Structure and Properties of Nucleic Acids (DNA & RNA)

Nucleic Acids

Overview of Nucleic Acids

Nucleic acids are large biomolecules essential for all known forms of life. They store and transmit genetic information. The two main types are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

• Polymer made of repeating nucleotides (monomers).

• Each nucleotide consists of three parts:

  • Phosphate group (negatively charged)

  • Pentose sugar (deoxyribose in DNA, ribose in RNA)

  • Nitrogenous base (four varieties in DNA: Adenine (A), Thymine (T), Cytosine (C), Guanine (G); in RNA, Uracil (U) replaces Thymine)

Nucleotide Structure

Pentose Sugar and Carbon Numbering

The pentose sugar in nucleotides has five carbon atoms, numbered 1' to 5' in a clockwise direction. The numbering is important for understanding how nucleotides connect to form nucleic acids.

• 1' (one prime) carbon: Attaches to the nitrogenous base.

• 3' (three prime) carbon: Has a hydroxyl group (-OH); forms a phosphodiester bond with the phosphate group of another nucleotide.

• 5' (five prime) carbon: Binds to the phosphate group.

Example: In DNA, the deoxyribose sugar lacks an oxygen atom at the 2' position compared to ribose in RNA.

DNA Directionality

Antiparallel Strands and 5' to 3' Orientation

DNA consists of two complementary strands that run in opposite directions, described as antiparallel. This orientation is crucial for DNA replication and function.

• One strand runs 5' → 3', the other runs 3' → 5'.

• The 5' end has a free phosphate group; the 3' end has a free hydroxyl group.

• DNA is "read" and synthesized in the 5' to 3' direction.

Example: During DNA replication, new nucleotides are added to the 3' end of the growing strand.

Bonds in Nucleic Acids

Types of Bonds

Two main types of bonds stabilize the structure of nucleic acids:

• Hydrogen bonds: Weak bonds that hold complementary nitrogenous bases together between the two DNA strands.

  • Adenine (A) pairs with Thymine (T) via two hydrogen bonds.

  • Cytosine (C) pairs with Guanine (G) via three hydrogen bonds.

• Phosphodiester bonds: Strong covalent bonds that link the 3' carbon of one nucleotide's sugar to the 5' phosphate group of the next nucleotide, forming the sugar-phosphate backbone.

Equation for Phosphodiester Bond Formation:

$\text{Nucleotide}_1\text{-}3'\text{OH} + \text{Nucleotide}_2\text{-}5'\text{P} \rightarrow \text{Nucleotide}_1\text{-}3'\text{O}-\text{P}-\text{O}-5'\text{Nucleotide}_2 + \text{H}_2\text{O}$

Base Pairing Rules

Pyrimidines and Purines

Nitrogenous bases are classified as either pyrimidines (single ring: Cytosine, Thymine, Uracil) or purines (double ring: Adenine, Guanine). Base pairing follows strict rules to maintain the uniform width of the DNA double helix.

• Pyrimidines (C, T, U) always pair with purines (A, G).

• Base pairing rules:

  • DNA: A pairs with T, C pairs with G

  • RNA: A pairs with U, C pairs with G

• Mnemonic: "CUT the Py" (C, U, T are pyrimidines); "PUR As Gold" (A, G are purines)

Example: In a DNA strand, if one sequence is 5'-ATCG-3', the complementary strand is 3'-TAGC-5'.

Comparison of DNA and RNA

Structural Differences

DNA and RNA differ in their sugars, bases, and structure. The following table summarizes the key differences:

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Strands	Double-stranded (usually)	Single-stranded (usually)
Location	Nucleus (mainly)	Nucleus & cytoplasm
Function	Genetic information storage	Protein synthesis, gene expression

Example: DNA contains thymine, while RNA contains uracil in its place.

Additional info: The antiparallel nature of DNA and the specific base pairing are essential for accurate DNA replication and transcription processes.