viral -ve sense RNA as template. +ve sense RNA is used as a template to synthesise more copies of viral -ve sense RNA or exported to cytoplasm as a template for translation of viral proteins using host cell ribosomes.

Glycoproteins (HA, NA) are synthesised in rER and glycosylated in Golgi apparatus.

Question 15

How does an influenza virus undergo Assembly and Maturation in the host cell?

Answer 15

HA and NA transported to cell membrane where they are inserted at a site which serves as exit points for viral release.

Viral proteins transported back to nucleus for assembly with viral -ve RNA to form nucleocapsids, leave the nucleus and transported to cell membrane.

Question 16

How does an Influenza virus undergo Exit/Release in the host cell?

Answer 16

Viruses obtain their viral envelopes from host cell membranes by budding at the exit points. The host cell membrane evaginates to surround the nucleocapsid and pinches off to form the viral envelope.

NA removes sialic acids from the sialic acid receptors on the surface of the host cell that can bind to the new viruses.

Question 17

How does HIV undergo Attachment to the host cell?

Answer 17

gp120 on the viral envelope of HIV binds to CD4 receptor on host T helper cells.

Question 18

How does HIV undergo Penetration into the host cell?

Answer 18

Binding of gp120 to CD4 receptor causes a conformational change to gp41 that brings viral envelope and host cell membrane close together for fusion, releasing nucleocapsid into cytoplasm. Uncoating releases the viral RNA and enzymes into cytoplasm.

Question 19

How does HIV undergo Replication in the host cell?

Answer 19

Reverse transcriptase transcribes ssRNA into dsDNA.
1. Synthesis of ssDNA using ssRNA as template, producing RNA-DNA hybrid.
2. Removes viral RNA.
3. Uses ssDNA to synthesise complementary DNA strand, forming dsDNA.

Viral DNA enters nucleus and is inserted into host DNA by HIV integrase.

Activation of host T helper cell during an immune response results in transcription of provirus. Viral mRNA leaves nucleus and serves as a genome for new viruses and template for translation of viral proteins.

Glycoproteins (gp120, gp41) synthesised in rER and glycosylated in Golgi apparatus.

HIV protease is used to cleave proteins from a polyprotein to form functional proteins.

Question 20

How does HIV undergo Assembly and Maturation in the host cell?

Answer 20

gp120, gp41 transported to cell membrane where they are inserted at a site which serves as exit points for viral release.

Viral proteins assemble with viral RNA to form the nucleocapsid in the cytoplasm.

Question 21

How does HIV undergo Exit/Release out of the host cell?

Answer 21

Viruses obtain their viral envelopes from host cell membranes by budding at the exit points. The host cell membrane evaginates to surround the nucleocapsid and pinches off to form the viral envelope.

Question 22

What is a provirus?

Answer 22

It is an host cell DNA genome that has been integrated with viral DNA. It is able to be replicated along with the host DNA when the cell undergoes cell division.

Question 23

What is a retrovirus?

Answer 23

Viruses that are able to transcribe ssRNA into dsDNA.

Question 24

What is the eclipse period?

Answer 24

The period of time between infection by a virus and the appearance of new mature virus within host cell.

Period of time between before Entry to Entry to Penetration

Eclipse period ends when intracellular infectious mature viruses appear.

Question 25

What is the latent period?

Answer 25

Latent period is the interval from infection until mature viruses are relased from host cell.

Period of time from before Entry to Release/Exit

Question 26

What is the burst period?

Answer 26

When there is an abrupt rise in the number of infectious phage particles released into the extracellular space.

After Exit of mature viruses

Interval from the end of latent period until all phages are released.

Question 27

What is antigenic drift?

Answer 27

Antigenic drift is a process that occurs when mutations to viral RNA coding for viral glycoproteins leads to changes in structural conformation of the glycoproteins.

When the structural conformation of glycoproteins in viruses are altered slightly, antibodies previously specific to a certain glycoprotein may not be effective against the new viruses.

Question 28

What is antigenic shift?

Answer 28

Antigenic shift is a major change to the virus structure to create a new subtype of virus. This happens when different types of influenza viruses infect the same cell at the same time. Resulting virus undergoes genetic reassortment and contain RNA segments from different virus types.

Question 29

What are the challenges in treating influenza?

Answer 29

Influenza virus has a high rate of mutation due to antigenic shift and antigenic drift, resulting in the occurence of different subtypes, making the development of vaccines difficult.

Question 30

What are the challenges in treating HIV?

Answer 30

• HIV reverse transcriptase is error-prone, resulting in higher chances of antigenic shift to occur.
• HIV may persist in latent state where virus does not assemble into infectious viral particles.