The Chromosome Replicating Team

Like all processes in living cells, a collection of specific proteins works together to perform functions which control and complete chromosome replication. All the proteins involved in DNA replication make and break bonds, so they are enzymes. The function of the main DNA replicating enzymes is described below.

Helicase: This is one of the enzymes that unwinds the double stranded DNA molecule. The unwound single strands no longer hydrogen bond to their complementary strand but the sugar-phosphate bonds remain intact. 

Topoisomerase: The unwinding work of Helicase creates tension in the double stranded DNA ahead of the replication fork. Topoisomerase relieves this tension by catalyzing a series of sugar phosphate bond breaking and making steps. Without this tension relief, the double stranded DNA could break and the intact chromosome cannot finish replicating. 

DNA Polymerase III (DNA pol III): This is the main DNA synthesizing enzyme. The enzyme uses single-stranded DNA as a template for creating a complementary DNA strand.  DNA pol III reads the template strand nucleotides in the 3′ to 5′ direction and builds the new complementary strand in the 5′ to 3′ direction, adding the next nucleotide to the 3′ end by catalyzing sugar-phosphate bonding. DNA pol III proofreads its own work while it builds the complementary strand and conducts mismatch repairs as needed.  

DNA pol III illustrates the specificity of enzymes several ways; it reads and proofreads the placement of new nucleotides to ensure accurate replication and yet it can only read in one direction (3’ to 5’) and build in one direction (5’ to 3’). These directional specifications mean that DNA pol III cannot start the replication process on the single stranded template. Another enzyme needs to be part of the in vivo replication team to prime the work of DNA pol III. 

Primase: Biochemists named this enzyme to describe its role in starting or priming the replication process. DNA (or RNA) primase is a special RNA polymerase. The enzyme reads the single stranded DNA template 3′ to 5′ and adds ribonucleic acid (RNA) nucleotides in the 5′ to 3′ direction. Once a few hundred RNA nucleotides are added, primase falls off the template strand and leaves the 3′ end that DNA Pol III needs to continue the process. 

DNA Polymerase I (DNA pol I): The DNA polymerase specializes in the removal of the RNA primers and replacing them with DNA nucleotides. The enzyme works with the same 3′ to 5′ reading and 5′ to 3′ building pattern of DNA polymerase III and primase.  

DNA Ligase: After the five enzymes described above have completed their work, some sugar phosphate bonds need to be re-sealed to finish the creation of the double stranded molecule. DNA ligase is the enzyme that seals this backbone sugar phosphate bond. 

Now that we have introduced the replication enzymes, we can describe the step-by-step action of this team of DNA replication enzymes. We will focus on the action that happens at one origin of replication.