The replication fork is a structure that forms within the nucleus during DNA replication. It is created by helices, which break the hydrogen bonds holding the two DNA strands together. The resulting structure has two branching "prongs", each one made up of a single strand of DNA.
Basically they say "null hypothesis" according to source:
http://www.statisticshowto.com/what-does-it-mean-to-reject-the-null-hypothesis/
They study the human body
