What were the old women doing?
In Percy jackosn

If it is the scene that I think you're referring to, they were sitting in a group on rocking chairs, cutting yarn.

Explanation:

in Greek mythology, these women are the three Fates, named Clotho, Lachesis, and Atropos. In mythology, a thread represented someone's lifeline, and when the Fates cut your thread, it meant your life was over and you died.

In this specific scene, from The Lightning Thief, the Fates are seen cutting a blue piece of yarn, which makes Percy's friend Grover nervous because he believes they've just cut Percy's lifeline.

this is math not biology xd

during the process of dna replication, brdu can replace thymidine and incorporate into the newly synthesized dna, cell proliferation thus can be reflected in the extent of brdu incorporation.