The genetic code is described as "degenerate", which means that:

Most amino acids are coded by more than one codon

One codon codes for several amino acids

The code differs between species

The Hershey–Chase experiment with bacteriophages established that:

DNA, not protein, is the genetic material

Protein is the genetic material

RNA is the genetic material in all organisms

Both DNA and protein are equally hereditary

In the lac operon, transcription is normally blocked by the:

Repressor protein binding the operator

RNA polymerase binding the operator

A single base insertion in a gene is especially damaging because the code is:

Non-overlapping and read in fixed triplets, causing a frameshift

Degenerate, so no amino acid changes

Molecular Basis of Inheritance — DNA, the Central Dogma and Gene Expression

This chapter is dense with landmark experiments and precise molecular facts, and NEET draws heavily from it. The spine is the central dogma — DNA → RNA → protein — and it helps to see everything else as either how DNA is structured, how it is copied, or how its message is read out. Two ideas anchor the details. First, DNA is antiparallel and complementary, and this single geometric fact drives base-pairing, semi-conservative replication, and transcription all at once. Second, the genetic code has fixed properties — triplet, degenerate, non-overlapping, universal — that each answer a specific style of question. Around this sit the classic experiments (Griffith, Hershey–Chase, Meselson–Stahl) and the lac operon, which are pure recall but reliably examined.

DNA Structure and Replication

DNA is a double helix of two antiparallel strands (one 5′→3′, the other 3′→5′). The bases pair specifically — A with T (two H-bonds), G with C (three H-bonds) — so the strands are complementary and one templates the other.
Chargaff's rule: in any DNA, A = T and G = C, so purines equal pyrimidines. The two strands wind with ~10 base pairs per turn and a pitch of 3.4 nm (each base pair 0.34 nm apart).
Replication is semi-conservative (each daughter DNA keeps one old strand) — proved by Meselson and Stahl with heavy-nitrogen labelling. The enzyme DNA polymerase builds the new strand only 5′→3′, giving a continuous leading and a discontinuous lagging strand (Okazaki fragments).

🎯 Antiparallel and complementary — one geometry explaining everything

Two small structural facts about DNA quietly explain most of the chapter. The strands are complementary: because A only pairs with T and G only with C, knowing one strand tells you the other exactly — which is how the molecule can be copied, and how RNA can be transcribed from it. The strands are also antiparallel: they run in opposite directions, one 5′→3′ and the other 3′→5′. That second fact has a consequence students often miss. DNA polymerase can add new nucleotides only to a 3′ end, i.e. it synthesises only in the 5′→3′ direction, so at a replication fork it can copy one template smoothly (the leading strand) but must copy the other in short backward pieces (the lagging strand, made of Okazaki fragments later joined by ligase). Complementarity is also why semi-conservative replication works and Meselson–Stahl could prove it: each old strand acts as a faithful template for a new one, so every daughter molecule is one old plus one new strand. Reasoning from "complementary + antiparallel" lets you derive base pairing, the copying mechanism and the leading/lagging asymmetry rather than memorising them separately.

The Central Dogma and the Genetic Code

Transcription copies a gene into RNA (in eukaryotes done by RNA polymerase; the message is then processed — introns removed by splicing, a cap and poly-A tail added). Translation reads the mRNA in triplets (codons) on the ribosome to build a protein.
The genetic code is a triplet: 64 codons for 20 amino acids. AUG is the start codon (also codes methionine); UAA, UAG, UGA are stop codons. tRNA is the adaptor that brings each amino acid to the codon via its anticodon.
Properties of the code: triplet, degenerate (most amino acids have more than one codon), non-overlapping, commaless, and nearly universal (the same codons mean the same amino acid across almost all life).

🧠 Each property of the code answers a different kind of question

The genetic code's properties are not a list to recite but a toolkit, each solving a specific problem the exam poses. Triplet — three bases per codon — is the answer to "why not two?": four bases in pairs give only 16 combinations, too few for 20 amino acids, whereas triplets give 64, enough with room to spare. Degenerate explains that spare room: with 64 codons for 20 amino acids, most amino acids are specified by more than one codon, so a change in the third base often does not change the amino acid — the basis of "silent" mutations. Non-overlapping and commaless means the ribosome reads the message in fixed, consecutive triplets with no gaps, which is why inserting or deleting a single base causes a frameshift that garbles everything downstream. Universal — the same codons mean the same amino acids in a bacterium and a human — is what makes genetic engineering possible, since a human gene can be read correctly by a bacterial cell. Match the property to the question: "why three bases" → triplet; "why does a mutation sometimes have no effect" → degenerate; "why is a single-base insertion so damaging" → non-overlapping/frameshift; "why can bacteria make human insulin" → universal.

