What is DNA?
DNA (Deoxyribonucleic Acid) is the genetic material that stores all biological information.
👉 Think like this:
DNA = instruction book of life
It tells:
How body develops
How cells function
What proteins to make
🔹 Where is DNA found?
In eukaryotes → nucleus (chromosomes)
In prokaryotes → nucleoid region
Also present in:
Mitochondria
Chloroplast
🧬 PART 1: NUCLEOTIDE — BASIC UNIT (VERY IMPORTANT)
DNA is made of repeating units called nucleotides.
🔹 Structure of ONE nucleotide
Each nucleotide has 3 components:
1. Nitrogen base
2. Sugar (deoxyribose)
3. Phosphate group
👉 So: Nucleotide = Base + Sugar + Phosphate
🔹 Types of Nitrogen Bases
🟢 Purines (BIG, double ring)
Adenine (A)
Guanine (G)
👉 Structure: double ring
🔵 Pyrimidines (SMALL, single ring)
Thymine (T)
Cytosine (C)
👉 Structure: single ring
🔹 Important Concept (NEET favourite)
👉 Purine always pairs with Pyrimidine
Why? → To maintain constant width of DNA
🧬 PART 2: NUCLEOSIDE vs NUCLEOTIDE
Nucleoside = Base + Sugar
Nucleotide = Base + Sugar + Phosphate
👉 Exam trick: 👉 No phosphate = nucleoside
🧬 PART 3: BONDS IN DNA (VERY IMPORTANT)
1. Glycosidic bond
Between base and sugar
2. Phosphoester bond
Between sugar and phosphate
3. Phosphodiester bond ⭐
Between two nucleotides
👉 Forms backbone of DNA
🧬 PART 4: POLYNUCLEOTIDE CHAIN
When many nucleotides join → polynucleotide chain
🔹 Backbone
Made of:
Sugar + Phosphate
👉 Called sugar-phosphate backbone
🔹 Orientation (VERY IMPORTANT)
Each strand has direction:
5′ end → phosphate present
3′ end → OH group present
👉 DNA always grows: 👉 5′ → 3′ direction
🔹 Why direction is important?
Because:
DNA replication works only in 5′ → 3′
RNA synthesis also 5′ → 3′
🧬 PART 5: DOUBLE HELIX STRUCTURE (FULL CONCEPT)
Proposed by James Watson and Francis Crick
🔹 Basic Structure
Two polynucleotide chains
Twisted → double helix
🔹 Antiparallel Nature
One strand → 5′ → 3′
Other strand → 3′ → 5′
👉 Opposite direction = antiparallel
🔹 Base Pairing (Chargaff Rule)
A = T (2 H-bonds)
G ≡ C (3 H-bonds)
👉 Important:
GC pair stronger (3 bonds)
AT pair weaker (2 bonds)
🔹 Complementarity
If one strand is:
ATG CCA
Other strand must be:
TAC GGT
👉 This is called complementary base pairing
🔹 Why complementarity is important?
👉 It helps in:
DNA replication
DNA repair
🧬 PART 6: GEOMETRY OF DNA (NEET IMPORTANT)
Distance between base pairs = 0.34 nm
10 base pairs = 1 turn
Length of 1 turn = 3.4 nm
👉 Helix is right-handed
🧬 PART 7: MAJOR & MINOR GROOVE
DNA has grooves:
Major groove
Minor groove
👉 Proteins bind here (important for gene regulation)
🧬 PART 8: STABILITY OF DNA
DNA is stable because:
Hydrogen bonds
Base stacking (hydrophobic interactions)
👉 More GC → more stable DNA
🧬 PART 9: SPECIAL POINTS FOR NEET
✔ DNA is negatively charged (phosphate)
✔ Backbone is outside
✔ Bases are inside
✔ Hydrogen bonds hold strands
✔ Complementary nature = key feature
