Spring 2009: Section 2 – lecture 6 Reading – viral replication

Single-stranded DNA virus 1. Single strand of linear DNA is injected into the cell by the virus particle. This strand is the ‘sense’ strand for transcription and is called the ‘+’ strand. 2. The initial strand is replicated to produce a ‘-‘ strand resulting in a doubled -stranded DNA molecule that can then form circular DNA.

3. Rolling circle replication then occurs using the ‘-‘ strand as the template to produce multiple copies of linear ‘+’ strands. These ‘+’ strands are then packaged into the viral particles.

Single-stranded RNA virus replication How do RNA viruses replicate? Since the cells the virus invades do not have the enzymes for RNA replication, the virus must carry the genetic information for the enzymes necessary for their replication. - RNA  RNA - RNA  DNA  RNA

RNA  RNA virus replication This type of virus carries an enzyme called RNA replicase that will replicated RNA. Because it is single-stranded, replication of RNA  RNA virus is similar to that of single-stranded DNA virus without the circular DNA step. An RNA virus can be either a plus (+) strand RNA virus or a minus (-) strand RNA virus

Steps involved in positive strand RNA RNA virus replication 1. Linear RNA (can be in multiple pieces) is injected into a host cell. Since it is ready to be translated it is called the ‘+’ strand. 2. The RNA is translated to produce replicase which then uses the positive strand to produce a minus strand.

With minus strand RNA virus, why is it necessary to carry in a replicase enzyme?

3. The replicase then uses the ‘-‘ stand to produce copies of the ‘+’ strand. 4. The ‘+’ RNA strands are then packaged into the viral particles.

Why can there be so much variation in RNA viruses or why are there so many strains of the flu virus? or Why do RNA viruses vary so much in their proteins or why do you need a flu shot every year?

RNA  DNA  RNA virus replication This virus is sometimes called a retrovirus. It carries the enzyme reverse transcriptase that allows it to synthesize a strand of DNA using RNA as the template. This type of virus may also incorporate into the host’s DNA once it is in DNA form.

So the replication goes against the central dogma of genetics. DNA  RNA  protein RNA  DNA  RNA  protein

In DNA form the virus can incorporate into the host DNA. In RNA form the virus can be packaged into new virus particles

Steps in retro-virus replication 1.The linear RNA enters the host cell and is translated to produce reverse transcriptase. The reverse transcriptase then uses the mRNA as a template to produce single- stranded DNA.

2.The host cell’s DNA polymerase or reverse transcriptase then replicates a complementary strand of DNA producing double-stranded DNA. Why would location of replication of the complimentary strand influence whether DNA polymerase or reverse transcriptase was involved in synthesizing the new DNA strand?

3.The viral DNA can now integrate into the host’s DNA.

4.The viral DNA is now replicated with the host’s DNA and all subsequently produced cells carry the virus. If the virus infects a germ cell it can be passed on to the next generation through the gametes. 5.The viral DNA can be transcribed while it is in the host DNA to produce new viral particles.

Examples of retro-viruses:

How would you stop successful infection by a retrovirus? How could you cure an individual who has become infected with a retrovirus?

Recombination in virus/phage - requires simultaneous co-infection - error in replication of the DNA or RNA - copy-choice error in replication - complementary pairing and recombination - error in packaging pieces of RNA

copy-choice error in replication example: replicase moving from one RNA strand to another RNA strand during replication.

complementary pairing and recombination example: complementary pairing and recombination between the two DNA molecules resulting in an exchange of DNA sequences. Viral particles carrying the DNA hybrids are called recombinants.

- error in packaging ex. mixing of pieces of RNA from two RNA virus