Landmark Experiments and Regulation

The experiments that proved DNA is the genetic material
Experiment	Showed	Key detail
Griffith (1928)	A "transforming principle" carries heredity	Heat-killed virulent + live avirulent Pneumococcus → live virulent
Avery–MacLeod–McCarty	The transforming principle is DNA	Only DNase abolished transformation
Hershey–Chase (1952)	DNA, not protein, is the genetic material	Radiolabelled phage: ³²P (DNA) entered, ³⁵S (protein) did not

ℹ️ The lac operon: a switch that reads the environment

Bacteria do not waste energy making an enzyme they do not need, and the lac operon is the classic example of how a gene is switched on only when useful. The operon is a cluster of genes for digesting lactose, controlled by a promoter and an operator. Normally a repressor protein sits on the operator and blocks transcription, so the enzymes are not made when there is no lactose to digest. When lactose is present, some of it binds the repressor and pulls it off the operator; RNA polymerase can now transcribe the genes, and the lactose-digesting enzymes are produced — exactly when they are needed. Remove the lactose and the repressor re-binds, switching the genes off again. So lactose acts as an inducer, and the whole system is a demonstration that gene expression is regulated, not constant: the cell reads its environment and turns genes on and off in response. The examinable points are the roles — repressor blocks, inducer (lactose) relieves the block — and that this is negative regulation, since the default state is off.

Key takeaways

DNA is antiparallel and complementary; A=T (2 H-bonds), G≡C (3 H-bonds); Chargaff: A=T and G=C.
Replication is semi-conservative (proved by Meselson–Stahl); DNA polymerase builds only 5′→3′ (leading + lagging strands).
Central dogma: DNA → (transcription) RNA → (translation) protein. tRNA is the adaptor molecule.
Code is triplet, degenerate, non-overlapping, commaless, nearly universal. AUG = start; UAA/UAG/UGA = stop.
Degeneracy → silent mutations; single-base insertion/deletion → frameshift; universality → genetic engineering works.
Hershey–Chase (³²P vs ³⁵S phage) proved DNA is the genetic material; Avery et al. showed the transforming principle is DNA.
Griffith showed transformation with heat-killed virulent + live avirulent Pneumococcus.
lac operon: repressor blocks the operator; lactose (inducer) relieves the block — negative regulation (default OFF).

Practice questions (8)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

In a DNA molecule, the base adenine always pairs with thymine through:
1. Two hydrogen bonds
2. Three hydrogen bonds
3. One covalent bond
4. An ionic bond
Show answer
Answer: A — Two hydrogen bonds
A pairs with T via two hydrogen bonds, while G pairs with C via three, which is why GC-rich DNA is more stable. Three hydrogen bonds belongs to the G–C pair, offered here to catch the swap.
The Meselson–Stahl experiment demonstrated that DNA replication is:
1. Semi-conservative
2. Conservative
3. Dispersive
4. Non-template based
Show answer
Answer: A — Semi-conservative
Using heavy and light nitrogen labelling, Meselson and Stahl showed each daughter DNA keeps one parental strand and one new strand — semi-conservative replication. Conservative replication would have kept the original duplex intact, which their density results ruled out.
The genetic code is described as "degenerate", which means that:
1. Most amino acids are coded by more than one codon
2. One codon codes for several amino acids
3. The code differs between species
4. Codons overlap one another
Show answer
Answer: A — Most amino acids are coded by more than one codon
Degeneracy means several codons can specify the same amino acid, so most amino acids have more than one codon. The reverse — one codon for several amino acids — would make the code ambiguous, which it is not, so that option inverts the definition.
The Hershey–Chase experiment with bacteriophages established that:
1. DNA, not protein, is the genetic material
2. Protein is the genetic material
3. RNA is the genetic material in all organisms
4. Both DNA and protein are equally hereditary
Show answer
Answer: A — DNA, not protein, is the genetic material
The ³²P-labelled DNA entered the bacteria and directed new phages, while the ³⁵S-labelled protein coat stayed outside, proving DNA is the genetic material. The result specifically excluded protein, which is why the second option is the classic wrong answer.
In the lac operon, transcription is normally blocked by the:
1. Repressor protein binding the operator
2. Inducer binding the operator
3. RNA polymerase binding the operator
4. Lactose binding the promoter
Show answer
Answer: A — Repressor protein binding the operator
The repressor sits on the operator and prevents transcription until lactose (the inducer) removes it. It is lactose that relieves the block, not causes it, so an option where the inducer binds the operator to switch things off reverses their roles.
The start codon that also codes for methionine is:
1. AUG
2. UAA
3. UGA
4. UAG
Show answer
Answer: A — AUG
AUG signals the start of translation and codes for methionine. UAA, UGA and UAG are the three stop codons, which end translation and code for no amino acid, so any of them as a "start" is wrong.
A single base insertion in a gene is especially damaging because the code is:
1. Non-overlapping and read in fixed triplets, causing a frameshift
2. Degenerate, so no amino acid changes
3. Universal across all species
4. Made of two-base codons
Show answer
Answer: A — Non-overlapping and read in fixed triplets, causing a frameshift
Because the ribosome reads consecutive, non-overlapping triplets, inserting one base shifts the reading frame and garbles every codon downstream — a frameshift mutation. Degeneracy might cushion a single-base substitution, but it cannot rescue a frameshift, so that option misapplies the property.
The molecule that acts as an adaptor, bringing a specific amino acid to the codon during translation, is:
1. tRNA
2. mRNA
3. rRNA
4. DNA polymerase
Show answer
Answer: A — tRNA
tRNA carries an amino acid at one end and an anticodon at the other, matching the codon on the mRNA — the adaptor role Crick predicted. mRNA carries the message and rRNA builds the ribosome, so neither performs the adaptor function itself.