DNA and RNA: Structure, Function, and Genetic Role

Posted on Feb 22, 2025 in Biology

Nucleic Acids: DNA and RNA

Components of Nucleic Acids

Nucleic acids are polymers formed by a sequence of nucleotides linked through phosphodiester bonds. Partial nucleotide hydrolysis yields orthophosphoric acid and a core consisting of an aldopentose and a nitrogenous base.

  • Pentose: β-ribofuranose or β-deoxyribofuranose.
  • Nitrogenous Bases: Heterocyclic compounds of two main types:
    • Pyrimidines: Derived from pyrimidine; these include cytosine, thymine, and uracil.
    • Purines: Adenine and guanine.

Nucleosides

Nucleosides are formed by a nitrogenous base and a β-aldopentose linked by an N-glycosidic bond between C1 of the pentose and N1 of the pyrimidine base or N9 of the purine base.

Nucleotides

Nucleotides are phosphate esters of nucleosides.

Important Dinucleotides:

  • FAD and FADH2: Consisting of adenosine phosphate and riboflavin phosphate.
  • NAD+ and NADP+: Consisting of adenosine phosphate and nicotinamide nucleotide linked by a pyrophosphate bond (5′-5′).

Deoxyribonucleic Acid (DNA)

DNA is a macromolecule formed by the polymerization of deoxyribose nucleotides A, G, C, and T as the fundamental bases, linked by a phosphodiester bond (3′-5′). It is found in the nucleus, mitochondria, and, in plants, chloroplasts. DNA structure is described in three levels: primary, secondary, and tertiary.

Primary Structure

The sequence of nucleotides in a single strand, differing in the sequence of nitrogenous bases along the phosphodeoxyribose backbone.

Secondary Structure

The arrangement in space of two polynucleotide strands in a double helix, with the nitrogenous bases facing each other and linked by hydrogen bonds.

DNA Features:

  • DNA in solution behaves as a strong acid due to the ionizable phosphate groups. Many divalent cations bind to the surface and core of the molecule, providing stability.
  • Denatured DNA can have its hydrogen bonds and hydrophobic interactions broken, altering the bases.
  • The molecule has the ability of self-replication (autoduplication). Due to the complementarity of the bases, the two strands separate and serve as templates for the synthesis of new complementary strands.
  • The base sequence contains hereditary information and can transfer it through transcription.

Tertiary Structure

Large DNA molecules are coiled around proteins, reducing space and preserving transcription. In eukaryotes, DNA is associated with histones, RNA, and other non-histone proteins. In viruses, it is associated with basic proteins.

DNA can be linear single-stranded, circular single-stranded, linear double-stranded, or circular double-stranded.

Ribonucleic Acid (RNA)

RNA macromolecules are formed by the polymerization of ribonucleotides A, G, C, and U, also linked in 5′ and 3′ sequences. Unlike DNA, RNA typically exists as a single polynucleotide chain, although it can have regions of double helix. RNA is generally less stable than DNA.

Types of RNA:

  • mRNA (messenger RNA): A single-stranded RNA molecule synthesized in the nucleus. Its function is to transmit genetic information to the ribosomes. It has a short lifespan, ranging from minutes to hours in prokaryotes and eukaryotes.
  • tRNA (transfer RNA): Small, single-stranded molecules folded into loops due to complementary base pairing. Its function is to capture activated amino acids from the cytoplasm and transport them to the ribosomes during protein synthesis, placing them in the correct sequence as indicated by mRNA.
  • rRNA (ribosomal RNA): The most abundant type of RNA. Different types exist, but all are associated with proteins to form the macromolecular structure of ribosomes. Ribosomes contain different channels, each of a specific size, which vary between prokaryotic and eukaryotic cells, with the latter being larger.

Role of Nucleic Acids

  • Storage of genetic information.
  • Transmission of genetic information.