🧬 1️⃣ Nucleic Acids – Molecules of Genetic Information

Nucleic acids are biomacromolecules responsible for storage, transmission, and expression of genetic information. They occur as polynucleotides, meaning long chains of repeating units called nucleotides.

🔹 What is a Nucleotide?

A nucleotide is the basic unit of nucleic acids and consists of three components:

1️⃣ Nitrogenous Base (Heterocyclic Compound)

  • Purines (double-ring): Adenine (A), Guanine (G)
  • Pyrimidines (single-ring): Cytosine (C), Thymine (T), Uracil (U)
  • Cytosine: Present in both DNA and RNA
  • Thymine: Present only in DNA
  • Uracil: Present only in RNA

🧠 Memory Trick:

CUT the PY → Cytosine, Uracil, Thymine are Pyrimidines

2️⃣ Pentose Sugar

  • Ribose: Found in RNA
  • 2′-Deoxyribose: Found in DNA (one oxygen less)

🧠 Key Line:

RNA = Ribose | DNA = Deoxy-ribose

3️⃣ Phosphate Group

  • Links sugars of adjacent nucleotides
  • Forms the sugar–phosphate backbone of nucleic acids

🔹 Nucleoside vs Nucleotide (Very Important)

  • Nucleoside = Sugar + Base (No phosphate)
  • Nucleotide = Sugar + Base + Phosphate

🧠 Exam Hack:

"Phosphate present? → Nucleotide"

🧬 Structure of DNA – Watson & Crick Model

James Watson and Francis Crick proposed the double helix model of DNA.

⭐ Key Features of B-DNA (NCERT Favourite)

  • Double-stranded helix
  • Strands are anti-parallel (5′ → 3′ and 3′ → 5′)
  • Sugar–phosphate backbone on the outside
  • Nitrogen bases face inward like ladder steps

🔗 Base Pairing (Complementary)

  • A = T → 2 hydrogen bonds
  • G ≡ C → 3 hydrogen bonds

🧠 Memory Line:

AT = 2 bonds, GC = 3 bonds

📏 Dimensions of B-DNA (Must Memorize)

  • One full turn (Pitch) = 34 Å (3.4 nm)
  • Base pairs per turn = 10
  • Distance between adjacent base pairs = 3.4 Å

🧠 Quick Recall:

10–34–3.4 rule

🧠 2️⃣ Structure of Proteins – Levels of Organization

Proteins are polymers of amino acids and show four levels of structural organization, each higher level adding complexity and function.

🔹 A. Primary Structure (1°)

  • Linear sequence of amino acids
  • Held together by peptide bonds
  • Gives positional information
  • Starts with N-terminal and ends with C-terminal amino acid

🧠 Key Point:

If primary structure is changed → protein becomes non-functional

🔹 B. Secondary Structure (2°)

Local folding of the polypeptide chain stabilized by hydrogen bonds.

Two common types:

  • α-Helix: Right-handed spiral structure
  • β-Pleated Sheet: Zig-zag sheet-like structure

🧠 NCERT Fact:

Proteins form only right-handed α-helices

🔹 C. Tertiary Structure (3°)

  • Further folding of the polypeptide into a compact 3D structure
  • Essential for biological activity
  • Stabilized by:
  • Hydrogen bonds
  • Disulfide bonds
  • Ionic bonds
  • Van der Waals forces

🧠 One-Line Recall:

Tertiary structure = Functional shape

🔹 D. Quaternary Structure (4°)

  • Association of two or more polypeptide chains
  • Each chain = subunit

⭐ Classic Example:

  • Haemoglobin → 4 subunits (α₂β₂)

🧠 Memory Line:

Multiple chains working together = Quaternary structure

💡 Questions & Answers

Q1. What are nucleic acids?

A1.

Nucleic acids are biomacromolecules made of nucleotides that store and transmit genetic information. DNA and RNA are the two types.

🔑 Key Points: Genetic material, polynucleotides.

Q2. Differentiate between nucleoside and nucleotide.

A2.

A nucleoside has only sugar and base, while a nucleotide has sugar, base, and phosphate.

🔑 Key Points: Phosphate distinguishes nucleotide.

Q3. Which nitrogen base is found only in RNA?

A3.

Uracil is found only in RNA.

Q4. What is the pitch of B-DNA?

A4. The pitch of B-DNA is 34 Å, which is the length of one complete turn of the helix.

Q5. Which protein structure gives positional information?

A5. The primary structure gives positional information of amino acids.

Q6. Why is tertiary structure important?

A6. Because the biological activity of a protein depends on its correct three-dimensional (tertiary) structure.

Q7. What is quaternary structure? Give an example.

A7. It is the association of multiple polypeptide chains to form a functional protein. Example: Haemoglobin.

Q8. What type of bonds stabilize secondary structure of proteins?

A8. Hydrogen bonds stabilize secondary structure.