Nucleic Acids: Structure and Properties

Concept: Nucleic Acids

Structure of Nucleic Acids

Nucleic acids are essential biomolecules that store and transmit genetic information. They are composed of nucleotides linked together to form long chains.

• Nucleotides are connected by phosphodiester bonds, forming the backbone of DNA and RNA.

• The individual strands of DNA are antiparallel to each other, meaning they run in opposite directions (5' to 3' and 3' to 5').

• RNA is less stable than DNA at high pH because of its 2' hydroxyl group, which makes it more susceptible to hydrolysis.

• Nucleotides and nucleic acids have a maximum light absorption at 260 nm (proteins at 280 nm).

Base Pairing and DNA Structure

The two strands of DNA are complementary due to the specificity of base pairing, which is crucial for the accurate transmission of genetic information.

• Base pairing: Adenine (A) pairs with Thymine (T) via two hydrogen bonds, and Guanine (G) pairs with Cytosine (C) via three hydrogen bonds.

• Strands of DNA with a higher GC composition are harder to separate and have a higher melting temperature due to the extra hydrogen bond in GC pairs.

Example: GC Content and Melting Temperature

• DNA with 60% GC content will have a higher melting temperature than DNA with 40% GC content.

Practice Questions

• What percentage of DNA is made of purines? Pyrimidines? Purines: Adenine (A) and Guanine (G) Pyrimidines: Cytosine (C) and Thymine (T) In double-stranded DNA, the percentage of purines equals the percentage of pyrimidines due to base pairing (A = T, G = C).

• If a piece of double-stranded DNA is made of 35% A and 15% C, what percent of its composition is T and G? Solution: Since A = T, T = 35%. Since G = C, G = 15%. Total: A + T + G + C = 35% + 35% + 15% + 15% = 100%.

Key Terms and Definitions

• Phosphodiester bond: The covalent bond that links the 3' carbon atom of one sugar molecule to the 5' carbon atom of another, forming the backbone of nucleic acids.

• Antiparallel: Refers to the opposite orientation of the two strands in a DNA double helix.

• Purines: Nitrogenous bases with a double-ring structure (Adenine and Guanine).

• Pyrimidines: Nitrogenous bases with a single-ring structure (Cytosine, Thymine, and Uracil in RNA).

Important Equations

• Chargaff's Rule: $\%A = \%T$ and $\%G = \%C$ in double-stranded DNA.

• Total base composition: $\%A + \%T + \%G + \%C = 100\%$

Additional info:

• DNA absorbs UV light most strongly at 260 nm due to the aromaticity of its bases.

• RNA's instability at high pH is due to the presence of the 2' hydroxyl group, which can participate in intramolecular reactions leading to strand cleavage.