Molecular Basis of Inheritance

Molecular Basis of InheritanceNEET Botany · Class 12 · NCERT Chapter 3

Overview Notes NEET Questions Interactive Learning

7 interactive concept widgets for Molecular Basis of Inheritance. Drag any slider, change any number, and watch the formula and the answer update live. Built so you understand how each NEET problem actually works, not just the final number.

On this page

Base pairing and Chargaff's rules calculator

Replication enzymes: helicase, primase, DNA Pol I, III, ligase

DNA to mRNA transcription simulator

Interactive genetic code table

Lac operon 4-state simulator (glucose/lactose)

VNTR gel patterns: paternity and forensics

Molecular Basis of Inheritance: 12-question NEET quiz

DNA base pairing and Chargaff's rules

Explore Watson-Crick base pairs (A-T: 2 H-bonds, G-C: 3 H-bonds). Use the Chargaff's rules calculator: set %A and all other base percentages auto-calculate.

DNA Structure

DNA base pairing and Chargaff's rules

Explore Watson-Crick base pairs and use Chargaff's rules to calculate unknown base percentages.

Select a base to see its complement:

Adenine (Purine)

— —

2 hydrogen bonds

Thymine (Pyrimidine)

A-T pairs have 2 H-bonds. AT-rich regions melt more easily (used as origins of replication).

Chargaff's rules calculator

In double-stranded DNA: A = T and G = C. Drag to set %A and the rest auto-calculates.

% Adenine (A): 30%

30.0%

20.0%

A + G = 50.0% (purines) = T + C = 50.0% (pyrimidines)

DNA structure key numbers for NEET:

0.34 nm (3.4 Å) between consecutive base pairs
3.4 nm (34 Å) per complete turn of helix
10 base pairs per turn
2 nm (20 Å) diameter of the helix
Right-handed B-form double helix

Try this

Set A = 20%: you get T = 20%, G = C = 30%. Now try A = 30%: T = 30%, G = C = 20%. This is exactly how NEET asks Chargaff's rule questions!
Click G to see it forms 3 H-bonds with C (stronger than A-T). That's why GC-rich regions are harder to melt — important for PCR primer design.

Replication fork enzyme map

Click each enzyme (helicase, SSB, primase, DNA Pol III, DNA Pol I, ligase) to see its role, location, and why it matters for NEET. Leading and lagging strand synthesis explained.

DNA Replication

Replication fork enzyme map

Click each enzyme at the replication fork to see its role, location, and why it matters for NEET.

Replication enzymes (click to explore):

Helicase

SSB Proteins

Topoisomerase (Gyrase)

Primase

DNA Pol III

DNA Pol I

DNA Ligase

Helicase

ROLE

Unwinds the double helix by breaking hydrogen bonds between base pairs. Uses ATP.

LOCATION AT FORK

At the replication fork — the "engine" that opens the helix.

Schematic of the replication fork:

3'─────────────────────────────── 5'   (parental template)
                    ↑ HELICASE (opens helix)
5'─────────────────── 3'                  (parental template)

LEADING strand (synthesised continuously toward fork):
5'─PRIMER─────────────────────────────→ 3'  (DNA Pol III)

LAGGING strand (synthesised away from fork as Okazaki fragments):
←─Fragment 3─|←─Fragment 2─|←─Fragment 1─   (each with own primer)
  DNA Pol I removes primers; Ligase joins fragments

Leading strand

Synthesised CONTINUOUSLY 5'→3' toward the replication fork. Only one RNA primer needed.

Lagging strand

Synthesised DISCONTINUOUSLY as Okazaki fragments (away from fork). Each fragment needs its own RNA primer.

Okazaki fragments

1,000-2,000 nt in bacteria; 100-200 nt in eukaryotes. Later joined by DNA ligase after RNA primers are replaced.

Try this

Click "Primase" — remember it is the ONLY enzyme that can start a new polynucleotide chain from scratch. DNA Pol III cannot.
Click "DNA Pol I" — it does TWO things: removes the RNA primer AND fills the DNA gap. It has 5'→3' exonuclease AND 5'→3' polymerase activities.
NEET trap: the lagging strand needs MULTIPLE RNA primers (one per Okazaki fragment), while the leading strand needs only ONE.

Transcription: DNA template to mRNA

Type a DNA template strand (3'→5') and see the coding strand and mRNA produced. Identifies start and stop codons in the output.

Transcription

Transcription: DNA template to mRNA

Enter a DNA template strand (3'→5') and see the coding strand and mRNA produced by RNA polymerase.

Enter DNA template strand (3'→5'). Use A, T, G, C only:

ATG start

GCT sample

TTA stop

Template strand (3'→5'):

DNA

Read by RNA polymerase 3'→5'

Coding strand (5'→3'):

DNA

Same sequence as mRNA (T→U); not transcribed

mRNA produced (5'→3'):

RNA

Complementary to template; used for translation

mRNA codons (groups of 3):

AUG(START)

CCC

GAU

Transcription rules to remember:

Template strand is read 3'→5'; mRNA is made 5'→3'
DNA A → RNA U (uracil replaces thymine in RNA)
DNA T → RNA A; DNA G → RNA C; DNA C → RNA G
The coding strand has the SAME sequence as mRNA (replace T with U)
AUG = start codon; UAA / UAG / UGA = stop codons

Try this

Try template: TAC — the mRNA will be AUG, the start codon. AUG codes for methionine, the first amino acid in all eukaryotic proteins.
Try ATT on the template → mRNA becomes UAA, which is the Ochre STOP codon.

