CHAPTER 21: Unit 2. Composition of Nucleic Acids

Like proteins and carbohydrates, nucleic acids are polymers. Proteins are polypeptides, carbohydrates are polysaccharides, and nucleic acids are polynucleotides. Each nucleotide has three parts: a five membered cyclic monosaccharide, a nitrogen-containing cyclic compounds known as nitrogenous base, and a phosphate group

(-OPO₃ ^2-),

All nucleic acids are linear, nonbranching polymers of nucleotides, and are therefore polynucleotides. DNA is double-stranded in virtually all organisms. (It is single-stranded in some viruses.) DNA occurs in many, but not all, small organisms as double-stranded and circular (without any ends). Higher organisms (eukaryotes) have approximately ten million base pairs or more, with the genetic material parceled out into multiple genetic pieces called chromosomes. For example, humans have twenty-three pairs of chromosomes in the nucleus of each somatic cell . Within the nucleus, the DNA molecules are found in “looped arrangements” that mimic the circular DNA observed in many prokaryotes.

All nucleic acids are polynucleotides, with each nucleotide being made up of a base, a sugar unit, and a phosphate. The composition of DNA differs from that of RNA in two major ways. Whereas DNA contains the bases guanine (G), cytosine (C), adenine (A), and thymine (T), RNA contains G, C, and A, but it contains uracil (U) in place of thymine. Both DNA and RNA contain a five-membered cyclic sugar (a pentose). RNA contains a ribose sugar. The sugar in DNA, however, is 2′-deoxyribose.

Nucleic acids are linear polymers, which means that they have a beginning and an end and they consist of repeating monomers called nucleotides. Each nucleotide contains a sugar molecule. In DNA molecules, the sugar is a deoxyribose sugar, which means its missing a hydroxyl group on the 2′ carbon atom. In RNA molecules, the sugar is a ribose sugar. The backbone of nucleic acids consists of repeating sugar-phosphate groups that are connected by phosphodiester bonds. A phosphodiester linkage is a connection between the 3′ carbon on one sugar molecule and the 5′ carbon an adjacent sugar molecule. The presence of a negative charge on the phosphate makes the backbone of the nucleic acid negatively charged and hydrophilic. The sugar molecule is also attached onto the nitrogenous base via the 1′ carbon atom. In DNA, the nitrogenous bases are adenine, guanine, cytosine and thymine. In RNA, the thymine is replaced with uracil. Notice that unlike the backbone of the nucleic acids, the bases do not remain constant but change as we move from one nucleotide to the next. Therefore, its the sequence of the bases that determines the genetic code that is stored in the nucleic acid.

What is the basic structure and function of nucleic acids?

Nucleic acids are macromolecules that store genetic information and enable protein production. Nucleic acids include DNA and RNA. These molecules are composed of long strands of nucleotides. Nucleotides are composed of a nitrogenous base, a five-carbon sugar, and a phosphate group.

Basic structure

Each nucleic acid contains four of five possible nitrogen-containing bases: adenine (A), guanine (G), cytosine (C), thymine (T), and uracil (U). A and G are categorized as purines, and C, T, and U are collectively called pyrimidines.

Components of Nucleic Acids

Nucleotide. The building block of nucleic acids, the nucleotide, is composed of a nitrogenous base, a pentose sugar, and one to three phosphates labeled as α, β, and γ.

There are two types of nucleic acids, deoxyribonucleic acid, DNA, and ribonucleic acid, RNA. DNA stores genetic information used for the synthesis of proteins including enzymes and is found in the nucleus and mitochondria. RNA has several functions and is found in the nucleus, cytosol and mitochondria. Messenger RNA (mRNA) carries genetic information obtained from DNA to sites that translate the information into a protein. Transfer RNA (tRNA) carries activated amino acids to sites where the amino acids are linked together to form polypeptides. Ribosomal RNA (rRNA) is a structural component of ribosomes, which serve as the sites for protein synthesis. Small nuclear RNA (snRNA) is a component of small nuclear ribonucleoprotein particles. These particles process heterogeneous RNA (hnRNA, the immature form of mRNA) into mature mRNA. In some viruses, HIV, influenza, polio, RNA functions as the storage house of genetic information.The study of the replication and expression of genetic information involves a unidirectional flow from DNA to RNA to Protein:

DNA ————->RNA————->Protein

A Review of The Bases

Pyrimidines:

Purines:

Nucleotide Composition

BaseRibonucleosideRibonucleotide (Base + Ribose)(Base + Ribose + Phos.)Adenine(A)AdenosineAdenosine 5′-monophosphate(AMP)Guanine(G)GuanosineGuanosine 5′-monophosphate(GMP)Cytosine(C)CytidineCytidine 5′-monophosphate(CMP)Uracil(U)UridineUridine 5′-monophosphate(UMP)

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