Acyclovir

Question 1

1. Describe the life cycle of a virus, explaining its parasitic nature in relation to the host cell.

Answer 1

Virus enters the host cell and the protein coat breaks up releasing the DNA.

2. Some DNA is replicated and other DNA is transcribed
3. mRNA from transcription is translated to produce protein for protein coat.
4. The protein coat and the DNA from replication assemble and exit to form new virus particles.

Question 2

Describe dna replication

Answer 2

Before replication can take place, DNA must UNRAVEL.
• TOPOISOMERASE - the enzyme that unravels the DNA.
• Then DNA helicase can break the hydrogen bonds and DNA polymerase can join the
new strand to the original strand

Question 3

How does acyclovir act as a nucleoside analogue

Answer 3

Acyclovir is similar in structure to deoxyguanosine but DOES NOT HAVE THE 3’ HYDROXYL GROUP.
• Therefore, acyclovir cannot attach to other nucleotides

Question 4

How does acyclovir become activated

Answer 4

Acyclovir is given in an inactive form and it only starts becoming active when it comes into contact with THYMIDINE KINASE. So it is only active in cells which the virus has invaded and released thymidine kinase.
ACTIVATION OF ACYCLOVIR
1. Acyclovir enters the cell in an inactive form but changes in the presence of Thymidine Kinase to form ACYCLO-GUANOSINE MONOPHOSPHATE. Also called Acyclo-GMP.
2. Then Acyclo-GMP is converted into ACYCLO-GUANOSINE DIPHOSPHATE (Acyclo-GDP) using GUANYLATE KINASE of the host cell.
3. The Acyclo-GDP is then converted to ACYCLO-GUANOSINE TRIPHOSPHATE (Acyclo- GTP) by PHOSPHOTRANSFERASES of the host cell. Acyclo-GTP is ACTIVE!
The Acyclo-GTP works well with Viral DNA Polymerase but does NOT work well with host polymerase. acyclovir gets incorporated into the viral DNA and because it doesn’t have a 3’ hydroxyl group it can’t continue the chain and join onto another phosphate of a different nucleotide.