Month: September 2022

FRAGMENT FROM NATURE continues from last weekend and concludes this weekend on a paper from the early days of untangling the structure of DNA, by Professor Alexander Rich from MIT, featuring the unraveling of left handed DNA. The story is written in crystal clear language, with unyielding logic, an historical milestone. https://link.springer.com/content/pdf/10.1007/978-94-009-7052-6.pdf View in LinkedIn

ZIG-ZAG ALTERNATION. “The left-handed helix has the phosphates arranged in a zig-zag array; hence the name Z-DNA. The same Z structure has been found in crystals of tetramers d(CpGpCpG) as well as in other hexamer crystals.” https://link.springer.com/content/pdf/10.1007/978-94-009-7052-6.pdf View in LinkedIn

CAUSE OF ZIG-ZAG. The unusual backbone is associated with guanosine residues which have an unusual conformation. NMR studies in solution have shown that purine residues can adopt the syn and anti conformations with equal ease. Another difference is found in the pucker of the sugar ring.” https://link.springer.com/content/pdf/10.1007/978-94-009-7052-6.pdf View in LinkedIn

ORIENTATION SWITCH. “One consequence of these differences in conformation is that the bases in Z-DNA have a different orientation relative to the sugar phosphate backbone than they do in B-DNA. In essence, the bases must "flip over" in order to go from one form to the other.” https://link.springer.com/content/pdf/10.1007/978-94-009-7052-6.pdf View in LinkedIn

THE STABILITY EXPLANTATION. “Since purine nucleotides can adopt the syn conformation more readily than pyrimidine nucleotides, it is reasonable to believe that the Z conformation will be favored in sequences with alternating purines and pyrimidines.” https://link.springer.com/content/pdf/10.1007/978-94-009-7052-6.pdf View in LinkedIn

STABILIZERS. “The major conclusion reached from these studies is that the equilibrium between the B form and Z form of poly(dG-dC) is strongly influenced by cations and by the substituents which are attached to the polymer. A number of these substituents profoundly influence the equilibrium between the two forms and favor the Z-DNA conformation.” https://link.springer.com/content/pdf/10.1007/978-94-009-7052-6.pdf View in LinkedIn

DNA METHYLATION. “One of the principal modifications of the nucleic acids, especially in higher eukaryotes, is methylation of cytosine on the 5 position where the cytosine is followed by a guanine.” https://link.springer.com/content/pdf/10.1007/978-94-009-7052-6.pdf View in LinkedIn

GENE CONTROL. “Extensive studies have been carried out which have shown that methylation of DNA is generally associated with turning off transcription; conversely, demethylation is associated with the initiation of transcription or the expression of the gene.” https://link.springer.com/content/pdf/10.1007/978-94-009-7052-6.pdf View in LinkedIn