STRUCTURE OF NUCLEIC ACIDS

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• Three characteristic components (a nitrogenous base, a pentose, and a phosphate) are found in nucleotides.
• A nucleoside is called a molecule without the phosphate group.
• Derivatives of two parent compounds, pyrimidine, and purine, are nitrogenous bases.
• Heterocyclic compounds are the bases and pentoses of the common nucleotides.
• An N-glycosyl bond covalently joins the base of a nucleotide to the 1 carbon of the pentose, and the phosphate is esterified to the 5 carbon.
• Two major purine bases (adenine and guanine) and two major pyrimidines (cytosine and either thymine or uracil) are contained in both DNA and RNA.
• DNA contains thymine, and RNA contains uracil. Thymine rarely occurs in RNA, and uracil rarely occurs in DNA.
• The structures of the five major bases are shown in figure 2, and the nomenclature of their corresponding nucleotides and nucleosides is summarized in Table.

  

• Two kinds of pentoses are found in nucleic acids: 2-deoxy-D-ribose in DNA and D-ribose in RNA.
• In nucleotides, both types of pentoses are in their furanose form.
• The pentose ring, which occurs in one of a variety of conformations generally described as “puckered,” is not planar.

• Minor bases, including methylated forms of major bases in DNA and altered or unusual bases, often have roles in regulating or protecting genetic information.
• Nucleotides with phosphate groups in positions other than on the 5 carbon are also present in cells.
• Ribonucleoside 2′,3′-cyclic monophosphates are isolatable intermediates, and ribonucleoside 3′-monophosphates are end products of the hydrolysis of RNA by certain ribonucleases.
• In cases where the purine or pyrimidine ring is substituted, the usual convention is to indicate the ring position of the substituent by its number.

• Minor bases of many types are also found in RNAs, especially in tRNAs.
• The successive nucleotides of both DNA and RNA have covalently linked through phosphate-group “bridges,” in which the 5-phosphate group of one nucleotide unit is joined to the 3-hydroxyl group of the next nucleotide, creating a phosphodiester linkage.
• Phosphodiester linkages are created between successive nucleotides of both DNA and RNA by joining the 5-phosphate group of one nucleotide unit to the 3-hydroxyl group of the next nucleotide through phosphate-group “bridges.”
• The covalent backbones of nucleic acids consist of alternating phosphate and pentose residues, and the nitrogenous bases may be regarded as side groups joined to the backbone at regular intervals.
• Alternating phosphate and pentose residues make up the covalent backbones of nucleic acids, with the nitrogenous bases being regarded as side groups joined to the backbone at regular intervals.
• The hydroxyl groups of the sugar residues form hydrogen bonds with water.
• Hydrogen bonds are formed between water and the hydroxyl groups of the sugar residues.
• The phosphate groups, with a pKa near 0, are completely ionized and negatively charged at pH 7, and the negative charges are generally neutralized by ionic interactions with positive charges on proteins, metal ions, and polyamines.
• Ionic interactions between positive charges on proteins, metal ions, and polyamines generally neutralize the negatively charged, completely ionized phosphate groups with a pKa near 0 at pH 7.
• The covalent backbone of DNA and RNA is subject to slow, nonenzymatic hydrolysis of the phosphodiester bonds.
• Slow, nonenzymatic hydrolysis of the phosphodiester bonds can occur in DNA and RNA covalent backbones.
• Cyclic 2′,3′-monophosphate nucleotides are the first products of the action of alkali on RNA and are rapidly hydrolyzed further to yield a mixture of 2′- and 3′-nucleoside monophosphates.
• The first products of the action of alkali on RNA are cyclic 2′,3′-monophosphate nucleotides, which are rapidly hydrolyzed further to yield a mixture of 2′- and 3′-nucleoside monophosphates.
• The nucleotide sequences of nucleic acids can be represented schematically, as illustrated below by a segment of DNA with five nucleotide units.
• A schematic representation of the nucleotide sequences of nucleic acids can be created, as illustrated below by a segment of DNA with five nucleotide units.
• The connecting lines between nucleotides (which pass through P) are drawn diagonally from the middle (C-3) of the deoxyribose of one nucleotide to the bottom (C-5) of the next.
• Diagonal lines that pass through P are used to draw the connecting lines between nucleotides, extending from the middle (C-3) of the deoxyribose of one nucleotide to the bottom (C-5) of the next.
• Some simpler representations of this pentadeoxyribonucleotide are pA-C-G-T-AOH, pApCpGpTpA, and pACGTA.
• pA-C-G-T-AOH, pApCpGpTpA, and pACGTA are some of the simpler representations of this pentadeoxyribonucleotide.
• Polymers containing 50 or fewer nucleotides are generally called oligonucleotides, while longer