VMC 211: General Veterinary Microbiology (1+1)

GENOME REPLICATION

• Most DNA virus genomes are replicated in the nucleus but some ds DNA virus genomes are replicated in cytoplasm. Genomes of most RNA viruses are replicated in the cytoplasm but those of the minus strand RNA viruses with segmented genomes are replicated in the nucleus. In retro and pararetro viruses RNA to DNA replication occurs in the cytoplasm and DNA to RNA replication occurs in the nucleus. 
• In the case of many RNA viruses the first nucleotide of the new strand base pairs with a nucleotide in the viral RNA. This initial nucleotide effectively acts as a primer for RNA replication.
• Some ss DNA viruses (parvoviruses) use self priming. There are regions at their 3’ end with complementarity sequences that can base pair. The hydroxyl group of the nucleotide at the 3’ end forms a linkage with the first nucleotide.
• Some DNA viruses (polyoma viruses) use cellular enzyme primase to synthesise their RNA primers. Others viruses such as herpes viruses encode their own primers.
• Retroviruses use the cellular t RNA to prime  DNA synthesis and then use the 3’ –OH in a partly degraded positive RNA template to prime positive DNA synthesis. RNA primer synthesized by cell primase is used for RNA synthesis from proviral DNA.
• Some viruses use hydroxyl group of a serine or tyrosine in a protein as a primer. Eg: DNA – adeno, RNA – picorna. Hepadna viruses use protein to initiate negative DNA replication and use RNA primer to initiate positive DNA.