Cytarabine (ara-C)

## 0–8 seconds: Arrival from the blood Blood carries cytarabine through this vessel to a leukemia cell, so treatment can reach the bone marrow. ## 8–25 seconds: A close imitation, then entry Cytarabine resembles a normal DNA ingredient, but its sugar carries an extra hydroxyl group. A membrane transporter carries this look-alike inside, using the same route as normal nucleosides. ## 25–49 seconds: Activation inside the cell Inside the cell, enzymes prepare cytarabine by adding phosphate groups one at a time. The same molecule moves between enzymes: one phosphate becomes two, then three. With three phosphates attached, activated cytarabine is called ara-CTP. The DNA copier can now use it. Ara-CTP can pass through a nuclear pore, reaching the DNA inside. ## 49–86 seconds: Normal copying, then a stall The DNA copier reads the existing strand and joins matching building blocks into a new strand. Each normal block joins the growing strand, so the copier can move on and add another. Ara-CTP also fits the copier and is added to the growing DNA strand. Its unusual sugar makes further extension difficult. Here the next block cannot join, so this copying site stalls. Many stalled copying sites can leave DNA unfinished. This is not an instant stop at every cytarabine molecule. ## 86–96 seconds: Damage activates self-destruction When unfinished DNA causes too much damage, the cell can activate its self-destruct program. Mitochondria release a signal, so caspases activate and dismantle the cell from inside. ## 96–108 seconds: Sealed packets and cleanup The damaged cell shrinks into sealed packets, so a cleanup cell can swallow what remains.