And in every one of the 100 trillion cells, the sequence of these four letters ... small loops of DNA. We've now zoomed in enough to see portions of the DNA strand. The DNA is wrapped around ...

Then a third enzyme, "polymerase," joins the party by attaching to the primer, moving along with the growing strand and adding new letters to form another double helix. In DNA replication ...

They can also be produced when DNA replication forks encounter DNA single-strand breaks or other types of lesion. They can occur at the termini of chromosomes due to defective metabolism of ...