The effect of 5-bromodeoxyuridine on DNA replication and cell division in Tetrahymena pyriformis

Possible effect of the cell synchronization method on the pattern of 5-bromodeoxyuridine incorporation into early synthesized DNA

Synchronously and asynchronously growing chick embryo fibroblasts have been used to study the pattern of 5-bromodeoxyuridine (BrdU) incorporation into DNA. In the synchronous cell system, the density of unifilarly substituted DNA is about 0.010 g/ml higher during first half of S phase than during second half of S phase. The density of unifilarly substituted DNA isolated from asynchronously growing cells is similar to that of DNA synthesized during the second half of S phase of synchronously growing cells for a given concentration of analogue in culture medium. Reassociation kinetics experiments have shown the oversubstitution to occur at the level of early synthesized repeated and/or intermediate DNA sequences. It is then assumed that the oversubstitution is due to some metabolic changes caused by the synchronization procedure itself. As BrdU incorporation into early replicating DNA is known to induce alterations of the cell metabolism, the implication of this phenomenon is discussed at the level of the inhibition of transformation which takes place when chick embryo fibroblasts are infected with Rous sarcoma virus during G1 and subsequently treated with BrdU during early S phase.

Extrusion of DNA from the macronucleus of Tetrahymena pyriformis GL

Administration of the thymidine analog 5-bromodeoxyuridine to exponentially growing cultures of Tetrahymena pyriformis GL in chemically defined medium results in inhibition of cell multiplication by at least one generation before DNA synthesis stops. Cell multiplication can be restored in these cultures, if they are transferred to fresh growth medium, but although most of the cells in the culture contain close to a G2-amount of DNA, a full DNA replication round is a prerequisite for renewed cell multiplication. Large extrusion bodies are found at the first division after transfer to fresh growth medium. Autoradiographic analysis has revealed that the DNA in the extrusion body is a representative of the DNA in the macronucleus indicating a random distribution of DNA between daughter nuclei and extrusion body.

Effect of 5-bromodeoxyuridine on differentiation. I. Probability distribution of BUdR-containing DNA-strands in subsequent divisions

The consequences on transcription of BUdR substitution of the normal thymidine in nuclear DNA are analyzed from a probabilistic point of view. The probability that at any stage subsequent to the administration of BUdR, a randomly chosen cell will be able to make good transcripts is calculated both under the assumption that incorporation of BUdR is complete and under the assumption that incorporation is partial. In the latter case both gradually increasing and decreasing incorporation rates are considered. The presented calculations may be of value for the investigation of any type of differentiating cells. Using the example of the sea urchin embryo it is shown how to calculate the probability that a particular group of cells, which in further development differentiate to a certain organ, will inherit as sufficient number of essential genes unaffected by BUdR in order to ensure a normal development. It is shown that the probability that the four macromeres (or micromeres) in the vegetal half of the 16-cell stage of the sea urchin embryo will contain at least one, two or four plus (or minus) strands of DNA uninfluenced by BUdR gradually decreases the earlier BUdR is administered. The theoretical calculations are in agreement with experimental observations on sea urchin embryos (described in part II).

Differences in the incorporation of bromodeoxyuridine by human lymphoblastoid cell lines

Long term human lymphoblastoid lines differ in their ability to grow in medium containing bromodeoxyuridine (BrdU) and to incorporate analog into their DNA. Eight Burkitt's lymphoma cell lines divided at least twice in BrdU-containing medium and made DNA in which over 90% of the thymidine residues were substituted with analog in both strands. Three infectious mononucleosis-derived lines and 24 lines transformed in vitro were inhibited by BrdU after one cell division and made only hybrid DNA in which one strand was substituted with analog. One out of eight normal individuals from whom long term lines were prepared gave cell lines which divided at least twice in BrdU and gave DNA in which both strands were substituted with analog. It would appear that intrinsic cellular factors regulate the response to BrdU and that Burkitt's tumor lines are characterized by their ability to make stable doubly substituted DNA containing a high proportion of halogenated analog.