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
Human Genome Project ( NCERT Explanation )
In the preceding sections you have learnt that it is the sequence of bases in DNA that determines the genetic information of a given organism.
Explanation :
English: DNA base sequence (A, T, G, C order) decides genetic information.
Hinglish: DNA ke andar bases ka jo order hota hai (A, T, G, C), wahi genetic information decide karta hai.
In other words,genetic make-up of an organism lies in DNA sequences.”
Explanation :
English: All traits depend on DNA sequence.
Hinglish: Humari body ke saare traits DNA sequence mein hi likhe hote hain.
“If two individuals differ, their DNA sequences should also different.
Explanation : Agar do log alag hain, toh unka DNA bhi kuch jagah alag hoga.
“These assumptions led to the quest of finding out the complete DNA sequence…”
Explanation :
English: Scientists wanted full DNA sequence of humans.
Hinglish: Isi wajah se scientists ne pura human DNA sequence jaanne ka goal banaya.
“With establishment of genetic engineering techniques… possible to isolate and clone DNA…”
Explanation :.Genetic engineering ki madad se DNA ko isolate aur copy (clone) karna possible ho gaya.
“Simple and fast techniques for determining DNA sequences…”
Explanation : DNA sequence nikalna easy aur fast ho gaya.
“A very ambitious project… launched in 1990.”
Explanation : Yeh bada project 1990 mein start hua.
“Human Genome Project (HGP) was called a mega project.”
It was very large and complex.
Explanation :
“Human genome has approx 3 × 10⁹ bp.”
Human DNA mein 3 billion base pairs hote hain.
“Cost of sequencing was $3 per bp…”
Sequencing was very expensive initially.
Hinglish: Pehle DNA sequencing bahut mehengi thi.
“Total cost ~9 billion dollars”
Whole project cost was huge.
Hinglish: Total project ka kharcha bahut zyada tha (~9 billion $).
Line:
“If stored in books… 3300 books required”
Hinglish: DNA ka data itna bada hai ki 3300 books lagenge
“Enormous data required computational devices…”
Hinglish: Itna data handle karne ke liye computer zaroori tha.
HGP was closely associated with the rapid development of a new area in biology called Bioinformatics.
Biology + Computer = Bioinformatics
GOALS OF HGP
(i) Identify genes (20,000–25,000)
Hinglish: Human body mein kitne genes hain, unhe identify karna
(ii) Determine sequence of 3 billion bp
Hinglish: DNA ke bases ka exact order pata lagana
(iii) Store information in databases
Hinglish: Data ko computer mein store karna
(iv) Improve tools for data analysis
Hinglish: Better software/tools banana
(v) Transfer technologies to industries
Hinglish: Knowledge ko industries mein use karna
(vi) Address ELSI issues
Hinglish: Ethical, legal aur social problems solve karna (DNA misuse avoid karna)
“HGP was a 13-year project…”
Hinglish: Project 13 saal chala
“Coordinated by US Department of Energy & NIH”
Hinglish: USA ke organizations ne lead kiya
“Wellcome Trust (UK)… Japan, France, Germany, China contributed”
Hinglish: Bahut countries ne milke kaam kiya
“Completed in 2003”
Hinglish: Project 2003 mein complete hua.
“DNA variation knowledge helps diagnose, treat diseases”
Hinglish: DNA knowledge se diseases ka treatment possible hua
“Besides understanding human biology…”
Hinglish: Sirf humans nahi, dusre organisms ko bhi samajhne mein help milti hai
“Applied in healthcare, agriculture, energy, environment”
Hinglish: Iska use medicine, farming, environment sab mein hota hai
“Many organisms sequenced: bacteria, yeast…”
Hinglish: Sirf human nahi, bacteria, yeast bhi study kiye gaye
Examples:
Caenorhabditis elegans → worm
Drosophila → fruit fly
Rice, Arabidopsis → plants
Hinglish: In sabka DNA bhi sequence kiya gaya
METHODOLOGIES
“Two major approaches…”
Hinglish: 2 tarike use kiye gaye
1. EST (Expressed Sequence Tags)
“Identify genes expressed as RNA”
Hinglish: Sirf active genes (jo RNA banate hain) ko study kiya
2. Sequence Annotation
“Sequence whole genome (coding + non-coding)”
Hinglish: Pura DNA sequence kiya (coding + non-coding dono)
Line:
“Assign functions later”
Hinglish: Baad mein har sequence ka kaam identify kiya
SEQUENCING PROCESS
“DNA is isolated and broken into fragments”
Hinglish: DNA ko chhote pieces mein tod diya
“Cloned in host using vectors”
Hinglish: DNA pieces ko bacteria/yeast mein daal ke copy kiya
“Amplification of DNA fragments”
Hinglish: Har DNA piece ki copies banayi gayi
“Hosts: bacteria & yeast”
Hinglish: DNA copy karne ke liye bacteria/yeast use hua
“Vectors: BAC & YAC”
Hinglish: Special DNA carriers use hue:
BAC = bacteria based
YAC = yeast based
“Sequenced using automated DNA sequencers”
Hinglish: Machines se DNA sequence read kiya gaya
“Based on Sanger method”
Hinglish: Frederick Sanger ka method use hua
“Sequences arranged using overlapping regions”
Hinglish: DNA pieces ko overlap dekh ke joda gaya
“Generate overlapping fragments”
Hinglish: Aise pieces banaye jo ek dusre se match ho sake
“Alignment not possible manually”
Hinglish: Insaan manually itna data arrange nahi kar sakta
“Computer programs used”
Hinglish: Software use hua (Bioinformatics)
“Sequences annotated and assigned to chromosomes”
Hinglish: DNA ko chromosomes ke according arrange kiya gaya
“Chromosome 1 completed in May 2006”
Hinglish: Sabse last chromosome (1) ka sequencing 2006 mein complete hua
Human → Cell → Chromosome → DNA → Sequencing → Computer
N Hinglish: Insaan → Cell → Chromosome → DNA → Sequence → Computer analysis
FINAL COMPLETE SUMMARY
- DNA sequence = genetic info
- HGP started 1990, completed 2003
- 3 billion base pairs
- 20,000–25,000 genes
- Huge data → Bioinformatics
- Methods → EST & Sequence Annotation
- Tools → BAC, YAC, Sanger method
- Result → Disease diagnosis, research, agriculture use.
