thymidine
Meaning of thymidine: The one building block that belongs to DNA alone, and a convenient handle for tracking which cells are dividing.
Definition of thymidine
Thymidine is a nucleoside, one of the small units from which DNA is assembled, made of the base thymine joined to the sugar deoxyribose. Adding phosphate groups turns it into the nucleotide that is stitched into a growing DNA strand. Cells build it from scratch and also recycle it, and it marks a difference between DNA and RNA, which carries uridine in its place.
How does a cell obtain it?
By two separate routes. The main one builds the thymine unit from a closely related uracil unit, a conversion needing folate as a helper. The second route is salvage: ready-made thymidine drifting in from broken-down DNA is picked up and phosphorylated by an enzyme, which is cheaper than building it. The first route matters far beyond textbook chemistry, because it is where two long-standing cancer drugs strike. Fluorouracil jams the enzyme that performs the conversion, and methotrexate starves that enzyme of the folate it needs. A dividing cell then runs out of this one unit while the other three remain plentiful, and DNA copying stalls.
Why is it used to track cell division?
Because a cell takes it up in quantity only when it is actually copying its DNA. Feeding cells a labeled version and then measuring how much ends up locked inside gives a direct count of how many were dividing, a technique that underpinned decades of laboratory work and still shapes how proliferation is described. Modern versions use a close chemical relative that antibodies can find on a slide, or a labeled relative that can be followed by a scanner as a measure of how fast a tumor is growing.
How do the related names line up?
| Name | What it is |
|---|---|
| Thymine | The base alone, one letter of the genetic code |
| Thymidine | That base joined to a deoxyribose sugar |
| Thymidine monophosphate | The same, plus a phosphate; the form built into DNA |
| Uridine | The counterpart used by RNA instead |
An older listing describes this compound as a treatment for mucositis, the raw painful lining of the mouth and gut that follows some chemotherapy. That overstates matters. Supplying thymidine can bypass a fluorouracil blockade and was investigated on exactly that reasoning, but it did not become an established treatment, and it has no approved therapeutic use of its own.
Also known as
Used in a sentence
Thymidine uptake was measured to estimate how rapidly the cells were dividing.
Good to know
Thymidine is a normal cell constituent and a laboratory marker of cell division rather than a medicine, and the claim that it treats mucositis reflects an investigational use that was never established.
Word origin
Where “thymidine” comes from
Named from thymine, which was first isolated from calf thymus tissue; the gland takes its name from the Greek thymos.