Intrinsic Control Mechanisms

Intrinsic control mechanisms ensure that the cycle is executed completely, so that, following cell division, both daughter cells are equipped with the same genetic information as far as possible. Of the many control mechanisms, the following are highlighted  

Mitosis is only initiated when the DNA has been completely rephcated during S phase. Mechanisms must exist that register completion of S phase and couple this to entry into M phase. 

Another control mechanism ensures that entry into S phase is only possible if preceded by mitosis. If the cell was able, during G2 phase, to enter a new S phase without mitosis taking place, this would lead to improgrammed multiplication of the chromosome set and thus to polyploidy. For S phase control, see 13.5. 

A further control mechanism, which is also intrinsic, tests whether the cells in Gj phase are large enough to initiate another round of cell division. The daughter cells produced by cell division must reach a critical size in the course of Gi phase before S phase can commence. 

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A cell that has suffered DNA damage can bring about a halt in the cell cycle with the help of intrinsic control mechanisms (DNA damage checkpoints, see 13.7). The aim is... 

Use of Hormones to Alter Normal Endocrine Function. Because of the intrinsic control mechanisms in the endocrine system, administration of exogenous hormones can often affect the normal release of hormones. This fact can be exploited in certain situations to cause a desired change in normal endocrine function. For instance, oral contraceptives containing estrogen and progesterone inhibit ovulation by inhibiting the release of LH and FSH from the anterior pituitary. 

Intrinsic resistance may than be defined as a natural, chromosomally controlled property of a bacterial cell that enables it to circumvent the action of a biocide (see Table 13.2). A summary of intrinsic resistance mechanisms is provided in Table 13.4. 

Many of the control mechanisms of the cell cycle are of intrinsic nature and are constitutive, i.e., they are operational in every cell cycle and ensure the ordering of the individual steps. However, other control mechanisms exist that are not active in every cell cycle these are only induced when defects are detected in central cell cycle events. These control mechanisms are known as checkpoints. An example of a checkpoint that is only activated when required is the DNA damage checkpoint (see 13.7). This is a biochemical pathway that detects DNA damage and creates a signal that arrests cells in the Gl, S or G2 phase of the cell cycle. 

An oscillating system is formed by the interplay of the three protein classes and the activity of this system makes up the specific biochemical functions of the individual phases of the cycle. The activity of the cychn-dependent protein kinases (CDKs) is central to the oscillating system. These create a signal that initiates downstream biochemical processes and thus determines the individual phases of the cycle. CDK activity is also the starting point for intrinsic and external control mechanisms. 

Unlike the majority of controlled-, sustained-, or extended-release systems in use, the extent of response in the lungs will be limited by intrinsic clearance mechanisms [110]. It will be surprising, but not unwelcome, if any deposited dosage form can extend the effective duration of action much beyond a day. Twenty-four hours would, in fact, be a significant achievement. 

Once Rp is determined by the EIS method, icorr is evaluated in the same way as with the polarization-resistance method (i.e., with Eq 6.28). Therefore, the Tafel constants still must be experimentally determined. The intrinsic value of the EIS method lies in the fact that extensive information is extracted (i.e., Rp, Rs, and C are all determined) and, ideally, interpreted to not only determine the corrosion rate but also the rate-controlling mechanisms at the material surface and within the electrolyte. 

The ability to control mechanical devices remotely by IR light with high speed and spatial precision offers many intriguing possibilities.The low recovery speed (up to several minutes) of thermo-responsive SMPs originates from their intrinsically low thermal conductivity (<0.3 W m ... 

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