DNA replication of Eukaryotes (cells with nucleus) occurs stepwise. Each step needs different enzymes. The DNA replication steps can be broken down as:
1. A notch is made by DNA gyrase. This looks like a bubble under a microscope, called Replication Bubble. Cracking up the hydrogen bonds in the double helix is the vital step and it occurs where more A-T bases exist. As there are only 2 H bonds between them as compared to C-G bases, it takes less energy to break.
2. Enzyme helicase uncoils the double stranded twisted DNA. Bases in the DNA get exposed.
3. Single stranded binding proteins (SSB) connect to each strand temporarily to hold the strands apart.
4. DNA polymerase stroll down the strand making freely available nucleotides (RNA primers) to combine with the exposed bases. This is done by base-pairing rule (ATTCGGC). To join the DNA nucleotides, RNA nucleotides are the primers.
5. DNA Polymerase depends on the template of original DNA strand to make new matching strand. There is difference in the elongation processes of the 2 templates. DNA polymerase can read the template only in 3-5 direction (meaning it begins from the 3' end of the template and reads the nucleotides to the 5' end of the template) and produces the matching strand in 5-3 direction as opposite ends attract each other to make antiparallel strands in the future double helix. Replicated strand on this template is in continuous form and is called leading strand while the other one is in discontinuous form and called the lagging strand. As DNA Polymerase cant read in 5-3 direction, so it has to jump back and forth to read and let the RNA primers to attach. This jumping of the DNA Polymerase back and forth causes breaks in the replicated chain making small parts of DNA. They are named as Okazaki fragments (a Japanese researcher discovered them).
6. The DNA Polymerase and DNA Ligase plug these breaks. Now each new double helix has one old and one new strand. The replicated DNA in new cells is said to be semi-conservative, as it has 50% of the original genetic matter from its parent but they have the exact DNA characteristics as of their parent cell.
7. There is a repair mechanism for checking the errors in the new strands and correcting them. Wrong nucleotides are replaced by nucleases (enzymes) and the DNA Polymerase combines the breaks.
8. New copies curl up themselves by design.
DNA Replication Steps for prokaryote cells are almost similar with some minor differences.