Virus

Question 1

Describe the non-living characteristics of viruses

Answer 1

1. Acellular; lack cellular components
2. Do not carry out metabolic processes outside a host cell
3. All cells contain double-stranded DNA genome, but viruses have either DNA/RNA genomes, which can be single/double-stranded
4. Incapable of movement

Question 2

Describe the living characteristics of viruses

Answer 2

1. Reproduce at a fast rate upon infecting living host cells
2. Possesses a genome capable of transmitting their genetic characteristics from one viral generation to the next

Question 3

Explain why viruses are obligate intracellular parasites

Answer 3

1. Lack essential cellular machinery
2. Cannot replicate independently of host cell
3. Upon infection, they hijack host cell machinery / exploit host raw materials
4. To carry out own replication cycle

Question 4

T4 phage Attachment

Answer 4

*The phage tail fibres bind to specific receptor on host bacterium cell surface membrane.

Question 5

T4 phage Penetration

Answer 5

• The tail sheath contracts, driving a hollow tube through the bacterial cell wall and cell membrane.
• dsDNA genome is injected into the bacterial cytoplasm.

Question 6

T4 phage Synthesis of viral genome and proteins

Answer 6

• Phage enzymes hydrolyse the cell’s DNA.
• The phage DNA directs synthesis of proteins and make new copies of genome.

Question 7

T4 phage Maturation

Answer 7

Phage genome is packaged inside the capsid.

Question 8

T4 phage Release

Answer 8

The phage directs the synthesis of lysozyme that breaks down the bacterial peptidoglycan 1cell wall.

Entry of water into the cell by osmosis causes the cell to swell and burst.

Phage particles are released.

Question 9

Describe the reproductive cycle of the T4 phage

Answer 9

Attachment

• The phage tail fibres bind to specific receptor on host bacterium cell surface membrane.

Penetration

• The tail sheath contracts, driving a hollow tube through the bacterial cell wall and cell membrane.
• dsDNA genome is injected into the bacterial cytoplasm.

Synthesis of viral genome and proteins

• Phage enzymes hydrolyse the cell’s DNA.
• The phage DNA directs synthesis of proteins and make new copies of genome.

Maturation

• Phage genome is packaged inside the capsid.

Release

• The phage directs the synthesis of lysozyme that breaks down the bacterial peptidoglycan 1 cell wall.
• Entry of water into the cell by osmosis causes the cell to swell and burst.
• Phage particles are released.

Question 10

lambda phage Attachment

Answer 10

The phage tail fibre binds to specific receptor on host bacterium cell surface membrane.

Question 11

lambda phage Penetration

Answer 11

The tail sheath contracts, driving a hollow tube through the host cell wall and cell membrane.

dsDNA genome is injected into the bacterial cytoplasm.

Question 12

lambda phage Synthesis of lambda repressor protein and viral genome

Answer 12

λ DNA genome circularizes and is incorporated into the bacterial chromosome, forming prophage.

lambda repressor is synthesized and blocks transcription of most other prophage genes.

Each time the host cell divides, it replicates λ DNA along with its own, and passes copies to daughter cells.

The phage continues to propagate without killing the host bacterial cells.

Question 13

lambda phage Prophage induction

Answer 13

Upon detection of host cell damage or stress, lambda repressor protein may be broken down. This results in expression of the phage genes.

The λ DNA genome of activated phage is excised from the bacterial chromosome and enters the lytic cycle.

Question 14

Describe the reproductive cycle of the lambda phage

Answer 14

Attachment

• The phage tail fibre binds to specific receptor on host bacterium cell surface membrane.

Penetration

• The tail sheath contracts, driving a hollow tube through the host cell wall and cell membrane.
• dsDNA genome is injected into the bacterial cytoplasm.

Synthesis of lambda repressor protein and viral genome

• λ DNA genome circularizes and is incorporated into the bacterial chromosome, forming prophage.
• lambda repressor is synthesized and blocks transcription of most other prophage genes.
• Each time the host cell divides, it replicates λ DNA along with its own, and passes copies to daughter cells.
• The phage continues to propagate without killing the host bacterial cells.

