NEET 2027 · Biology · Class 12 Chapter 5 · File MBI-00

Molecular Basis of Inheritance

The chapter split into eleven topics and ranked by real NEET weightage, with the study order, the format shift, and the three diagrams that have to be drawable from memory.

NCERT lebo105 · rationalised edition · weightage from 89 NEET questions, 2015–2025

What this file is. The chapter split into eleven teachable topics, each given a priority band based on how often NEET has actually asked from it. Read this page first, then work through the concept files in the order given at the bottom.

Why this chapter gets more time than any other

Molecular Basis of Inheritance is the single highest-yielding chapter in the whole NEET Biology syllabus. Across the eleven sittings from 2015 to 2025 it produced roughly 89 questions — an average of eight to nine per paper, worth 32 to 36 marks. That is close to a tenth of the entire Biology score from one chapter. The second-placed chapter, Biotechnology: Principles and Processes, produces about half as many.

NEET sitting	Questions from this chapter	Marks
2015 & re-test	7	28
2016 & phase 2	7	28
2017	8	32
2018	8	32
2019 & Odisha	7	28
2020 & phase 2	8	32
2021	8	32
2022	9	36
2023	9	36
2024 & re-exam	9	36
2025	9	36

The count has been climbing, not falling: seven questions in the mid-2010s, locked at nine since 2022.

The eleven topics, ranked by what NEET actually asks

Sub-topic	Share of the chapter's NEET questions	Priority
Transcription — transcription unit, RNA polymerases, hnRNA processing	20%	P1
DNA replication	17%	P1
Translation and protein synthesis	13%	P1
Structure of DNA and RNA	12%	P1
Regulation of gene expression — the lac operon	11%	P1
The historical experiments — search for the genetic material	9%	P2
Genetic code and mutations	8%	P2
DNA packaging — nucleosome and chromatin	4.5%	P2
Human Genome Project	2%	P3
DNA fingerprinting	2%	P3
Properties of genetic material and the RNA world	folded into assertion–reason items	P3

The priority bands

P1Highest yield — about three quarters of the chapter's marks

Five topics. If time ever runs short, these five are the chapter.

Structure of DNA and RNA — nucleotide chemistry, Chargaff, the dimensions, why RNA differs
DNA replication — semiconservative proof, the enzymes, leading versus lagging strand
Transcription — the transcription unit and its polarity, the three eukaryotic RNA polymerases, capping, tailing and splicing
Translation — tRNA as adapter, the ribosome, initiation to termination, untranslated regions
The lac operon — the four genes, their products, and the operon's state under each condition

P2High yield — about a fifth of the marks

Genetic code and mutations — the properties, the exceptions, frameshift arithmetic, sickle cell
The search for the genetic material — Griffith, Avery–MacLeod–McCarty, Hershey–Chase
DNA packaging — the nucleosome, histones, euchromatin and heterochromatin

P3Moderate — small but very cheap marks

Properties of genetic material and the RNA world — why DNA beat RNA for the job
Human Genome Project — goals, the two methods, the salient-feature numbers
DNA fingerprinting — VNTRs, the six steps in order, Southern blotting

These three together have produced only about four questions in ten years. Learn the headline numbers and the ordered step list, then stop. Memorising chromosome-by-chromosome gene counts is a poor trade.

The format has changed, and that is the real difficulty

The chapter's content has barely moved in a decade. What has moved is how it is asked. Single-fact MCQs have fallen from about 85% of the chapter's questions to about 40%, and the space has been taken by formats that require judging several statements at once.

Question format	2015–2018	2022–2025	What it means for Aamirah
Straight single-fact MCQ	85%	40%	Recall alone no longer carries the chapter
Assertion–reason	2%	20%	A ten-fold rise. This is her weakest format.
Multi-statement (“how many are correct”)	3%	23%	Each clause must be judged separately
Match the column	8%	15%	Four facts recalled for one mark
Numerical	2%	2%	Free marks if the three types are drilled

Direct relevance to Aamirah. Assertion–reason has been an unstable format for her — errors have gone in both directions across papers, which points to an unsettled procedure rather than a fixed bias. Roughly four or five of the nine questions from this chapter will now arrive in assertion–reason or multi-statement form. That is sixteen to twenty marks resting on one procedure. Fix the procedure here and the gain spreads across the whole paper.

The fixed procedure, used in every worked solution in this pack: judge the assertion alone and write true or false. Judge the reason alone and write true or false. Only then ask whether the reason explains the assertion. Never read the pair as a single sentence.

Three diagrams that must be drawable from memory

Around a fifth of this chapter's questions depend on picturing a diagram that is not printed in the question. Three carry almost all of that load.

The replication fork — which strand is continuous, which is fragmented, and why
The transcription unit — where the promoter sits, which strand is the template, which way everything points. This is the chapter's single biggest source of avoidable error.
The lac operon — the linear order i, p, o, z, y, a and what the repressor is doing in each state

Practise until each can be drawn from blank paper in under a minute.

The three numerical types

This is one of very few Biology chapters that produces genuine calculations. They appear about once a year, they are mechanical, and most candidates skip them.

Type	Method	Worked in this pack
Length of a DNA molecule	length = number of base pairs × 0.34 nm	Q1, Q2
Chargaff percentages	A=T and G=C; the four values follow from any one	Q3, Q4
Frameshift codon counting	count complete codons before the deletion point, then re-divide the rest by three	Q15

Where this chapter leaks into other chapters

Other chapter	The shared idea
Biotechnology: Principles and Processes	PCR repeats replication in a tube — heat replaces helicase, primers replace primase. Restriction enzymes rely on base pairing.
Principles of Inheritance and Variation	Sickle cell anaemia is a single base substitution here and an autosomal recessive pattern there.
Cell: The Unit of Life	Euchromatin and heterochromatin, and the nucleolus as the site of rRNA synthesis.
Biomolecules	N-glycosidic bonds joining base to sugar; phosphodiester bonds joining nucleotide to nucleotide.
Evolution	23S rRNA acting as a ribozyme is the molecular evidence for the RNA world.

Suggested order of work

MBI-01, Priority 1 concepts. Structure → replication → transcription → translation → lac operon. Do not reorder: each one is built on the one before.
MBI-02, Priority 2 and 3 concepts. Genetic code → historical experiments → packaging, then the three short P3 topics.
MBI-03, the one-page fact sheet. Every number, exception, scientist and rule on one printable page.
MBI-04, twenty worked questions. Attempt each one closed-book first, then read the solution card.

Syllabus check. Nothing has been cut from this chapter in the rationalised NCERT, and the NTA syllabus lists every section: search for genetic material, structure of DNA and RNA, DNA packaging, replication, central dogma, transcription, genetic code, translation, gene expression and regulation with the lac operon, genome and Human Genome Project, DNA fingerprinting, and protein biosynthesis. There is no removed content to worry about here. There is beyond-NCERT content that NEET has still asked; it is flagged with this same banner style wherever it appears in the concept files.