Complementary chains

Section 28.14 The nucleotide sequence of DNA can be deter-mined by a technique in which a short section of single-stranded DNA is allowed to produce its complement in the presence of dideoxy analogs of ATP, TTP, GTP, and CTP DNA for-mation ter-minates when a dideoxy analog is incorporated into the growing polynucleotide chain. A mixture of polynucleotides differing from one another by an incremental nucleoside is produced and analyzed by electrophoresis. From the observed sequence of the complementary chain, the sequence of the original DNA is deduced. 

The two major types of nucleic acids are DNA and RNA. Nucleic acids are polyphosphate esters containing the phosphate, sugar, and base moieties. Nucleic acids contain one of five purine or pyrimidine bases that are coupled within double-stranded helices. DNA, which is an essential part of the cell s chromosome, contains the information for the synthesis of protein molecules. For double-stranded nucleic acids, as the two strands separate, they act as a template for the construction of a complementary chain. The reproduction or duplication of the DNA chains is called replication. The DNA undergoes semiconservative replication where each of the two new strands contains one of the original strands. 

DNA is a linear polymer of covalently joined deoxyribonucleotides, of four types deoxyadenylate (A), deoxyguanylate (G), deoxycytidy-late (C), and deoxythymidylate (T). Each nucleotide, with its unique three-dimensional structure, can associate very specifically but non-covalently with one other nucleotide in the complementary chain A always associates with T, and G with C. Thus, in the double-stranded DNA molecule, the entire sequence of nucleotides in one strand is complementary to the sequence in the other. The two strands, held together by hydrogen bonds (represented here by vertical blue lines) between each pair of complementary nucleotides, twist about each other to form the DNA double helix. In DNA replication, the two strands separate and two new strands are synthesized, each with a sequence complementary to one of the original strands. The result is two double-helical molecules, each identical to the original DNA. 

There are 16 possible pairs in one chain. These can be designated as in the following examples (AA/TT), (CG,CG), and (AG,CT). The first two letters within the parentheses represent the sequence (from 5 to 3 ) in one chain while the second pair of letters represent the sequence (again from 5 to 3 ) in the complementary chain. The rules state that (AA,TT) or (TT,AA) repeated in a sequence will stabilize die B form of DNA. Repetition of (CC,GG) or (GG,CC) will favor conversion to the A form. Repetitions of (CG,CG) favor the Z form, especially if an alternating sequence of purines and pyrimidines is present throughout the (G + C)-rich region. 

In a transversion a purine in one chain is replaced by a pyrimidine, while the pyrimidine in the complementary chain is replaced by a purine ... 

The sequence AATCCGTAGC appears on a DNA strand. What would be the the sequence on (a) the complementary chain of this DNA and (b) a transcribed RNA chain ... 

Described in 1953 by James Watson and Francis Crick, the double helix of DNA (deoxyribonucleic acid) is the cellular storehouse of genetic information. This biopolymer consists of a pair of complementary chains approximately 2.4 nanometers (9.5XlO-8 inches) in diameter and composed of... 

The head of each spectrin chain interacts with the head of the complementary chain of another heterodimer. In the tetramer, there are paired interactions (Fig. 5-31), while in higher oligomers, a closed loop is formed, as the head region is quite flexible. This is shown for the hexamer in Fig. 5-39, but this self-association may continue indefinitely. 

A model for DNA incorporating base pairing between complementary strands and consistent with x-ray diffraction data was developed by J. Watson and F. Crick in 1953. Basic to the structure was the twisting of the two strands around one another to give a right-handed helix (the double helix), and to achieve a structure consistent with data available at the time, it was necessary to orient the complementary chains in opposite directions (Fig. 7-3). Direct proof for this opposite polarity in chain direction was achieved about 10 years later. 

Then he and Watson went back to work. Within a few months, the two of them pulled off the scientific coup of the century, coming up with a new DNA structure made of two complementary chains wound around each other to form a spiral—they called it a double helix—each of which, when separated, could form another, identical chain. This was exactly what Pauling had described as a likely feature of the genetic material four years earlier. 

The normal base pairs in DNA. Adenine in one polynucleotide chain pairs with thymine in the complementary chain guanine pairs with cytosine. A-T base pairs are joined by two hydrogen bonds G-C base pairs are Joined by three hydrogen bonds. 

Proteolysis of DEBS dimers in which the thiols at the active sites of co-operating ACP and KS residues had been crosslinked by prior treatment with 1,3-dibromopropanone gave a different pattern of fragments consistent with the formation of crosslinks between two complementary protein chains [40]. It was therefore concluded that the KS of one chain co-operated with the ACP residing in the complementary chain rather than in its own, as had been demonstrated earlier for the animal FAS. 

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