Genetic code explorer: all 64 codons

Build any mRNA codon position by position and find which amino acid it codes for. Start and stop codons highlighted. Key codons (AUG, UAA, UAG, UGA, UUU, UGG) clickable.

Genetic Code

Genetic code explorer: all 64 codons

Build any mRNA codon base by base and find which amino acid it codes for. Spot start and stop codons instantly.

Build your codon (pick each position):

1st base (5' end)

2nd base

3rd base (3' end)

Methionine (START)

START codon

Must-know codons for NEET:

AUG

START / Met

UAA

STOP (Ochre)

UAG

STOP (Amber)

UGA

STOP (Opal)

UUU

Phe — 1st decoded

UGG

Trp — only 1 codon

Genetic code properties (NEET must-know):

TRIPLET: 3 bases per codon → 64 combinations (4³)
DEGENERATE: 20 amino acids but 61 sense codons → multiple codons per amino acid
UNAMBIGUOUS: one codon → only one amino acid (never ambiguous)
UNIVERSAL: same code in almost all organisms (exceptions: mitochondria)
NON-OVERLAPPING and COMMALESS: read sequentially without gaps

Try this

Click "AUG" in the key codons box — this loads the start codon. AUG is the only start codon and also codes for methionine (Met).
UGG codes for tryptophan and is the only amino acid with just ONE codon (not degenerate). Try it!
UAA, UAG, UGA are the three stop codons. None of them codes for an amino acid.

Lac operon: toggle glucose and lactose

The Jacob-Monod model. Toggle glucose and lactose switches to see all 4 operon states — repressor binding, CRP-cAMP activation, and structural gene transcription level.

Gene Regulation

Lac operon: toggle glucose and lactose

The classic Jacob-Monod model. Toggle glucose and lactose to see all 4 operon states.

Glucose present

Lactose present

YES

Operon ON (full activity)

Repressor: FREE (released)

CRP-cAMP: ACTIVE (no glucose)

Lactose present: allolactose binds the repressor → operator is FREE. No glucose: cAMP levels are HIGH → cAMP binds CRP → CRP-cAMP activates the lac promoter strongly. BOTH conditions for full transcription are met. All three structural genes (lacZ, lacY, lacA) are highly expressed. The cell uses lactose as its energy source.

Operon structure (opacity shows transcription level):

RNA Pol binds here

Repressor binds here

lacZ

β-galactosidase

lacY

Permease

lacA

Transacetylase

mRNA transcribed →

All 4 lac operon states (NEET summary):

Glucose	Lactose	Repressor	CRP-cAMP	Transcription
+	-	Bound	Inactive	OFF
-	-	Bound	Active	OFF
+	+	Free	Inactive	LOW
-	+	Free	Active	HIGH ✓

Try this

Set Glucose OFF + Lactose ON — this is the MAXIMUM transcription state (high cAMP-CRP activation + repressor released).
Set both OFF — no inducer means repressor blocks the gene even though CRP-cAMP would activate it. Two control points must BOTH be satisfied.

DNA fingerprinting: reading gel electrophoresis bands

Read VNTR band patterns across gel lanes for paternity tests and forensic cases. Reveal answers and understand how band-sharing works in related vs unrelated individuals.

DNA Fingerprinting

DNA fingerprinting: reading gel electrophoresis bands

Each lane shows the VNTR band pattern for one individual. Bands at the same position indicate matching repeat sequences.

Paternity test

Forensic crime scene

Child A has a disputed father. Two men are tested. Which man is the biological father?

High MW

Low MW

Mother

Child A← whose child?

Man 1

Man 2

Mother

Child A

Man 1

Man 2

Reveal answer

How DNA fingerprinting works:

Extract DNA from blood, hair root, saliva, or other cells.
Cut with restriction enzymes at fixed sites flanking VNTR regions.
Run on agarose gel electrophoresis — smaller fragments travel further.
Southern blotting: transfer DNA from gel to nylon membrane.
Hybridise with a labelled probe that binds to VNTR sequences.
Autoradiography or chemiluminescence reveals the band pattern.
Compare band patterns — identical patterns = same individual; shared bands = related.

Try this

In the paternity test: compare Child A's bands with Mother first. Bands not in the mother MUST come from the biological father. Then check which man has all those remaining bands.
DNA fingerprinting was invented by Alec Jeffreys in 1984 and first used in a UK immigration case and then famously in a forensic case in Leicestershire, UK.

Molecular Basis of Inheritance NEET quiz

12-question scored quiz covering Hershey-Chase, Chargaff's rules, nucleosome, Meselson-Stahl, replication enzymes, transcription, genetic code, stop codons, lac operon, HGP, and DNA fingerprinting.

Chapter Quiz