Circularisation of DNA: DNA Replication during Lytic Cycle
Circularisation of DNA: Lambda DNA circularises after infection, so genes form a single transcriptional unit. Cos elements, or cohesive ends, are 5’ overhangs located on linear DNA that are complementary to one another. By forming base pairs between single-stranded ends, linear DNA molecule can circularise.
DNA Replication during Lytic Cycle: Bidirectional/Theta (θ) replication Circularised DNA required. Replication begins at specific site, produces two replication forks, which move in opposite directions around DNA circle and meet at opposite sides. Function is to increase templates available for transcription for next stage of replication. Rolling circle replication Generates long concatemers required to package DNA into phage head. Begins with nick/ single-stranded break (SSB) at origin. 5’ end displaced from the strand. Using unnicked (3’ end) strand as a template, DNA polymerase replicates around DNA molecule, displacing the nicked strand as single-stranded DNA. Multiple copies made, all joined together.
Packing of DNA in phage head Long concatemers cut at cos sites by phage encoded nuclease to generate singlestranded 5’ ends. Enzyme only works on double cos sites (only found in concatemers) and makes overhangs complementary. Tail added to packaged phage head.
DNA Replication during Lytic Cycle: Bidirectional/Theta (θ) replication Circularised DNA required. Replication begins at specific site, produces two replication forks, which move in opposite directions around DNA circle and meet at opposite sides. Function is to increase templates available for transcription for next stage of replication. Rolling circle replication Generates long concatemers required to package DNA into phage head. Begins with nick/ single-stranded break (SSB) at origin. 5’ end displaced from the strand. Using unnicked (3’ end) strand as a template, DNA polymerase replicates around DNA molecule, displacing the nicked strand as single-stranded DNA. Multiple copies made, all joined together.
Packing of DNA in phage head Long concatemers cut at cos sites by phage encoded nuclease to generate singlestranded 5’ ends. Enzyme only works on double cos sites (only found in concatemers) and makes overhangs complementary. Tail added to packaged phage head.
