Molecular Basis of Inheritance is the single heaviest chapter in NEET Biology. It is long, full of experiments, enzymes and numbers, and NTA asks from every corner of it. This molecular basis of inheritance NEET PYQ guide shows which facts repeat, so you can revise the chapter in the order that scores.
For the bigger plan, see how this chapter fits into the strategy to score 360 in Biology. This article focuses on what past papers actually ask.
Want to test yourself first? Solve the Molecular Basis of Inheritance PYQs for free, then use this guide to fix the gaps.
Table of Contents
Why This Chapter Tops the Biology Paper
NEET usually asks five to seven questions from this chapter alone. No other Biology chapter gives that many marks from one reading of NCERT.
The questions are direct but detailed, so line-by-line reading matters. Keep the Molecular Basis class 12 notes beside you, and use our guide to memorising NCERT Biology diagrams for the replication fork, the transcription unit and the operon model. For the full unit plan, read the genetics preparation strategy guide.
Molecular Basis of Inheritance NEET PYQ Analysis: The Repeated Question Types
When you sort a decade of NEET biology PYQs from this chapter, seven question types cover almost everything.
| Question type | Frequency | What it tests |
|---|---|---|
| DNA structure and packaging | Very frequent | Chargaff’s rules, dimensions, nucleosome |
| Search for genetic material | Very frequent | Griffith, Avery, Hershey–Chase |
| DNA replication | Very frequent | Meselson–Stahl, enzymes, Okazaki fragments |
| Transcription and translation | Most frequent | Strands, RNA polymerases, processing, tRNA |
| Genetic code | Regular | Codon properties, start and stop codons |
| Lac operon | Regular | Genes, repressor, inducer |
| HGP and DNA fingerprinting | Regular | Numbers, VNTRs, steps |
For a quick warm-up before the full set, try these five quick molecular basis questions. The Mendelian side of the unit is covered in our Principles of Inheritance PYQ patterns article.
DNA Structure: Numbers NTA Loves
- Adenine pairs with thymine by two hydrogen bonds; guanine pairs with cytosine by three.
- Chargaff’s rule: A = T and G = C, so purines = pyrimidines.
- Pitch of the helix = 3.4 nm, with about 10 base pairs per turn; the distance between base pairs is 0.34 nm.
- A nucleosome is DNA wrapped around a histone octamer and contains about 200 base pairs.
- Histones are rich in the basic amino acids lysine and arginine.
- Euchromatin is loosely packed and active; heterochromatin is dense and inactive.
The Experiments
| Scientist(s) | Organism | Conclusion |
|---|---|---|
| Griffith (1928) | Streptococcus pneumoniae | A “transforming principle” exists |
| Avery, MacLeod, McCarty | Same bacterium | The transforming principle is DNA |
| Hershey and Chase (1952) | Bacteriophage | DNA is the genetic material (³²P entered, ³⁵S did not) |
| Meselson and Stahl (1958) | E. coli with ¹⁵N | Replication is semiconservative |
DNA Replication in Short
- The main enzyme is DNA-dependent DNA polymerase, which adds nucleotides only in the 5’→3′ direction.
- Replication is continuous on one strand and discontinuous on the other.
- The short pieces on the lagging strand are Okazaki fragments, joined by DNA ligase.
Because copying is semiconservative, after two generations in ¹⁴N medium, half the DNA is hybrid and half is light.
Transcription and Translation
Transcription
- A transcription unit has a promoter, a structural gene and a terminator.
- The template strand runs 3’→5′. The coding strand has the same sequence as the RNA, with T in place of U.
- In eukaryotes, RNA polymerase I makes rRNA (28S, 18S, 5.8S), RNA polymerase II makes hnRNA, and RNA polymerase III makes tRNA, 5S rRNA and snRNA.
- hnRNA is processed by capping (methyl guanosine triphosphate at the 5′ end), tailing (200–300 adenylate residues at the 3′ end) and splicing (introns removed, exons joined).
Genetic code and translation
- The code is triplet, degenerate, unambiguous and nearly universal.
- 61 codons code for amino acids; UAA, UAG and UGA are stop codons.
- AUG codes for methionine and also acts as the start codon.
- tRNA is the adapter molecule. Charging of tRNA is also called aminoacylation.
The Lac Operon
| Gene | Product |
|---|---|
| i | Repressor protein |
| z | β-galactosidase |
| y | Permease |
| a | Transacetylase |
Lactose (allolactose) is the inducer. It binds the repressor and frees the operator, so RNA polymerase can transcribe the genes. Control by the repressor is called negative regulation.
HGP and DNA Fingerprinting
- The human genome has about 3164.7 million base pairs and roughly 30,000 genes.
- Less than 2% of the genome codes for proteins.
- The largest known human gene is dystrophin. Chromosome 1 has the most genes (2968) and the Y chromosome the fewest (231).
- DNA fingerprinting was developed by Alec Jeffreys and uses VNTRs, a type of satellite DNA, detected by Southern blotting.
Worked PYQ-Pattern Questions
Q1. If a double-stranded DNA has 20% adenine, what is the percentage of cytosine?
Answer: 30%. A = T = 20%, so G + C = 60% and C = 30%.
Q2. Which RNA polymerase transcribes tRNA in eukaryotes?
Answer: RNA polymerase III.
Q3. In the lactose operon of E. coli, which gene codes for permease? (a) i (b) z (c) y (d) a
Answer: (c) y.
Q4. In the Hershey–Chase experiment, which radioactive label was found inside the bacteria?
Answer: ³²P, which labelled the viral DNA. ³⁵S labelled the protein coat and stayed outside.
Q5. A coding strand reads 5′-ATGC-3′. What is the mRNA sequence?
Answer: 5′-AUGC-3′. The mRNA matches the coding strand, with U in place of T.
Common Traps in This Chapter
- Swapping the template strand and the coding strand
- Mixing up the products of RNA polymerases I, II and III
- Counting 64 codons as coding for amino acids (it is 61)
- Confusing the z, y and a gene products
- Writing capping at the 3′ end and tailing at the 5′ end
Practise Every Molecular Basis PYQ
This guide gives you the facts and tables. Practice makes them automatic. Every previous year question from this chapter is available chapter-wise, free:
👉 Solve all Molecular Basis of Inheritance NEET PYQs
After each set, mark the NCERT line behind every wrong answer. In this chapter, the same lines are asked year after year.
Conclusion
This chapter rewards careful reading. Use this molecular basis of inheritance NEET PYQ analysis to master the experiments, the replication and transcription machinery, the genetic code, operon regulation and the HGP numbers. Then practise until each fact comes without effort.
FAQ
Q: How many questions come from Molecular Basis of Inheritance in NEET? A: Usually five to seven questions, which makes it the highest-weightage chapter in NEET Biology.
Q: What is the most important topic in this chapter? A: Transcription and translation are tested most, followed by DNA replication, the classic experiments and the lac operon.
Q: Is NCERT enough for this chapter? A: Yes. Almost every question is a direct NCERT line, figure or number, so read the chapter line by line and solve past questions.
Q: What is the difference between the template strand and the coding strand? A: The template strand (3’→5′) is copied by RNA polymerase. The coding strand (5’→3′) is not copied but has the same sequence as the RNA, with T instead of U.
Q: How should I revise this chapter for NEET? A: Use a molecular basis of inheritance NEET PYQ list to find the repeated facts, make short tables for enzymes and experiments, and solve past questions chapter-wise.
Q: Where can I practise these PYQs for free? A: You can solve every previous year question from this chapter for free using the practice link in this article.