The Timer

The timer is connected to the scaler, and its purpose is to start and stop the scaler at desired counting time intervals. The front panel of a typical timer is shown in Fig. 1.7e. Some models combine the timer with the scaler in one module. 

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Simultaneously start the timer and release the crimped hose to begin cake formation. Maintain agitation during cake formation and move the leaf as may be required to ensure that sohds do not settle out in any part of the container. It is not necessary to try to simulate the velocity with which the full-scale unit s filtration surface passes through the slurry in the filter tank. 

Note A similar situation may be found in an aulomalic Y/A starter when the setting of the timer, to changeover from star to della, is al a higher value lo allow a longer starling lime to pick up speed. Then also the motor wotild meet the same fate if the starting time exceeded the thermal withstand time of the motor. 

Electronic Flow Calibrators These units are high accuracy electronic bubble flowmeters that provide instantaneous air flow readings and a cumulative averaging of multiple samples. These calibrators measure the flow rate of gases and report volume per unit of time. The timer is capable of detecting a soap film at 80... 

Wait 1 minute, then measure the temperature and start the timer. Record this temperature in Data Table 1. 

Drop a piece of the antacid tablet into the water. Immediately start the timer. Stir the contents of the test tube throughout the reaction. 

Mailer I have some speculations. The other thing that goes on in an embryo that is independent of protein synthesis is centrosome replication. We showed last year that if cyclin E/Cdk activity is blocked, so is centrosome replication. One possibility is that the timer is actually monitoring centrosomes, as a sort of autonomous thing that replicates. The two things that are replicated in cells are the genome and centrosomes very little is known about centrosome replication. 

Mailer Murakami and Vande Woude published a nice paper a couple of years ago where they took a Mos arrest ed blastomere and re-started the cell cycle with Ca2+, getting a new cycle of tyrosine phosphorylation equivalent to cycle one, even though the embryo was actually entering cycle two or three. This would suggest that MAP kinase sets the timer back to zero. Of course, MAP kinase is very high in the unfertilized egg, the cell in which the timer is activated. 

Newport We actually got some Mos from George at one point. That was the only thing we could find that would stop the timer once the eggs had been fertilized. 

Although the timer is cell-intrinsic, it requires signals from other cells to operate normally. It depends on PDGF, for example, the main mitogen for OPCs. The... 

It is uncertain whether the intrinsic timer in OPCs primarily controls the onset of differentiation, the cessation of proliferation, or both. It seems likely, however, that the timer at some point interacts with the cell-cycle control system that regulates progress through the cell cycle. As the cells stop dividing and differentiate in Gl, it is the part of the control system that operates in G1 that is most likely to be relevant. In principle, the components of the cell-cycle control system could be components of the timer. The timer, for example, could depend on a decrease in one or more positive intracellular regulators, such as a cyclin or a Cdk... 

Nurse Martin Raff has set up two general questions that I think we should try to address in this discussion. One concerns the nature of this timer (and we could add to this, why is the timer needed). The second is this relationship between stem cells and progenitors, and what we mean by this. But I ll start with a specific question. If it is only 6-7% of the cells that undergo one or two more extra divisions, is that sufficient to account for the p27 l mutant phenotype with respect to cell number ... 

Reik If you consider an organ that is already fully differentiated at birth, and then it simply grows, is the timer off in that situation ... 

Edgar I have two simple questions about the timer. First, what starts it ticking and, second, does it have any influence on cell growth ... 

Raff I don t know the answer to the second question. I suspect that the timer starts ticking when the precursors first arise from multipotential CNS stem cells, but we don t know. 

There are also monomeric G-proteins. Just like the trimeric G-pro-teins, they are involved as signal relays and timers. The Ras superfamily relays signals from receptor tyrosine kinases to downstream elements that eventually regulate transcription. Rho and Rac relay signals from cell-surface receptors to the cytoskeleton, while Rab regulates intracellular transport of vesicles. Regardless of what they do, they use the timer mechanism provided by the G-protein. Three-letter acronyms (TLA), such as Ras, Rho, and Rab, are difficult to remember, sometimes even when you know what the letters stand for. Unfortunately, there s nothing you can do about this except to memorize them. 

The electrical system provides power for the operation of the fan motor, the gear motor and magnetic clutches in the reversing drive, the timer motor, and the air solenoid valve. The manually initiated timer was adjustable for processing times up to 15 min. 

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