THE MOLECULAR COMPOSITION OF CELLS (NUCLEIC ACIDS)

 Nucleic acids, specifically DNA and RNA, are the primary informational molecules in cells. Deoxyribonucleic acid (DNA) plays a central role as the genetic material, primarily located in the nucleus of eukaryotic cells. On the other hand, various forms of ribonucleic acid (RNA) are involved in numerous cellular functions. Messenger RNA (mRNA) carries genetic instructions from DNA to ribosomes, where it acts as a template for protein synthesis. Additionally, ribosomal RNA (rRNA) and transfer RNA (tRNA) are directly involved in the process of protein synthesis, while other RNA types facilitate the processing and transport of RNA and proteins. RNA not only serves as an informational molecule but also has the ability to catalyze specific chemical reactions, particularly in protein synthesis and RNA processing.

DNA and RNA are polymers made up of nucleotides, which consist of nitrogenous bases linked to phosphorylated sugars. DNA contains four bases: two purines (adenine and guanine) and two pyrimidines (cytosine and thymine). In RNA, adenine, guanine, and cytosine are also present, but uracil replaces thymine. The nitrogenous bases attach to sugars—2′-deoxyribose in DNA and ribose in RNA—to form nucleosides. Nucleotides are created when one or more phosphate groups are added to the sugar. Nucleic acids, therefore, consist of purine and pyrimidine bases linked to phosphorylated sugars.

The formation of nucleic acids occurs through the polymerization of nucleotides, where phosphodiester bonds form between the 5′ phosphate group of one nucleotide and the 3′ hydroxyl group of another. Short chains of nucleotides are called oligonucleotides, while longer chains, known as polynucleotides, make up the DNA and RNA found in cells, often containing thousands to millions of nucleotides. These polynucleotide chains have directionality, with one end terminating in a 5′ phosphate group and the other in a 3′ hydroxyl group.

Polynucleotides are synthesized in the 5′ to 3′ direction, meaning new nucleotides are always added to the 3′ hydroxyl end of the growing chain. This directionality is reflected in how the sequence of bases in DNA and RNA is written, always in the 5′ to 3′ orientation, corresponding to the process of nucleic acid synthesis.

Nucleotide polymerization occurs through the formation of phosphodiester bonds. These bonds are created between the 3′ hydroxyl group of one nucleotide and the 5′ phosphate group of another, linking them into a polynucleotide chain. This chain has a specific directionality, with one end terminating in a 5′ phosphate group and the other in a 3′ hydroxyl group, giving it a defined sense of orientation.

The sequence of bases along the polynucleotide chain of DNA and RNA encodes the genetic information. DNA, a double-stranded molecule, consists of two polynucleotide chains running in opposite directions. The bases are positioned inside the molecule, where hydrogen bonds between complementary base pairs join the two strands—adenine pairs with thymine, and guanine pairs with cytosine. This complementary base pairing is crucial, as it allows one strand of DNA (or RNA) to serve as a template for synthesizing a complementary strand.

The unique ability of nucleic acids to direct the formation of complementary strands enables DNA and RNA to self-replicate. This capability underlies their role as the cell’s primary informational molecules. DNA and RNA direct the synthesis of specific proteins, which in turn regulate and control most cellular processes and activities.