Replication Bubble

Develop a model which explains the major steps to replication, specifically a replication bubble.

To form a replication bubble, Helicase will come in first at the origin point of said replication bubble, and will unwind the two strands by breaking hydrogen bonds between nitrogenous bases. Single-strand binding proteins keep the fork open, and topoisomerase makes sure the DNA strands do not become damaged from the stress of unwinding. Topoisomerase does its job by breaking and swiveling pieces of DNA, which will later be rejoined. Next, primase comes in and places an RNA primer at the 5’ end of the leading strand, and on the 5’ end of each Okazaki fragment of the lagging strand. This allows new DNA to be created in the 5’-3’ direction. DNA polymerase III places new nucleotides on the parental DNA that complements the template in the 3’ to 5’ direction. So, the new DNA is formed in the 5’-3’ direction, but the new nucleotides are placed starting at the 3’ end of the old DNA strand that is being used as a template. DNA polymerase II removes any RNA nucleotide primers that were placed in the leading and lagging strand(s), replacing them with DNA nucleotides. Lastly, DNA ligase will clean up the disjointed areas and make sure all the Okazaki fragments, breaks in DNA sequencing, etc. are glued and smoothed out. At this point, the DNA strand will be fully replicated. Multiple replication bubbles occur at the same time, which is why DNA can be replicated so quickly, otherwise the time it would take for this to happen via one single replication bubble would be too great.

Mastering Biology | Pearson; Campbell Biology