Synthesis of RNA from DNA templates

Question 1

mRNA synthesis is required before proteins are produced

What enzymes are responsible for production of mRNA?

Answer 1

hose cell RNA polymerase II produces cellular mRNA from DNA template, including proviral DNA of retroviruses

In contrast, viral RNA polymerases transcribe the large DNA genomes of viruses that replicate in the cytoplasm, such as poxviruses.

Question 2

What is different between RNA and DNA viruses, with regards to expression of viral genes?

Answer 2

DNA viruses have strictly defined, reproducible sequences, based on having enzymes and regulatory proteins.

RNA polymerase binds to DNA promoter sequences, which governs transcription order. Means that proteins are produced in order of need.

Starting with regulator proteins, to DNA synthesis, and then structural proteins of viral particles

RNA viruses genes are continually expressed

Question 3

Eukaryotes have three transcriptional systems, using three different enzymes, that are all very similar in structure

What is the function of each enzyme?

RNA polymerase I
RNA polymerase II
RNA polymerase III

Answer 3

RNA polymerase usually refers specifically to DNA-dependent RNA polymerase. These are utilised by DNA viruses, using the DNA template

RNA viruses usually bring their own enzymes - RNA-dependent RNA polymerase. As they cannot use the host DNA-dependent enzyme as it only works on DNA

• RNA polymerase I - produce ribosomal RNAs (rRNAs) which form the major sections of the ribosome
• RNA polymerase II - makes precursors to mRNA, as well as precursors of small, regulatory RNA molecules (microRNAs)
• RNA polymerase III - produce transfer RNAs (tRNAs) which transfer specific amino acids to growing polypeptide chain at ribosomal site of protein synthesis during translation

Question 4

What is the benefit of having temporal expression of viral gene expression?

Answer 4

Optimal production

co-ordination of structural proteins and viral DNA - facilitate genome encapsidation and assembly of viral progeny. Need both to be produced at same time

prevent pre-mature release of incomplete virions

reduce competition of finite cellular resources - substrates for RNA/DNA synthesis

restriction of synthesis of cytotoxic viral proteins that facilitate release of viral particles to the end of the infectious
cycle