Question

Why the process of licensing is necessary for replication of eukaryotic chromosomes but not prokaryotic chromosomes? Describe the main steps of licensing process.

Answer 1

Answer

Replication licensing is the process that checks for the chromosome replication to occur only once in a cell cycle. So that the chromosome number can be maintained.

Steps in licensing the replication process:

The steps involved in Replication-licensing process are as follows:

The origin recognition complex (ORC) binds at the region of initiation on the chromosome.

This is followed by the binding of two more proteins Cdc6 and Cdt1 at that site.

The binding of all these proteins at the initiation site is regulated by cyclin-dependent kinases (CDKs) which, activate these proteins whenever required and inactivate when not required by the cell.

This entire complex enables a special complex of polypeptides called mini-chromosome-maintenance proteins Mcm2-7 to bind at the region of replication origin. This complex function like that of helicase enzyme and results in the formation of a pair of replication forks.

The formation of replication forks is the license provided for the process of replication.

Immediately, after the formation of replication forks the Mcm2-7 complex dissociates. This prevents the re-formation of replication forks so that, the replication does not initiate again at the origin.

Inhibition of ORC and proteins Cdc6 and Cdt1 prevents the re-licensing of origin and thus, preventing the chromosome replication more than once in a cell-cycle.

As the prokaryotic chromosomes bind to a large protein complex and possess single replication origin mostly the replication licensing process is not essential for it.

In eukaryotes there are number of origins for its replication all over the large genome and the origin recognition complex remains on the DNA throughout replication. So, it requires replication licensing process to prevent repeated replication in one cell-cycle