Unit 5: Nucleic Acids

1. Structure of Purines, Pyrimidines, Nucleosides, and Nucleotides

Nucleic acids are the informational molecules of the cell, composed of long chains of nucleotides.

Purines and Pyrimidines

These are nitrogenous bases that form the core of nucleic acids.

• Purines: Double-ring structures including Adenine (A) and Guanine (G).
• Pyrimidines: Single-ring structures including Cytosine (C), Thymine (T) in DNA, and Uracil (U) in RNA.

Nucleosides vs. Nucleotides

• Nucleoside: A nitrogenous base linked to a pentose sugar (Ribose or Deoxyribose).
• Nucleotide: A nucleoside plus one or more phosphate groups. Nucleotides are the monomers of DNA and RNA.

Base Pairing

Bases pair specifically through hydrogen bonds: A pairs with T/U, and G pairs with C. Hoogsteen base pairs are alternative base pairings that occur in certain triple-helix or specialized DNA structures.

2. Watson and Crick Model of B-DNA

In 1953, James Watson and Francis Crick proposed the double-helix model for B-DNA.

Key Features of B-DNA

• Double Helix: Two polynucleotide strands wound around a common axis in a right-handed manner.
• Antiparallel Strands: One strand runs 5' to 3', while the other runs 3' to 5'.
• Sugar-Phosphate Backbone: Located on the outside of the helix.
• Internal Bases: Hydrophobic nitrogenous bases are stacked inside, perpendicular to the axis.
• Complementary Pairing: A=T (2 hydrogen bonds) and G≡C (3 hydrogen bonds).
• Dimensions: Each turn contains 10.5 base pairs and is approximately 3.4 nm long.

3. Structure and Functions of Different Types of RNA

RNA is typically single-stranded and contains ribose sugar and uracil instead of thymine.

Major Types of RNA

4. Denaturation, Renaturation, and Cot Curves

Physical properties of DNA change with environmental conditions like heat.

Denaturation and Renaturation

• Denaturation (Melting): Separation of double-stranded DNA into single strands by breaking hydrogen bonds, usually by heat or high pH.
• Renaturation (Annealing): The process where complementary single strands reform the double helix upon cooling.

Hyperchromaticity vs. Hypochromaticity

Hyperchromaticity: The increase in UV light absorption (at 260 nm) when DNA is denatured into single strands.
Hypochromaticity: The decrease in UV absorption when single strands re-anneal into a double helix.

Cot Curves

A Cot curve is a graph used to analyze DNA complexity. It plots the fraction of DNA renatured against the product of initial DNA concentration (Co) and time (t). Highly repetitive DNA sequences renature faster than unique sequences.

5. Exam Focus Enhancements