10.5.2. Structure of Nucleic Acids

Complete hydrolysis of DNA (or RNA) yields a pentose sugar, phosphoric acid and nitrogen containing heterocyclic compounds (called bases). In DNA molecules, the sugar moiety is b-D-2-deoxyribose whereas in RNA molecule, it is b-D-ribose.

Nucleic acids, which include DNA and RNA, are fundamental components of cellular life. When we perform complete hydrolysis (a process that breaks down molecules into smaller units), we obtain three main components: a pentose sugar, phosphoric acid, and nitrogenous bases. In DNA, the sugar component is specifically 'b-D-2-deoxyribose', meaning it is a five-carbon sugar that lacks one oxygen molecule compared to RNA. RNA, on the other hand, has 'b-D-ribose' which contains all oxygen atoms. This distinction between the sugar components of DNA and RNA is crucial for their different roles in biological systems.

Think of nucleic acids like a recipe. Just as a recipe consists of ingredients like flour (sugar), water (phosphoric acid), and yeast (bases), nucleic acids contain their own specific ingredients: sugars, acids, and nitrogenous bases. The specific combination and types of these ingredients dictate what kind of nucleic acid you will end up with, just like how different recipes produce different foods.

A unit formed by the attachment of a base to 1¢ position of sugar is known as a nucleoside. When nucleoside is linked to phosphoric acid at 5¢-position of sugar moiety, we get a nucleotide. Nucleotides are joined together by phosphodiester linkage between 5¢ and 3¢ carbon atoms of the pentose sugar.

Nucleotides are the building blocks of nucleic acids. A nucleoside is created when a nitrogenous base is attached to the sugar at the 1' (one prime) position. When a phosphoric acid is then attached to the nucleoside at the 5' (five prime) position, we have a complete nucleotide. The nucleotides link together to form chains by phosphodiester bonds, which connect the 5' carbon of one nucleotide to the 3' carbon of another. This structure creates the backbone of the nucleic acid strand, with the bases protruding from the sugar-phosphate backbone.

Imagine building a long chain of beads where each bead represents a nucleotide. The sugar in DNA and RNA acts like a string, while the beads are the bases that hang off the string. When you link the beads together (just like joining nucleotides), they form a long, usable strand—like the process of threading pearls on a fine chain to create a beautiful necklace.

The two strands are complementary to each other because the hydrogen bonds are formed between specific pairs of bases. Adenine forms hydrogen bonds with thymine whereas cytosine forms hydrogen bonds with guanine.

DNA undergoes a unique secondary structure where two strands twist around each other to form a double helix. This structure arises from specific base-pairing rules: adenine pairs with thymine, and cytosine pairs with guanine, facilitated by hydrogen bonds. The pairing ensures that the DNA strands are complementary; for every adenine on one strand, there is a thymine on the opposite strand. This specific pairing is essential for processes like DNA replication and transcription, ensuring that genetic information is accurately copied and passed on.

Think of this pair-bonding in DNA like matching socks. Each sock (nucleotide) is designed to fit perfectly with its matching pair (complementary base)—adenine with thymine and cytosine with guanine. When you put them together, they create a stable structure, just like how matching socks keep your feet cozy and ensure you look put together.

RNA molecules are of three types and they perform different functions. They are named as messenger RNA (m-RNA), ribosomal RNA (r-RNA) and transfer RNA (t-RNA).

RNA plays several critical roles in the cell. The three primary types of RNA are messenger RNA (mRNA), which carries genetic instructions from DNA to the protein-making machinery of the cell; ribosomal RNA (rRNA), which forms the core structural and functional components of ribosomes; and transfer RNA (tRNA), which brings amino acids to the ribosome during protein synthesis. Each type of RNA has a specific structure that suits its function, ensuring that cells can efficiently translate genetic information into proteins.

Consider RNA as the crew of a production team. mRNA acts like the script that directors read (it carries the genetic instructions), rRNA serves as the stage upon which actors perform (it constitutes the ribosome), and tRNA is like the prop master who brings the items needed for the show (it delivers the amino acids) to ensure that everything comes together for the final production, which in this case are proteins.

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