Eukaryotic Linear DNA Synthesis: Mechanism Of Telomere Addition

Note: The 3’ end of the chromosome will always be longer, but now all sequences in the chromosome will be replicated

Courtesy: Alberts et. al. Molecular Biology of the Cell

Prokaryotic DNA Replication: Rolling Circle Replication

• Rolling circle replication is used to replicate some plasmids

• Plasmids are small circular DNA molecules that are double stranded

• For rolling circle replication, the outer strand is considered the (+) strand and the inner strand is the (-) strand

• To start rolling circle replication, the (+) strand is nicked (cut)

• In rolling circle replication, the (+) strand serves as a template for the (-) strand and the (-) strand serves as a template to make the (+) strand

Prokaryotic DNA Replication:Rolling Circle Replication

• The nick in the (+) strand will create a free 3’ OH and allow for synthesis of a new (+) strand– Synthesis of a new (+) strand will start at the free 3’OH using the (-) strand as a

template– Synthesis of a new (+) strand is continuous lending the name rolling circle because we

go around the circle

• Synthesis of a new (-) strand uses the (+) strand as a template

• Synthesis of a new (-) strand occurs in a discontinuous replication

Prokaryotic DNA Replication: Evidence For Bi-Directional Replication In Bacteria

• Although there are two replication forks created, there are two possible ways that the circular Bacterial chromosome could be replicated– Uni-directional (DNA is replicated at only one of the

forks)– Bi-Directional (DNA is replicated at both forks)

• Elizabeth Gyurasits and R.B. Wake showed that DNA replication in B. subtilis is bi-directional

• Grew B. subtilis in the presence of 3H-Thymidine (weak radioactivity) for a short time

• Then switch the B. subtilis to media with a more strongly radiolabeled nucleotide (32P)

• Exposed the replicating DNA to autoradiography

• Both forks picked up the strongly radiolabeled nucleotide, and must have been replicating at the point where the shift occured