Nuclear clocks

The half-life of isotopes provides scientists with a nuclear clock that can be used to date objects. The concept is based on knowing the fraction of original material that is present in a sample. For instance, if half of the original isotope is present in the sample, then the sample s age is equivalent to the isotope s half-life. If one-fourth of the original material is present, then the sample s age is 2 half-lives. Because the use of radio-metric dating involves making accurate measurements of parent and daughter activities in the sample, it is assumed that parent... 

Because the half-life of any particular nuclide is constant, the half-life can serve as a nuclear clock to determine the age of objects. The method of dating objects based on their isotopes and isotope abundances is called radiometric dating. 

Apart from the activity ratios of the radon-222 decay product radionuclides, the residence time of tropospheric aerosols can be derived from the activity ratios of the fission product radionuclides released into the atmosphere during the explosions from nuclear weapons testing or nuclear reactor accidents, such as Sr/ Sr and " Ba/ Sr. These nuclide ratios are considered as nuclear clocks. The applicability of the radionuclide ratios depends on whether steady-state conditions hold at the time and place of measurement and on the kind of sample, whether surface air or precipitation (rain or snow), used for the radioisotope activity determination. 

Right breast 10 o clock Breast carcinoma, ductal type, modified Black s nuclear grade III (poorly differentiated), ER 50%, PR 30%. 

Nasmyth There is a similar situation in Drosophila, where you also have a high cytoplasminucleus ratio. The only problem in Nenopus is that the whole cell cycle clock is occurring throughout the entire cytoplasm, whereas Drosophila has a certain degree of nuclear autonomy. Even there, these controls are missing to some extent. 

Because radioactive decay is a nuclear process, the rate of radioactive decay is totally unaffected by any external factors. Unlike chemical reactions, therefore, there is no dependency on temperature, or pressure, or any of the other environmental factors which affect the rate at which normal chemical reactions occur. This is the reason why radioactive decay chronometers, such as 14C, Ar-Ar, and U-series methods, are so important in geology and archaeology - they provide an absolute clock . 

There is growing evidence that clock proteins are regulated dynamically in both temporal (production and degradation) and spatial (nuclear and cytoplasmic) dimensions. The phosphorylation of mPERl and mPER2 by casein kinase le (CKIe) is known as an important step for the accumulation of negatively active clock proteins (Lowrey et al 2000) as in Drosophila (Kloss et al 1998). 

FIG. 1. Expression of clock proteins in mouse SCN. (a) Immunostaining of mouse SCN sampled at beginning (CTO) and end (CT12) of circadian day reveals constitutive expression of mCLOCK, and rhythmic expression of mCRY (scale bar 500 /tm). Inset high power confocal views of mCLOCK-ir and Hoescht DNA stain confirm nuclear localization of mCLOCK-ir. 

Koshash In the simple experiments of the clock protein profiles in the wild-type, do you see PER build up cytoplasmically before you see a nuclear PER ... 

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