Dividing cells, synchronization

In the synchronously dividing cells of early Xenopus and sea urchin embryos, the concentration of mitotic cyclins... 

In some cases, changes in auxin levels show a positive correlation with changes in mitotic activity. Nishinari and Yamaki (1976) noted that, in synchronized tobacco cell cultures, there is an increase in the activity of lAA-synthesiz-ing enzymes preceding an increase in the level of auxin, and that these events occur just prior to the increase in the number of dividing cells. The authors concluded that synthesis of free lAA may initiate mitosis. A dependence of cell division on internal auxin concentration in Acer pseudoplatanus was also shown by Lequay and Guern (1977). 

The material used in our work was the dividing sea-urchin egg, the advantages of which have already been discussed. Mitosis in synchronously dividing cell populations was arrested with what we hope is a minimum of biochemical damage by immersion of the cells in 30% ethanol at —10 . We were generally unsuccessful in attempts to isolate the mitotic apparatus cleanly by direct mechanical means, and so we turned to a less orthodox approach, which we refer to as selective solvbtlization. The problem was to find conditions under which we could dissolve or disperse the membrane and cytoplasmic contents other than the mitotic apparatus, leaving the latter behind. 

The advantages of HU as an agent for chemical ablation are derived from its biological activity. Relatively low doses of HU can be administered orally (de Belle and Heisenberg 1994 see below) or by injection (Truman and Booker 1986 Prokop and Technau 1994). Within approximately 3 hours, HU accumulates to maximum concentrations in tissues and is metabolized to an inactive molecule approximately 1 hour thereafter (Timson 1975). The lethal effects of HU are only realized in dividing cells. All remaining cells are held synchronous in the Gj phase of the cell cycle until metabolism of HU is complete. 

Although the precise mechanism of action of paclitaxel in enhancing radiation effects is not clear, one of the proposed mechanisms is its effect on the cell cycle. Its effects on the microtubules result in an M-phase arrest (55). Experiments done on synchronously dividing Chinese hamster cells have shown that cells in M and G2 are most sensitive to the effects of radiation (56). 

In higher organisms intercellular signaling pathways have the important task of coordinating and regulating cell division. The pathways ensure that cells divide synchronously and, if necessary, arrest cell division and enter a resting state. 

After 20-30 min the cells divide synchronously. Again this method is not universally applicable as not all cell lines show reversible colcemid inhibition. 

How many divisions In the development pathway of C. elegans, cell division is initially synchronous—that is, all cells divide at the same rate. Later in development, some cells divide more frequently than do others. How many times does each cell divide in the synchronous period Refer to Figure 2.26. 

Hydroxyurea inhibits the enzyme ribonucleotide reductase, leading to suppression of DNA synthesis, elimination of cells in the S phase of the cell cycle, and synchronization in the Gj or pre-DNA synthesis phase. The drug is usually administered daily and can be initiated at 40 to 50 mg/kg per day in divided doses until the WBC count falls below 10,000/mm. At that point, the dose can be decreased to a maintenance level of 20 mg/kg per day, but increasingly is tapered and stopped once imatinib therapy is started. Imatinib normalizes blood counts based on suppression of the malignant clone in the bone marrow, an effect not seen with hydroxyurea. Suppression of the malignant clone in the bone marrow is the desired therapeutic effect because of its impact on survival. ... 

As an example, consider the batchwise growth of a synchronized population of cells. A culture is said to be synchronized when all cells divide at the same time for our purposes, we say that a culture is synchronized when all cells have the same age. 

A comparison of the outcomes of this approach to single-cell protein synthesis kinetics determination with the alternative method which has been widely used in many previous studies of cell cycle and cell kinetic behavior of many organisms is instructive. In the latter methods, synchronous culture or an equivalent experimental technique are used to make measurements of the time variation of protein content or some other cellular variable versus time as the cell grows from a newborn daughter cell to a mature and dividing mother cell. Then, by estimating the slope of... 

---