Prophage induction

• Upon detection of host cell damage or stress, lambda repressor protein may be broken down. This results in expression of the phage genes.
• The λ DNA genome of activated phage is excised from the bacterial chromosome and enters the lytic cycle.

Question 15

Influenza Attachment

Answer 15

Haemagglutinin on the viral envelope binds to specific glycoprotein receptors / sialic acid receptors on the host cell surface membrane

Question 16

Influenza Penetration and uncoating

Answer 16

Viral particle enters host cell by receptor-mediated endocytosis as the host cell surface membrane invaginates and pinches off, enclosing the viral particle in a coated vesicle / endosome

The drop in pH in the endosome triggers the viral envelope to fuse with the membrane of the endosome.

The capsids are degraded by cellular enzymes, releasing the ssRNA (-) genome segments and viral RNA-dependent RNA polymerase into the cytoplasm.

The viral RNA polymerase and ssRNA (-) are transported into the nucleus

Question 17

Influenza Synthesis of viral genome and proteins

Answer 17

ssRNA (-) used as a template to transcribe complementary ssRNA (+) via viral RNA-dependent RNA polymerase

ssRNA (+) used as

a. template to make new copies of ssRNA (-) genome

b. mRNAs for eventual translation into viral proteins in cytoplasm

Viral mRNA is exported from the nucleus to be translated into viral proteins (i.e. viral RNAdependent RNA polymerase, capsid proteins, haemagglutinin, neuraminidase)

Newly synthesized viral proteins are

a. Transported back into nucleus (i.e. capsid proteins and RNA-dependent RNA polymerase)

b. Secreted through GA and incorporated into cell surface membrane (i.e. neuraminidase and haemagglutinin)

Question 18

Influenza Maturation

Answer 18

In the nucleus, capsid proteins assemble and the ssRNA (-) strands and RNA-dependent RNA polymerase are packaged to form new viral particles

Question 19

Influenza Release

Answer 19

New viral particles leave the nucleus and move towards cell surface membrane where neuraminidase and haemagglutinin have clustered

Viral particles bud off at ‘exit points’

Neuraminidase cleaves the glycoprotein receptors on host cell

Question 20

Describe the reproductive cycle of the influenza virus

Answer 20

Attachment

• Haemagglutinin on the viral envelope binds to specific glycoprotein receptors / sialic acid receptors on the host cell surface membrane

Penetration and uncoating

• Viral particle enters host cell by receptor-mediated endocytosis as the host cell surface membrane invaginates and pinches off, enclosing the viral particle in a coated vesicle / endosome
• The drop in pH in the endosome triggers the viral envelope to fuse with the membrane of the endosome.
• The capsids are degraded by cellular enzymes, releasing the ssRNA (-) genome segments and viral RNA-dependent RNA polymerase into the cytoplasm.
• The viral RNA polymerase and ssRNA (-) are transported into the nucleus

Synthesis of viral genome and proteins

• ssRNA (-) used as a template to transcribe complementary ssRNA (+) via viral RNA-dependent RNA polymerase
• ssRNA (+) used as

a. template to make new copies of ssRNA (-) genome

b. mRNAs for eventual translation into viral proteins in cytoplasm

• Viral mRNA is exported from the nucleus to be translated into viral proteins (i.e. viral RNAdependent RNA polymerase, capsid proteins, haemagglutinin, neuraminidase)
• Newly synthesized viral proteins are

a. Transported back into nucleus (i.e. capsid proteins and RNA-dependent RNA polymerase)

b. Secreted through GA and incorporated into cell surface membrane (i.e. neuraminidase and haemagglutinin)

Maturation

• In the nucleus, capsid proteins assemble and the ssRNA (-) strands and RNA-dependent RNA polymerase are packaged to form new viral particles

Release

• New viral particles leave the nucleus and move towards cell surface membrane where neuraminidase and haemagglutinin have clustered
• Viral particles bud off at ‘exit points’
• Neuraminidase cleaves the glycoprotein receptors on host cell