Structure and Function of Nucleic Acids

Antiparallel Strands and Hydrogen Bonding

A critical feature of the DNA molecule is that the two strands are antiparallel. This means they run in opposite directions; if one strand is oriented from the $5'$ end to the $3'$ end, the complementary strand will run from the $3'$ end to the $5'$ end.

The nitrogenous bases on opposite strands are held together by relatively weak hydrogen bonds. While a single hydrogen bond is weak, the millions of bonds across a DNA molecule provide significant stability to the structure.

Nitrogenous Bases and Pairing Rules

The nitrogenous bases are categorized into two structural groups based on their ring shapes:

• Purines: These have a double-ring structure. The purines are Adenine (A) and Guanine (G).
• Pyrimidines: These have a single-ring structure. The pyrimidines are Thymine (T) and Cytosine (C).

Because of their shapes, specific pairing rules (often called Chargaff's rules) must be followed to ensure the width of the DNA ladder remains constant:

• Adenine always pairs with Thymine ($A-T$).
• Guanine always pairs with Cytosine ($G-C$).

Study Tip: To remember the difference between the shapes, remember: "Pure As Gold" (Purines = Adenine and Guanine) and "CUT the Py" (Cytosine, Uracil, and Thymine are Pyrimidines). Note that Uracil is only used in RNA!

RNA: The Messenger

Ribonucleic acid, or RNA, is the second major type of nucleic acid. While DNA acts as the permanent storage of information, RNA functions to carry messages from the DNA to the cell's machinery to facilitate protein synthesis.

RNA differs from DNA in three primary ways:

1. Strand Structure: RNA is typically single-stranded, whereas DNA is double-stranded.
2. Sugar Type: RNA contains a ribose sugar, while DNA contains deoxyribose.
3. Nitrogenous Bases: RNA uses Uracil (U) instead of Thymine.

The four bases found in RNA are Adenine (A), Guanine (G), Cytosine (C), and Uracil (U).

Energy Carriers: ATP

Not all nucleotides are used for genetic storage. Some specialized nucleotides function as the primary energy currency of the cell. The most important of these is Adenosine Tri-Phosphate, or ATP.

ATP is a nucleotide that stores energy in the bonds between its phosphate groups. When a cell requires energy to perform work, it undergoes a phosphorylation reaction (specifically a hydrolysis reaction). During this process, the terminal (last) phosphate group is removed from the ATP molecule, releasing a burst of usable energy for the cell.