Introducing Nucleic Acids

By the end of this lesson and the next few, you should be able to:

  • Describe the structure of nucleic acids and define the two types of nucleic acids
  • Explain the structure and role of DNA
  • Explain the structure and roles of RNA

Nucleic Acids

Nucleic acids are the most important macromolecules for the continuity of life. In fact, they carry the genetic blueprint of a cell and carry instructions for the functioning of the cell.

DNA and RNA

The two main types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA is the genetic material in all living organisms, ranging from single-celled bacteria to multicellular mammals. It is found in the nucleus of eukaryotes and in the organelles, chloroplasts, and mitochondria. However, in prokaryotes, the DNA is not enclosed in a membranous envelope.

Genome

We refer to the entire genetic content of a cell as its genome, and the study of genomes is genomics. In eukaryotic cells but not in prokaryotes, DNA forms a complex with histone proteins to form chromatin, the substance of eukaryotic chromosomes. A chromosome may contain tens of thousands of genes. Many genes contain the information to make protein products. Other genes code for RNA products. In general, DNA controls all of the cellular activities by turning the genes “on” or “off.”

The other type of nucleic acid, RNA, is mostly involved in protein synthesis. The DNA molecules never leave the nucleus. Instead, they use an intermediary to communicate with the rest of the cell. This intermediary is the messenger RNA (mRNA). Other types of RNA—like rRNA, tRNA, and microRNA—are involved in protein synthesis and its regulation.

Nucleotides

DNA and RNA consist of monomers which we refer to as nucleotides. The nucleotides combine with each other to form a polynucleotide, DNA or RNA. Each nucleotide consists of three components: a nitrogenous base, a pentose (five-carbon) sugar, and a phosphate group (see image below). Each nitrogenous base in a nucleotide is attached to a sugar molecule, which is attached to one or more phosphate groups.

nucleotide

Carbon residues in the pentose are numbered 1′ through 5′ (the prime distinguishes these residues from those in the base, which are numbered without using a prime notation). The base is attached to the 1′ position of the ribose, and the phosphate is attached to the 5′ position. When a polynucleotide forms, the 5′ phosphate of the incoming nucleotide attaches to the 3′ hydroxyl group at the end of the growing chain. Image Attribution: OpenStax Biology

The nitrogenous bases, important components of nucleotides, are organic molecules. In fact, they have that name because they contain carbon and nitrogen. They are bases because they contain an amino group that has the potential of binding an extra hydrogen, and thus, decreases the hydrogen ion concentration in its environment, making it more basic. Each nucleotide in DNA contains one of four possible nitrogenous bases. These are adenine (A), guanine (G) cytosine (C), and thymine (T).

Purines and pyrimidines

Adenine and guanine are classified as purines. The primary structure of a purine is two carbon-nitrogen rings. Cytosine, thymine, and uracil are classified as pyrimidines which have a single carbon-nitrogen ring as their primary structure (see image above). Each of these basic carbon-nitrogen rings has different functional groups attached to it. In molecular biology shorthand, we simply represent the nitrogenous bases by their symbols A, T, G, C, and U. DNA contains A, T, G, and C whereas RNA contains A, U, G, and C.

Deoxyribose and ribose

The pentose sugar in DNA is deoxyribose, and in RNA, the sugar is ribose (see image above). The difference between the sugars is the presence of the hydroxyl group on the second carbon of the ribose and hydrogen on the second carbon of the deoxyribose. We number the carbon atoms of the sugar molecule as 1′, 2′, 3′, 4′, and 5′ (we read 1′ as “one prime”).

The phosphate residue joins to the hydroxyl group of the 5′ carbon of one sugar and the hydroxyl group of the 3′ carbon of the sugar of the next nucleotide, which forms a 5′–3′ phosphodiester linkage. Unlike the other linkages connecting monomers in macromolecules, a simple dehydration reaction does not form the phosphodiester linkage. Actually, its formation involves the removal of two phosphate groups. Generally, a polynucleotide may have thousands of such phosphodiester linkages.

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