Thymine base-pairing with adenine

DNA wound around one another. The sugar-phosphate backbone is on the outside of the helix, and complementary pairs of bases extend into the center of the helix. The base pairs are held together by hydrogen bonds. Adenine base pairs with thymine, and cytosine base pairs with guanine. The two strands of DNA in the helix are antiparallel to one another. RNA is single stranded. 

In molecular biology, a set of two hydrogen-bonded nucleotides on opposite complementary nucleic acid strands is called a base pair. In the classical Watson-Crick base pairing in DNA, adenine (A) always forms a base pair with thymine (T) and guanine (G) always forms a base pair with cytosine (C). In RNA, thymine is replaced by uracil (U). 

DNA, adenine (A) pairs with thymine (T) and cytosine (C) pairs with guanine (G). Therefore, when there is an A in one strand of the double-stranded DNA molecule, there is a T in the other strand. When the genetic code is copied from DNA to RNA, the two strands of DNA molecule separate, and the RNA nucleotides pair with nucleotides on each strand of DNA. In this case, the nucleotide that pairs with adenine (A) on the DNA is uracil (U) because RNA does not contain thymine (T). Because of the exact nature of base pairing, the genetic code can be transmitted accurately at each stage of the process. 

In this structure, the bases on opposite strands pair up with each other in a very specific way adenine (A) pairs with thymine (T), and guanine (G) pairs with... 

Fig. 3. For replication, ihc two strands of the parent DNA molecule I light gray) separate as the base pairs detach. The replicated (daughter) strands (dark gray 1 form as guanine (G) pairs with cytosine (C) and adenine (At pairs with thymine (T)...

Figure 6.2. Molecular architecture of DNA. Each strand of DNA is composed of alternating pentose sugar (deoxyribose) and phosphate moieties linked to each other via phosphodiester linkage. The first carbon position of the sugar is attached to one of the four nitrogenous bases (A, T, G, or C). The two strands are in opposite orientation to each other with respect to a 5 or 3 phosphate group attached to the sugar moiety. Cytosine (C) pairs with guanine (G) via three hydrogen bonds, and adenine (A) pairs with thymine (T). (Reproduced from Textbook of Biochemistry with Clinical Correlations, T. M. Devlin, ed., Wiley, New York, 1982.)...

Hoechst 33258 binds to the minor groove of double-stranded DNA with a preference for the A-T sequence (Pjura et al. 1987). Interaction between DNA and proteins very often induces structural modifications in both interacting molecules. Such modifications in DNA can be characterized with 2-aminopurine (2AP), which is a highly fluorescent isomer of adenine. 2AP does not alter the DNA structure. It forms a base pair with thymine and can be selectively excited, since its absorption is red-shifted compared to that of nucleic acids and aromatic amino acids. In addition, its fluorescence is sensitive to the conformational change that occurs within the DNA (Rachofsky et al. 2001). 

Similarly to proteins, both DNA and RNA have a secondary and a tertiary structure. The secondary structure of DNA shows two chains running in opposite directions, coiled in a left-handed (double) helix about the same axis. All the bases are inside the helix, and the sugar phosphate backbone is on the outside (see e.g. [1]). The chains are held together by hydrogen bonds between the bases with adenine always paired with thymine and guanine paired with cytosine. The base pairing in DNA is shown below ... 

Adenine on one chain forms a base pair with thymine on the other chain. 

Base pairing The hydrogen bonds formed between complementary bases that are part of the polynucleotide chains of nucleic acids. The base pairing is specific in that adenine will base pair with thymine (uracil in RNA) and guanine will pair with cytosine. 

There is a large variability possible in the structures of double stranded DNA due to the fact that (compared to polypeptides) many more bonds can be rotated in the backbone of each monomer (Scheme 14). The most common and physiologically most important structure is the B-DNA helix. It consists of two polynucleotide chains running in opposite direction which coil around a common axis to form a right-handed double helix. In the helix, the phosphate and deoxyribose units of each strand are on the outside, and the purine and pyrimidine bases on the inside. The purine and pyrimidine bases are paired by selective hydrogen bonds adenine is paired with thymine, and guanine with cytosine (Scheme 15). The structure is very flexible and can form a supercoil with itself, or around proteins. It can form a left-handed supercoil around histones to form nucleosomes which assemble in yet another helical structure to form chromatin. ... 

Fig. N-9. The spiral structure of deoxyribonucleic acid, or DNA— the basic building block of life on earth. It s a double helix (a double spiral structure), with the sugar (deoxyribose)-phosphate (phosphoric acid) backbone represented by the two spiral ribbons. Connecting the backbone are four nitrogenous bases (a base is the nonacid part of a salt) adenine (A) paired with thymine (T), and guanine (G) paired with cytosine (Q with the parallel spiral ribbons held together by hydrogen bonding between these base pairs. Adenine and guanine are purines, while thymine and cytosine are pyrimidines.

The mode of action has been a subject for research for a number of years. While it was originally thought that maleic hydrazide replaced uracil in the RNA sequence, it has been deterrnined that the molecule may be a pyrimidine or purine analogue and therefore base-pair formation is possible with uracil and thymine and there exists the probabiHty of base-pair formation with adenine however, if maleic hydrazide occurs in an in vivo system as the diketo species, then there remains the possibiHty of base-pairing with guanine (50). Whatever the mechanism, it is apparent that the inhibitory effects are the result of a shutdown of the de novo synthesis of protein. 

The two strands which make up DNA are held together by hydrogen bonds between complementary pairs of bases adenine paired with thymine and guanine paired with cytosine. The integrity of the genetic code (and of life as we know it) depends on error-free transmission of base-pairing information. 

DNA is made up ot two intertwined strands. A sugar-phosphate chain makes up the backbone of each, and the two strands are joined by way of hydrogen bonds betwen parrs of nucleotide bases, adenine, thymine, guanine and cytosine. Adenine may only pair with thymine and guanine with cytosine. The molecule adopts a helical structure (actually, a double helical stnrcture or double helix ). 

The rules of base pairing (or nucleotide pairing) in DNA are adenine (A) always pairs with thymine (T) cytosine (C) always pairs with guanine (G). 

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