DNA as a double helix

The Watson and Crick model for DNA as a double helix is only a generalized model to describe much more complex structures. Along with the typical double helix there exist structural elements such as supercoils, kinks, cruciforms, bends, loops, and triple strands as well as major and minor grooves. Each of these structural elements can vary in length, shape, location, and frequency. Even the simple DNA double helix can vary in pitch (number of bases per helical turn), sugar pucker conformation, and helical sense (whether the helix is left-or right-handed). 

The spectacular discoveries in the area of nucleic acids started with the work of the British scientists Francis Crick, James Watson, and Maurice Wilkins, who were the first to describe the molecular structure of DNA as a double helix made up of two twisted strands. They were jointly awarded the 1962 Nobel Prize for Physiology or Medicine for their discoveries. 

One of the most exciting biological discoveries is the recognition of DNA as a double helix (Watson and Crick, 1953) of two antiparallel polynucleotide chains with the base pairings between A and T, and between G and C (Watson and Crick s DNA structure). Thus, the nucleotide sequence in one chain is complementary to, but not identical to, that in the other chain. The diameter of the double helix measured between phosphorus atoms is 2.0 nm. The pitch is 3.4 nm. There are 10 base pairs per turn. Thus the rise per base pair is 0.34 nm, and bases are stacked in the center of the helix. This form (B form), whose base pairs lie almost normal to the helix axis, is stable under high humidity and is thought to approximate the conformation of most DNA in cells. However, the base pairs in another form (A form) of DNA, which likely occurs in complex with histone, are inclined to the helix axis by about 20° with 11 base pairs per turn. While DNA molecules may exist as straight rods, the two ends bacterial DNA are often covalently joined to form circular DNA molecules, which are frequently supercoiled. 

Watson and Crick s discovery of the structure of DNA revolutionized scientists understanding of heredity and genetics. Find out how Watson and Crick showed that the structure of DNA is a double helix. How did Rosalind Franklin s work in X-ray crystallography play a role Present your findings as an annotated, illustrated time line. 

Figure 5-35 Stereoscopic drawings illustrating the binding of a dimeric molecule of the Trp repressor protein to a palindromic sequence in DNA. (A) Schematic view showing structures of the aporepressor (partly shaded gray) and the holorepressor with bound tryptophan (unshaded) are superimposed. Cylinders represent the a helices in (B). From Zhang et al.wi (B) MolScript ribbon diagram with a few side chains that interact with the DNA shown. Two tandemly bound dimeric repressor molecules are shown. Two bound molecules of tryptophan are visible in each dimer. The DNA is drawn as a double helix with lines representing the base pairs. From Lawson and Carey.405...

Chargaff s Studies of DNA Structure The chapter section DNA Is a Double Helix that Stores Genetic Information includes a summary of the main findings of Erwin Chargaff and his coworkers, listed as four conclusions ( Chargaff s rules p. 278). In this problem, you will examine the data Chargaff collected in support of these conclusions. 

Deoxyribonucleic acid (DNA) carries the genetic information of all cellular forms of life. DNA is usually found as a double helix, in which the nucleoside bases of the single strands are stacked upon each other, forming strong hydrogen bonds with... 

There are single strands of DNA, but the biological polymer is mostly found as a double helix in which two DNA strands spiral around each other as shown in Figure 11. The two strands are not duplicates of each other. Instead, they are complementary. This means that where an adenine (A) is found in one strand, thymine (T) is found in the other. Likewise, a guanine (G) in one strand is matched with a cytosine (C) in the other. 

DNA uses adenine, thymine, cytosine, and guanine as bases RNA uses uracil rather than thymine. DNA has deoxyribose RNA has ribose. DNA usually can be found as a double helix RNA is commonly found single stranded and nonhelical. 

The structures of both DNA and RNA depend on the sequence of the nucleotides (i.e. the amine bases). Watson and Crick showed that DNA is a double helix composed of two strands with complementary bases, which hydrogen bond to one another. A and T form strong bonds to one another, as does C and G. 

This chapter urges for the combination of both of these target outcomes, early mechanical engineering content and competency mastery, as essential in mechanical engineering education. Figure 3-1 below depicts the mental model of content and competency development as a double helix DNA. The relationship between both content and competency development is spiraled and interdependent meaning that both outcomes are needed and will co-exist and support each other. 

Both DNA and RNA are polymers their monomer units are called nucleotides and consist of three parts a phosphate group, a pentose (5-carbon sugar) and a nitrogen-containing base. The DNA molecules vary in size, but they can have as many as 30,000 base pairs, and they are the largest of all natural molecules. We now know that the three-dimensional staicture of DNA is a double helix a form resembling a spiral staircase. The outside handrails are the sugar-phosphate... 

Polysaccharides (monomers, sugars), proteins (monomers, amino acids), and nucleic acids (monomers, nucleotides) are natural polymers. DNA occurs as a double helix, with bases in each strand H-bonded to specific bases in the other. The base sequence of an organism s DNA determines the amino-acid sequences of its proteins, which determine the proteins structure and function. 

We have seen that the macromolecule of DNA is a double helix in the form of a spiral staircase in which the links between the different nucleic adds form the banisters 2uid the purine and pyrimidine bases form the steps. As we have seen, each step is formed hy one of the two combinations adraiine-thymine (or thymine-adenine) and guanine-cytosine (or cytosine-guanine). This means that a particular sequence of bases on one of the banisters is paired to one definite complementary sequence on the other. 

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