E ExamMaster

CBSE Class 12 · Biology

Molecular Basis of Inheritance

Official NCERT chapter from Biology (book code lebo1). ExamMaster notes are original teaching at CBSE Class 12 depth.

This lesson follows the official chapter “Molecular Basis of Inheritance” in Biology. The words below are ExamMaster’s teaching, not a paste from the book. Use the NCERT chapter for the classroom sequence; use these notes to hold the idea without copying exercises or figures.

  • CBSE Class 12
  • Medium level
  • 8 concepts

1The DNA

DNA: a double chain of nucleotides as taught — A with T, G with C as framed. DNA is the information-look, not a “life juice”. One double-line mention is enough.

A protein as DNA because both sit in a nucleus-talk is a miss.

Figure. Not a helix — a pairing ladder. The top rail reads 5' to 3'; the bottom rail is 3' to 5' so the partners face. A pairs with T by two hydrogen bonds; G pairs with C by three. That pairing is why Chargaff measured A = T and G = C in double-stranded DNA.

How it works

  1. Name the double chain and the pairsDNA.
  2. Keep information as the jobThe heading.
  3. Refuse a juice-sloganA sequence.

2The Search for Genetic Material

Search for genetic material: Griffith, Avery, Hershey–Chase as taught — looks that forced DNA (or the lesson’s order). A search is a look-plus-claim, not a biography dump. One transform or phage look is enough.

A name-list with no look is a mute history.

Figure. Griffith's four treatments. Live smooth (S) kills; live rough (R) does not; heat-killed S does not; heat-killed S plus live R kills — something from the dead S transformed the live R. Avery, MacLeod and McCarty showed that something is DNA, not protein.

How it works

  1. Name the look (transform, phage, …)The search.
  2. Say what claim it forcedDNA as the material.
  3. Keep the biography shortA look-story.

3RNA World

RNA world: a taught leftover that RNA can store and act as framed. RNA world is a why-RNA-matters sentence, not a second DNA. One honest sentence is enough.

A DNA-only slogan as the whole history is a skip if this heading sat.

Figure. RNA world: one polymer that can store a sequence and catalyse a reaction. Later life splits the jobs — DNA keeps the archive, protein does most catalysis — but RNA still sits in the middle as mRNA, tRNA and ribozyme.

How it works

  1. Name RNA as store-and-act as taughtThe leftover.
  2. Keep it a why-sentenceThis class.
  3. Refuse a university dumpOne sentence.

4Replication

Replication: DNA copies as taught — semi-conservative if named; a fork and polymerases as framed. Replication is a copy-job, not a translation. One unzip-and-copy sentence is enough.

A protein-make labelled replication is a miss.

Figure. Meselson–Stahl densities after a 15N to 14N switch. Generation 0 is all heavy–heavy. Generation 1 is all hybrid — one old strand plus one new — which kills conservative replication. Generation 2 is half hybrid and half light–light, the semi-conservative signature.

How it works

  1. Name unzip and copy as taughtReplication.
  2. Keep semi-conservative if askedThe extra.
  3. Refuse a protein-write hereThat is translation.

5Transcription

Transcription: DNA to RNA as taught. A gene-look becomes a message. Transcription is a write-into-RNA, not a protein yet. One school sentence is enough.

A ribosome labelled transcription is a miss.

Figure. RNA polymerase reads the template 3' to 5' and writes mRNA 5' to 3'. Template TAC GGA becomes AUG CCU. The coding strand is 5' ATG CCT 3' — the same sense as the mRNA except T becomes U.

How it works

  1. Name DNA → RNATranscription.
  2. Keep it a message-makeThe job.
  3. Refuse a protein as this leftoverNext.

6Genetic Code

Genetic code: a triplet as taught — three bases for one amino acid; start/stop if named. The code is a dictionary, not a language-slogan. One codon-sentence is enough.

A single base as a codon is a miss.

Figure. Four bases and twenty amino acids. A two-letter codon gives only 16 words — too few. A three-letter codon gives 64, which is enough: 61 sense plus UAA, UAG, UGA as stops. AUG is both methionine and the usual start.

How it works

  1. Name three bases → one amino acidThe code.
  2. Keep start/stop if taughtThe extra.
  3. Refuse a word-slogan as the testA triplet.

7Translation

Translation: RNA to a protein as taught — a ribosome reads codons. Translation is a protein-make, not a DNA-copy. One school sentence is enough.

A nucleus-copy labelled translation is a miss.

Figure. The ribosome reads mRNA in threes. AUG in the P site is Met on a tRNA with anticodon UAC; UUU in the A site is Phe on a tRNA with anticodon AAA. Peptide-bond formation joins Met-Phe; the chain grows as the ribosome steps one codon.

How it works

  1. Name RNA → protein at a ribosomeTranslation.
  2. Keep codon-read as the jobThe extra.
  3. Refuse a DNA-copy hereThat was replication.

8Regulation of Gene Expression

Regulation of gene expression: a gene can sit on or off as taught — lac operon if named. Regulation is a switch-look, not a second code. One on/off sentence is enough.

Every gene always-on as the leftover this heading refuses.

Figure. The lac map is i, promoter, operator, then z, y, a. Off: repressor sits on the operator so polymerase cannot read the three structural genes. On: lactose inactivates the repressor, the operator is free, and one transcript covers z (β-galactosidase), y (permease) and a.

How it works

  1. Name on/off as taughtRegulation.
  2. Keep lac only if listedHonest extra.
  3. Refuse a university dumpThis class.
Transcription is
  1. DNA to RNA as taught
  2. RNA to protein
  3. DNA to DNA

The message-write.

Notes

  • Mapped to the official NCERT chapter “Molecular Basis of Inheritance”. Original teaching only — no textbook sentences.
  • Science here is Physics, Chemistry and Biology ideas at this class, never a language or social-science chapter.

Recap

Hold these pegs from the official chapter “Molecular Basis of Inheritance”. The wording is ExamMaster’s teaching, not a textbook recap.

The DNA
DNA: a double chain of nucleotides as taught — A with T, G with C as framed.
The Search for Genetic Material
Search for genetic material: Griffith, Avery, Hershey–Chase as taught — looks that forced DNA (or the lesson’s order).
RNA World
RNA world: a taught leftover that RNA can store and act as framed.
Replication
Replication: DNA copies as taught — semi-conservative if named; a fork and polymerases as framed.
Transcription
Transcription: DNA to RNA as taught. A gene-look becomes a message. Transcription is a write-into-RNA, not a protein yet. One school sentence is enough.
Genetic Code
Genetic code: a triplet as taught — three bases for one amino acid; start/stop if named.

Practise Molecular Basis of Inheritance

Reading is free and needs no account. Practice, mocks and progress live in the app.

  • A 4-question practice set that ends the chapter
  • 1 quick check with worked explanations
  • Timed mocks scored with the real marking scheme
  • Readiness tracked per topic, kept on your device
Continue with Google — freeNo card, no trial. Works offline once installed.