DNA Replication Explained Step-By-Step

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DNA replication is the process of copying DNA and creates a new, identical copy of DNA. DNA replication happens right before cell division as both copies of the cell need DNA.

DNA replication is carried out in 3 main steps:

1. First, helicase unzips the double helix of DNA by breaking the hydrogen bonds between the nucleotides on opposing strands. SSB or single-stranded binding proteins keep the strands separated and topoisomerase prevents the DNA strand from supercoiling.

2. Second, primase creates a RNA primer on each strand which you can think of as a “helicopter landing platform”. DNA polymerase (III) can then attach to each primer and build 2 new strands of DNA in the 3’-5’ direction. Now, all of this is happening simultaneously as the helicase is unzipping the double helix. Therefore, the DNA polymerase on the 5’-3’-strand has to do it in increments, creating so called Okazaki fragments, while the leading strand can do it in “one go”. For this reason, we call the 3’-5’ strand the “leading strand” and the 5’-3’ strand the “lagging strand”.

3. Finally, RNAse H removes the “excess” RNA primers and DNA polymerase (I) fills in the corresponding DNA. Ligase then glues together all of these fragments into one DNA strand.

After this whole process you end up with 2 identical new DNA molecules that contain 1 “original” strand and 1 “new” strand. The process is therefore said to be semi-conservative.