Viruses

Question 1

Compare replication cycle of influenza and HIV [4]

and RELEASE [2]

Answer 1

1. Attachment
2. Penetration
   » receptor-mediated ENDOCYTOSIS involving invagination of PM vs. fusion
3. Replication
   » RNA-dependent RNA pol vs. RTase
4. Integration into host genome > PROVIRUS formed

RELEASE

S1: Viral envelope derived from budding off from PM
S2: May not kill host cell
S3: Viral glycoproteins embedded into PM

D1: NA cleaves sialic acid
D2: HIV protease cleaves polyproteins

Question 2

Describe how influenza acquires its envelope [3]

Answer 2

1. HA and NA transported via Golgi vesicles
   » incorporated into PM
2. Assembly of viral proteins + RNA
3. PM embedded with HA surround virion
   » buds off, taking PM as its envelope
4. NA cleaves sialic acid receptor

Question 3

Describe the roles of: [4]

1. Tail fibres
2. Tail sheath
3. Phage DNA
4. Capsid

Answer 3

1. Tail fibres - recognise and adsorb to complementary receptor sites on the bacterial surface
2. Tail sheath - contracts to thrust hollow core tube through CW to inject phage DNA
3. Phage DNA
   » replication&raquo_space; many copies of phage DNA
   » template for synthesis of phage proteins
4. Capsid - protect and introduce phage genome

Question 4

Explain why it is sufficient for phages to contain only DNA [2]

Answer 4

1. FUNCTION OF DNA - genetic material coding for phage proteins
2. Host cells have:
   » RNA pol - transcribe phage DNA into phage mRNA
   » ribosomes - translation

Question 5

Describe an advantage of the lysogenic cycle (AND phage infection in general) to bacteria [2]

Answer 5

SPECIALISED TRANSDUCTION
» prophage inaccurately excised&raquo_space; genes transferred
» increases genetic variation
» increases the chance of survival under different selection pressures

Question 6

Compare entry of phage vs. animal virus [2]

Answer 6

1. Attachment

2. Entry (injection vs fusion/endocytosis)

Question 7

Describe entry of HIV [2]

Answer 7

1. gp120 binds to CD4 receptors
2. Envelope fuses with PM, facilitated by gp41
3. Nucleocapsid released directly into the cytoplasm
4. Capsid breaks down&raquo_space; releases viral enzymes + genome