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Carbohydrates and Nucleic Acids Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 16, 2011

Nucleic Acid

The nucleic acids, RNA (ribonucleic acid) and DNA (deoxyribonucleic acid), are carbohydrate biopolymers with phosphate backbones. The repeating sugar in RNA is ribose, and in DNA it is 2-deoxyribose.

Nucleosides are glycosides of ribofuranose or deoxyribofuranose. Ribofuranose is the monosaccharide ribose in the furanose form. The nitrogenous bases (below) are bonded to the anomeric carbon (C1) of the sugar.

Nucleic Acid

Adenine and guanine are known as the purine bases. Cytosine, thymine, and uracil are the pyrimidine bases.

Nucleotides are phosphate esters of nucleosides, formed at the CH2OH group of the sugar. Adenosine triphosphate (ATP) is the triphosphate ester of adenosine, formed from adenine and ribose.

Nucleic Acid

Nucleic acids are polymeric nucleotides in which phosphate esters link ribose or deoxyribose molecules through the C1–OH of one and the C3–OH of the other. In RNA, the aglycone nitrogen bases are cytosine, adenine, guanine, and uracil. In DNA, thymine replaces uracil. The RNA polymer is like that of DNA, except that in DNA an H replaces the OH group on C2 of the ribose ring.

DNA, a constituent of the cell nucleus, consists of two strands of polynucleotides that are coiled to form a double helix. The strands are held together by H bonding between the nitrogen bases. The pyrimidines always form H bonds with a specific purine; i.e., cytosine with guanine and thymine with adenine. However, in RNA the pairing is between uracil and adenine.

Through its sequence of nitrogen bases, DNA stores the genetic information for cell function and division, and for biosynthesis of enzymes and other essential proteins. In protein synthesis, the information in the DNA is transcribed onto messenger RNA (mRNA), which moves from the nucleus to the ribosomes in the cytoplasm of the cell. Here the information is transferred to ribosomal RNA (rRNA). Transfer RNA (tRNA) carries amino acids to the surface of the rRNA, where the protein is "grown." A specific three-term sequence of bases in the mRNA, called a codon, calls up a tRNA carrying the specific amino acid that is to be the next unit in the growing protein chain. For example, the codon cytosine. uracil. guanine is translated as "leucine."

The existence of 64 – 20 = 44 excess codons allows a valuable redundancy in the genetic code. It also permits the signaling for the start and end of the protein chain.

Practice problems for these concepts can be found at: Carbohydrates and Nucleic Acids Practice Problems

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