The twisting of the two strands around each other results in the formation of uniformly spaced major and minor grooves (Figure 3). Only the pairing between a purine and pyrimidine can explain the uniform diameter. The diameter of the DNA double helix is 2 nm, and it is uniform throughout. Therefore, ten base pairs are present per turn of the helix. Each base pair is separated from the other base pair by a distance of 0.34 nm, and each turn of the helix measures 3.4 nm. The sugar and phosphate of the nucleotides form the backbone of the structure, whereas the nitrogenous bases are stacked inside. The two strands are anti-parallel in nature that is, the 3′ end of one strand faces the 5′ end of the other strand. The base pairs are stabilized by hydrogen bonds adenine and thymine form two hydrogen bonds and cytosine and guanine form three hydrogen bonds. Adenine and thymine are complementary base pairs, and cytosine and guanine are also complementary base pairs. Base pairing takes place between a purine and pyrimidine namely, A pairs with T and G pairs with C. Watson and Crick proposed that DNA is made up of two strands that are twisted around each other to form a right-handed helix. The phosphate backbone of DNA has a negative charge as a result of the presence of hydrogen bonds formed between both the phosphorus and oxygen atoms in the. (credit a: modification of work by Marjorie McCarty, Public Library of Science) Scientist Rosalind Franklin discovered (b) the X-ray diffraction pattern of DNA, which helped to elucidate its double helix structure. The work of pioneering scientists (a) James Watson, Francis Crick, and Maclyn McCarty led to our present day understanding of DNA. Unfortunately, by then Franklin had died, and Nobel prizes are not awarded posthumously.įigure 2. In 1962, James Watson, Francis Crick, and Maurice Wilkins were awarded the Nobel Prize in Medicine. Watson and Crick were able to piece together the puzzle of the DNA molecule on the basis of Franklin’s data because Crick had also studied X-ray diffraction (Figure 2). covalent bonds are formed between the phosphate of one nucleotide and. In Wilkins’ lab, researcher Rosalind Franklin was using X-ray diffraction methods to understand the structure of DNA. The base and phosphate are both linked together by covalent bonds to the pentose sugar. Pauling had discovered the secondary structure of proteins using X-ray crystallography. Other scientists like Linus Pauling and Maurice Wilkins were also actively exploring this field. In the 1950s, Francis Crick and James Watson worked together to determine the structure of DNA at the University of Cambridge, England. The phosphate residue is attached to the hydroxyl group of the 5′ carbon of one sugar of one nucleotide and the hydroxyl group of the 3′ carbon of the sugar of the next nucleotide, thereby forming a 5′-3′ phosphodiester bond. The nucleotides combine with each other by covalent bonds known as phosphodiester bonds or linkages. The sugar is deoxyribose in DNA and ribose in RNA Each nucleotide is made up of a sugar, a phosphate group, and a nitrogenous base. Note that several other enzymes are also involved in DNA repair, a series of highly co-ordinated processes that ensure the integrity of the genetic code is not disrupted.Figure 1. Essentially they rejoin breaks in the DNA backbone.ĭNA ligase joining the sticky ends of DNA fragments that have been cut with a restriction enzyme.ģ-D representation of DNA ligase repairing DNA after chromosomal damage (double-strand break). The nitrogenous bases form weak hydrogen bonds that hold the two strands together, while the sugar-phosphate backbone is held together by stronger covalent bonds known as phosphodiester bonds.ĭNA ligases link DNA fragments by inducing the formation of a phosphodiester bond between the 3' hydroxyl end of one nucleotide with the 5' phosphate end of another nucleotide (shown right). A DNA double helix resembles a twisted ladder in which the nitrogenous bases (A, T, C, G) form the runs of the ladder and the repeating sugar-phosphate groups form the sides or the ‘backbone’ of the DNA molecule. Each nucleotide consists of a sugar, phosphate and nitrogenous base. They are involved in DNA repair, DNA replication (link Okazaki fragments together) and are also used for ‘sticking together’ (ligating) DNA stands that have been cut by restriction enzymes.Ī DNA molecule is comprised of repeating subunits known as nucleotides. DNA ligases are a class of enzymes that can fix breaks or link together DNA strands.
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