Pairing in DNA

Figure 7.4 The edges of the base pairs in DNA that ate in the major groove are wider than those in the minor groove, due to the asymmetric-attachment of the base pairs to the sugar-phosphate backbone (a). These edges contain different hydrogen bond donors and acceptors for potentially specific interactions with proteins (b).

A clue to the chemical basis of base pairing in DNA came from the analysis of the base composition of various DNAs by Erwin Chargaff in the late 1940s. His data showed that the four bases commonly found in DNA (A, C, G, and T) do not occur in equimolar amounts and that the relative amounts of each vary from species to species (Table 11.3). Nevertheless, Chargaff noted that certain pairs of bases, namely, adenine and thymine, and guanine and cytosine, are... 

As indicated in Chapter 11, the base pairing in DNA is very specific the purine adenine pairs with the pyrimidine thymine the purine guanine pairs with the pyrimidine cytosine. Further, the A T pair and G C pair have virtually identical dimensions (Figure 12.10). Watson and Crick realized that units of such similarity could serve as spatially invariant substructures to build a polymer whose exterior dimensions would be uniform along its length, regardless of the sequence of bases. 

Based on your analysis, is it likely that tautomeric equilibria involving the nucleotide bases will interfere noticeably with base pairing in DNA Explain. 

Insert An additional length of base pairs in DNA, generally introduced by the techniques of recombinant DNA technology. 

Groenhof G, Schafer LV, Boggio-Pasqua M, Goette M, Grubmuller H, Robb MA (2007) Ultrafast deactivation of an excited cytosine-guanine base pair in DNA. J Am Chem Soc 129 6812—6819... 

Adenine (A) A nitrogenous purine base, which bonds with thymine (T) to form the A-T base pair in DNA and the A-U base pair in RNA. 

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). 

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). 

Hereditary nonpolyposis colorectal cancer results from a deficiency in the ability to repair mismatched base pairs in DNA that are accidentally introduced during replication. 

The nitrogenous base pairs in DNA (A-T and C-G) link through the formation of hydrogen bonds within the double-stranded (ds) structure for DNA. 

FIGURE 10.8 Hydrogen bonding between preferred base pairs in DNA. Top structures illustrate the number of hydrogen bonds and the bottom the bond distances between preferred base pairs. 

Change in Base Sequence in DNA Causing Mutation A change in single base pair in DNA can cause substitution of Bio-organic Chemistry... 

As with proteins, the nucleic acid polymers can denature, and they have secondary structure. In DNA, two nucleic acid polymer chains are twisted together with their bases facing inward to form a double helix. In doing so, the bases shield their hydrophobic components from the solvent, and they form hydrogen bonds in one of only two specific patterns, called base pairs. Adenine hydrogen bonds only with thymine (or uracil in RNA), and guanine pairs only with cytosine. Essentially every base is part of a base pair in DNA, but only some of the bases in RNA are paired. The double-helix structure... 

FIG U RE 3.4 Watson-Crick base pairing in DNA. Adenine is complementary to thymine, and guanine is complementary to cytosine. 

Base Pairing in DNA In samples of DNA isolated from two unidentified species of bacteria, X and Y, adenine makes up 32% and 17%, respectively, of the total bases. What relative proportions of adenine, guanine, thymine, and cytosine would you expect to find in the two DNA samples What assumptions have you made One of these species was isolated from a hot spring (64 °C). Suggest which species is the thermophilic bacterium. What is the basis for your answer ... 

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