H

OH OH OH H

Ribose Deoxyribose

10.9 A nucleotide contains either a ribose sugar (in RNA) or a deoxyribose sugar (in DNA). The atoms of the five-sided ring are assigned primed numbers.

The sugars of DNA and RNA are slightly different in structure. RNA's ribose sugar has a hydroxyl group attached to the 2'-carbon atom, whereas DNA's sugar, called deoxyribose, has a hydrogen atom at this position and contains one oxygen atom fewer overall. This difference gives rise to the names ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). This minor chemical difference is recognized by all the cellular enzymes that interact with DNA or RNA, thus yielding specific functions for each nucleic acid. Further, the additional oxygen atom in the RNA nucleotide makes it more reactive and less chemically stable than DNA. For this reason, DNA is better suited to serve as the long-term repository of genetic information.

The second component of a nucleotide is its nitrogenous base, which may be of two types — a purine or a pyrimidine ( FIGURE 10.10). Each purine consists of a six-sided ring attached to a five-sided ring, whereas each pyrimidine consists of a six-sided ring only. DNA and RNA both contain two purines, adenine and guanine (A

0 0

Post a comment