Origin of replication
The origin of replication is a sequence in DNA at which initiation of replication occurs.
DNA has 4 nitrogen bases which are Adenine(A) , Guanine(G) , Thymine(T) , Cytosine(C). RNA also has 4 bases but in RNA Uracil (U) is present instead thymine. Adenine always pair with thymine by two hydrogen bonds in DNA. Guanine pair with cytosine by three hydrogen bonds.
High AT-content determines if a region of DNA is origin of replication.
Origin of replication usually have AT rich region because AT pairs are joined by two hydrogen bonds and GC pairs are joined by three hydrogen bonds. Therefore, AT repeats easier to separate than GC rich region.
DNA Helicases are the enzymes that unwind the DNA helix by breaking the hydrogen bond. Therefore, it unwinds the helix.
SSB proteins (single-stranded binding proteins) bind single-stranded DNA after strands are separated by helicases
Topisomerase corrects the overwinding by breaking and rejoining the DNA strands.