Replacement synthesis

Energy for maintenance is the energy required for survival, or non-growth related purposes. It includes activities such as active transport across membranes and turnover (replacement synthesis) of macromolecules. 

Although the solid-phase technique was first developed for the synthesis of peptide chains and has seen considerable use for this purpose, it has also been used to synthesize chains of polysaccharides and polynucleotides in the latter case, solid-phase synthesis has almost completely replaced synthesis in solution. The technique has been applied less often to reactions in which only two molecules are brought together (nonrepetitive syntheses), but many examples have been reported. 

PROBLEMS Classify the five balanced equations above as single replacement, double replacement, synthesis, or decomposition. 

Replacement synthesis, most useful for specific labeling of one of the strands of the duplex, less used than other techniques... 

Labeling through sequential exonuclease-polymerase activities ( replacement synthesis )... 

Replacement synthesis after digestion of single strands from duplex by exonuclease... 

Experiments show that it is on the ribosomes that protein synthesis actually occurs. If cells synthesizing protein from radioactive amino acids are studied, the radioactivity is first found bound to the ribosomes, and is only later released from them as soluble protein. A cell suspension from which ribosomes have been removed can never be made to synthesize protein, whilst if they are subsequently replaced synthesis can proceed rapidly. But the ribosomes alone are inadequate. In order to incorporate radioactive amino acids into new protein there needs to be added to the ribosomes a preparation of soluble cell material which contains certain enzymes, the soluble low-molecular-weight messenger RNA (m-RNA), transfer RNA (t-RNA), ATP, GTP, and ions like magnesium and potassium. So what role do these various substances perform in protein synthesis ... 

The DuPont process not only eliminates the use of phosgene as a starting material, but also avoids the production of large amounts of hydrochloric acid as an unwanted by-product. In this method, methyl-amine reacts with carbon monoxide to yield the corresponding aldehyde, which is then catalytically converted to isocyanate. This phosgene-free replacement synthesis also supports the trend in the chemical process industry to seek to reduce inventories on plant sites of hazardous synthetic reagents. Methyl isocyanate produced from this process is converted in situ to an agrochemical product. 

As discussed in section 8.3, even in severe undernutrition, the rate of protein breakdown remains more or less constant, while the rate of replacement synthesis falls, as a result of the low availability of metabolic fuels. It is only in cachexia (section 8.4) that there is increased protein catabolism as well as reduced replacement synthesis. 

McGhee WD, Riley D, Kevin C, Pan Y, Pamas B (1995) Carbon dioxide as a phosgene replacement synthesis and mechanistic studies of urethanes from amines, CO2, and alkyl chlorides. J Org Chem 60 2820-2830... 

Duplex DNAs with 3 -protruding ends can also be labeled using Pol Ik in a two-step procedure. First, the 3 ends are exonucleolytically digested in the absence of added dNTPs by the 3 — 5 -exonuclease activity and, second, dNTPs containing a labeled nucleotide are provided to start a replacement synthesis. Note that the replacement synthesis method of labeling can be more efficiently performed by the use of T4 DNA Pol whose exonuclease activity is substantially stronger. 

The 3 end of DNA can be labeled more extensively by means of replacement synthesis (32). In this method, duplex DNA is first digested in the absence of dNTPs by the 3 — 5 -exonuclease activity, e.g., up to 30-40% of the length from each end. (Note that about 50% digestion will lead to dissociation of the two single-stranded halves and result in a loss of the template due to rapid degradation.) Subsequently, four dNTPs containing one [a- P]dNTP are added to the reaction mixture in order for the polymerase to extend the 3 ends to the length of the template. 

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