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Chemistry LibreTexts

9.1: Nucleotides

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    342759
    • Contributed by Anonymous
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    Learning Objectives

    • Identify the components of nucleosides and nucleotides.
    • Identify structural differences between the nitrogenous bases.
    • To identify the different molecules that combine to form nucleotides.
    • Differentiate between the components in DNA and RNA.
    • Demonstrate naming nucleosides and nucleotides.

     

    Nucleic acids are molecules that store and replicate information for cellular growth and reproduction. The two types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).  The repeating, or monomer, units that are linked together to form nucleic acids are known as nucleotides.

     

    Structural Components of Nucleotides

    The deoxyribonucleic acid (DNA) of a typical mammalian cell contains about 3 × 109 nucleotides. Nucleotides can be further broken down to phosphoric acid (H3PO4), a pentose sugar (a sugar with five carbon atoms), and a nitrogenous base (a base containing nitrogen atoms).

     

    \[\mathrm{nucleic\: acids \underset{down\: into}{\xrightarrow{can\: be\: broken}} nucleotides \underset{down\: into}{\xrightarrow{can\: be\: broken}} H_3PO_4 + nitrogen\: base + pentose\: sugar}\]

     

    If the pentose sugar is ribose, the nucleotide is more specifically referred to as a ribonucleotide, and the resulting nucleic acid is ribonucleic acid (RNA). If the sugar is 2-deoxyribose, the nucleotide is a deoxyribonucleotide, and the nucleic acid is DNA.

     

    ribose and deoxyribose
     
    Figure \(\PageIndex{1}\): Structures of pentose sugar found in nucleic acids: ribose (left) and 2-deoxyribose (right).
     

    The nitrogenous bases found in nucleotides are classified as pyrimidines or purines. Pyrimidines are heterocyclic amines with two nitrogen atoms in a six-member ring and include uracil, thymine, and cytosine. Purines are heterocyclic amines consisting of a pyrimidine ring fused to a five-member ring with two nitrogen atoms. Adenine and guanine are the major purines found in nucleic acids (Figure \(\PageIndex{2}\)).

     

    Figure 2.jpg

    Figure \(\PageIndex{2}\): The Nitrogenous Bases Found in DNA and RNA

     

    The formation of a glycosidic bond between C1′ of the pentose sugar and N1 of the pyrimidine base or N9 of the purine base joins the pentose sugar to the nitrogenous base. In the formation of this bond, a molecule of water is removed. Table \(\PageIndex{1}\) summarizes the similarities and differences in the composition of nucleotides in DNA and RNA.

     

    The numbering convention is that primed numbers designate the atoms of the pentose ring, and unprimed numbers designate the atoms of the purine or pyrimidine ring.

     

    Table \(\PageIndex{1}\): Composition of Nucleotides in DNA and RNA

    Composition DNA RNA
    purine bases adenine and guanine adenine and guanine
    pyrimidine bases cytosine and thymine cytosine and uracil
    pentose sugar 2-deoxyribose ribose
    inorganic acid phosphoric acid (H3PO4) H3PO4

     

    Nucleosides

    A nucleoside is produced during the condensation reaction between the pentose and nitrogenous base.

    nucleoside

    Figure \(\PageIndex{3}\): Structure of a nucleoside formed with the pentose sugar (shaded orange) and adenine base (shaded green).

     

    Nucleosides are named according to the identity of the nitrogenous base, by changing the ending of the name. If the base is a purine, the -ine ending is replaced -osine. If the base is a pyrimidine, the -ine or -il ending is replaced with -idine.  If the nucleotide represents a deoxyribonucleotide, the word deoxy- is added to the front of the name. Using these guidelines, the name of the nucleoside shown in Figure \(\PageIndex{3}\) is deteremined to be adenosine.

     

    Naming Nucleotides

    Nucleotides are named by adding monophosphate (sometimes shown as 5’-monophosphate) to the end of the name of the corresponding nucleoside.  In addition to the full name, abbreviations can be used to indicate the composition of the nucleotide. If deoxyribose is present, a lower case d is used.  The names (full and abbreviated) and structures of the major ribonucleotides and one of the deoxyribonucleotides are given in Figure \(\PageIndex{4}\).

    Figure 3.jpg

    Figure \(\PageIndex{4}\): The Pyrimidine and Purine Nucleotides

     

    Apart from being the monomer units of DNA and RNA, the nucleotides and some of their derivatives have other functions as well. Adenosine diphosphate (ADP) and adenosine triphosphate (ATP), shown in Figure \(\PageIndex{5}\), have a role in cell metabolism. Moreover, a number of coenzymes, including flavin adenine dinucleotide (FAD), nicotinamide adenine dinucleotide (NAD+), and coenzyme A, contain adenine nucleotides as structural components.

    151590641374775478.jpg

     

    Figure \(\PageIndex{5}\): Structures of Two Important Adenine-Containing Nucleotides

     

    Example \(\PageIndex{1}\)

    Classify each compound as a pentose sugar, a purine, or a pyrimidine.  Indicate whether it can exist in DNA, RNA, or both.

    1. adenine

    2. guanine

    3. deoxyribose

       

    Solution

    1. purine, it can exist in both DNA and RNA

    2. purine, it can exist in both DNA and RNA

    3. pentose sugar, it can only exist in DNA

     

    Exercise \(\PageIndex{1}\)

    Classify each compound as a pentose sugar, a purine, or a pyrimidine.  Indicate whether it can exist in DNA, RNA, or both.

    1. thymine

    2. ribose

    3. cytosine

     

    Summary

    Nucleosides are composed of a pentose sugar (ribose or deoxyribose) and a nitrogen-containing base (adenine, cytosine, guanine, thymine, or uracil). Nucleosides containing a purine base end with osine and those containing a pyrimidine end with idine. If the deoxyribose sugar is present, deoxy is added to the front of the name.

    Nucleotides are composed of phosphoric acid, a pentose sugar (ribose or deoxyribose), and a nitrogen-containing base (adenine, cytosine, guanine, thymine, or uracil). Ribonucleotides contain ribose, while deoxyribonucleotides contain deoxyribose. Nucleotides are named by adding monophosphate to the end of the nucleoside name.