Question

1.Describe the synthesis of the leading strand for DNA replication.

2. How the lagging strand differ from the leading strand?

3. what is semiconservative replication?

Answer 1

1. The parental strand that runs in the 3'->5' direction is synthesized continuously in the 5'->3' direction. This is call the leading strand synthesis. The DNA replication is initiated in a definite sequence called the origin of replication. The initiator protein Dna recognises the oriC and denatures a small segment of the oriC. First the two hexameric DnaB protein bind to the lagging strand template and start to unwound the ds-DNA. This creates two potential replication forks. DNA gyrase binds ahead of the advancing replication fork and promotes negative superhelicity to remove positive superhelicity. Then the RNA polymerase enzyme synthesizes primer fmon the leading strand for DNA polymerase to act on as DNA polymerase cannot initiate replication de-novo. Then DNA polymerase starts adding the nucleotide chain in a reaction were it adds the incoming nucleotide triphosphate to the free 3'-OH end of the primer. After that in continuously adds the nucleotides until the twoo replication forks meet at a terminal point. During leading strand synthesis lagging strand is also synthesized in the 5'->3' direction in a discontinuous manner. After replication is complete the RNA primers are removed by DNA polymerase 1 and the it synthesizes the gap and ligase seals the nick.

2. Leading strand is synthesized continuously but lagging strand is synthesized in short segments called the okazaki fragments. Later this fragments are joint after primer removal and gap synthesis.

The leading strand grows in a 5'->3' direction but the lagging strand grows in the 3'-5' direction. But each okazaki fragment is synthesized in the 5'-3' direction as DNA polymerase can add nucleotides only in the 5'-3' direction. The leading Strand is formed on the 3'-5' running parental strand. And the lagging strand is synthesized on the 5'-3' running parental strand.

3. In semiconservative mode of replication eact replicated daughter molecule contains one parental strand and one newly synthesized daughter strand. That is the two parntal strands act as template for the daughter molecule synthesis. This model ensures faithfull copying of the genetic material at each cell division.