Nucleotides extra

Although several enzymes can in theory convert purine ribonucleotides to ribonucleosides, in only a few cases is it known exactly which is acting. Certain strains of Bacillus svbtilis accumulate nucleotides extra-cellularly, but mutants lacking 5 -nucleotidase or alkaline phosphatase, or both, accumulate smaller quantities of nucleotides than cells which have both enzymes. 5 -Nucleotidase appeared to be quantitatively the more important for nucleotide dephosphorylation in these cells 23). Baer and Drummond 12) compared the rates of dephosphorylation of the 2 -, 3 -, and 5 -phosphates of adenosine by perfused rat heart and concluded that only a specific 5 -nucleotidase was actively in contact with the blood. More evidence for the action of specific, rather than nonspecific phosphatases, are the observations that in one system or another, adenylate, inosinate, xanthylate, or guanylate appears to be the nucleotide most rapidly de-phosphorylated. 

Two proteins are initially involved in the nonho-mologous rejoining of a ds break. Ku, a heterodimer of 70 kDa and 86 kDa subunits, binds to free DNA ends and has latent ATP-dependent helicase activity. The DNA-bound Ku heterodimer recruits a unique protein kinase, DNA-dependent protein kinase (DNA-PK). DNA-PK has a binding site for DNA free ends and another for dsDNA just inside these ends. It therefore allows for the approximation of the two separated ends. The free end DNA-Ku-DNA-PK complex activates the kinase activity in the latter. DNA-PK reciprocally phos-phorylates Ku and the other DNA-PK molecule, on the opposing strand, in trans. DNA-PK then dissociates from the DNA and Ku, resulting in activation of the Ku helicase. This results in unwinding of the two ends. The unwound, approximated DNA forms base pairs the extra nucleotide tails are removed by an exonucle-... 

A scanner with two lasers for Cy3 and Cy5 labeling is fairly good enough for most of the microarray experiments. However, multiple lasers are necessary for simultaneous detection of all four nucleotide polymorphisms in chip-based SNPs detection. Besides, an extra third flurophore attached to a sequence that specifically binds to a linker region of the DNA spots could be used for spotting quality control. 

In an effort to reduce the competition encountered with naturally occurring tRNAs, even when rare codons are used, Hardesty and co-workers have devised a strategy based on 4-nucleotide codon-anticodon pairs [48]. An extra thymidine was inserted either 5 or 3 to the rare arginine codon AGG to yield TAGG... 

The nucleotide components required for transcription and replication have to be imported into the nucleus from the cytoplasm, incorporation of these components into RNA leads to primary products, which are then altered by cleavage, excision of introns, and the addition of extra nucleotides (RNA maturation see p. 242). it is only once these process have been completed that the RNA molecules formed in the nucleus can be exported into the cytoplasm for protein synthesis (translation see p. 250). 

Ribonuclease II [EC 3.1.13.1], also called exoribo-nuclease II, catalyzes the exonucleolytic cleavage of the polynucleic acid, preferring single-stranded RNA, in the 3 - to 5 -direction to yield 5 -phosphomononucleotides. The enzyme processes 3 -terminal extra-nucleotides of monomeric tRNA precursors, following the action of ribonuclease P. Similar enzymes include RNase Q, RNase BN, RNase PHI, and RNase Y. Ribonuclease T2 [EC 3.1.27.1] is also known as ribonuclease II. 

This system [EC 3.1.26.5] catalyzes the endonucleolytic cleavage of RNA, removing the 5 -extra-nucleotide from the tRNA precursor. This step is essential for tRNA processing. It generates the 5 -termini of mature tRNA... 

Why are there two pyridine nucleotides, NAD+ and NADP+, differing only in the presence or absence of an extra phosphate group One important answer is that they are members of two different oxidation-reduction systems, both based on nicotinamide but functionally independent. The experimentally measured ratio [NAD+] / [NADH] is much higher than the ratio [NADP+] / [NADPH]. Thus, these two coenzyme systems also can operate within a cell at different redox potentials. A related generalization that holds much of the time is that NAD+ is usually involved in pathways of catabolism, where it functions as an oxidant, while NADPH is more often used as a reducing agent in biosynthetic processes. See Chapter 17, Section I for further discussion. 

Although the rates of spontaneous mutation are low, they can be greatly increased by mutagenic chemicals (Chapter 27) or by irradiation. It is perfectly practical to measure the rates of both forward and back mutation. When this was done, it was found that certain chemicals, e.g., acridine dyes, induce mutations that undergo reverse mutation at a very much lower frequency than normal. It was eventually shown that these mutations resulted either from deletions of one or more nucleotides from the chain or from insertions of extra nucleotides. Deletion and insertion mutations often result from errors during genetic recombination and repair at times when the DNA chain is broken. 

These are designated, according to their sedimentation constants, as 5S, 16S, and 23S and contain about 120, 1700, and 3300 nucleotides, respectively. All three pieces appear in cells as parts of larger pre-rRNA precursor molecules with extra nucleotide sequences at both the 3 and 5 ends.207 208... 

The structure of the precursor to the minor E. coli tyrosine tRNA, is shown in Fig. 28-10. This is encoded by the amber suppressor gene SupF (see Chapter 29). Transcription of its gene is initiated by GTP 43 bp upstream of the 5 end of the mature tRNA and usually terminates at a p-dependent signal 225 bp beyond the CCA terminus of the tRNA. An endonuclease cuts the transcript a few nucleotides beyond the CCA end. It is then trimmed to an 130-nucleotide piece still containing 2-3 extra nucleotides at the 3 end. This intermediate is cut by RNase P at the 5 end after which final trimming is done at the 3 end (Fig. 28-10).226 227... 

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