The base pairs in DNA are adenine with thymine and cytosine with guanine. A hydrogen bond is a weak chemical bond that occurs between hydrogen atoms and more electronegative atoms, like oxygen, nitrogen and fluorine. The participating atoms can be located on the same molecule adjacent nucleotides or on different molecules adjacent nucleotides on different DNA strands.
Hydrogen bonds do not involve the exchange or sharing of electrons like covalent and ionic bonds. The weak attraction is like that between the opposite poles of a magnet. Hydrogen bonds occur over short distances and can be easily formed and broken. They can also stabilize a molecule. Sign up for our Newsletter! Mobile Newsletter banner close. Mobile Newsletter chat close. The nucleotide is named depending on which nitrogenous base is present.
The nitrogenous base can be a purine such as adenine A and guanine G , characterized by double-ring structures, or a pyrimidine such as cytosine C and thymine T , characterized by single-ring structures. In polynucleotides the linear polymers of nucleotides the nucleotides are connected to each other by covalent bonds known as phosphodiester bonds or phosphodiester linkages. Nucleotide Structure : Each nucleotide is made up of a sugar, a phosphate group, and a nitrogenous base.
In their mononucleotide form, nucleotides can have one, two, or three phosphates attached to them. When linked together in polynucleotide chains, the nucleotides always have just one phosphate. A molecule with just a nitrogenous base and a sugar is known as a nucleoside. Once at least one phosphate is covalently attached, it is known as a nucleotide. Watson and Crick proposed that DNA is made up of two polynucleotide strands that are twisted around each other to form a right-handed helix.
The two polynucleotide strands are anti-parallel in nature. That is, they run in opposite directions. The sugars and phosphates of the nucleotides form the backbone of the structure, whereas the pairs of nitrogenous bases are pointed towards the interior of the molecule. The twisting of the two strands around each other results in the formation of uniformly-spaced major and minor grooves bordered by the sugar-phosphate backbones of the two strands.
The two anti-parallel polynucleotide strands are colored differently to illustrate how they coil around each other. B is a cartoon model of DNA, where the sugar-phosphate backbones are represented as violet strands and the nitrogenous bases are represented as color-coded rings.
C is another spacefill model, with the sugar-phosphate atoms colored violet and all nitrogenous base atoms colored green. The major and minor grooves, which wrap around the entire molecule, are apparent as the spaces between the sugar-phosphate backbones. The diameter of the DNA double helix is 2 nm and is uniform throughout.
Only the pairing between a purine and pyrimidine can explain the uniform diameter. That is to say, at each point along the DNA molecule, the two sugar phosphate backbones are always separated by three rings, two from a purine and one from a pyrimidine. The two strands are held together by base pairing between nitrogenous bases of one strand and nitrogenous bases from the other strand.
Base pairing takes place between a purine and pyrimidine stabilized by hydrogen bonds: A pairs with T via two hydrogen bonds and G pairs with C via three hydrogen bonds. The interior basepairs rotate with respect to one another, but are also stacked on top of each other when the molecule is viewed looking up or down its long axis. Each carbon of ribose is numbered as shown. In an aqueous environment, like inside the cell, the phosphate groups are negatively charged, as drawn in the figure above.
Although each nucleotide in DNA contains identical sugar and phosphate groups, there are four different bases and thus four different nucleotides that can be incorporated into DNA. The four bases are adenine , cytosine , guainne , and thymine , and their structures are shown below.
We often shorten this notation to A, C, G, and T. Note that two pairs of bases have similar structures. A and G both have two carbon-nitrogen rings and are known as purines. In contrast, C and T have a single carbon-nitrogen ring and belong to a class of molecules called pyrimidines. Copyright , Nature Education.
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