In the Hershey–Chase experiment, bacteria infected by phages labelled with radioactive phosphorus became radioactive after infection. What does this result show?

Phage DNA entered the bacteria.

The phage protein coat entered the bacteria.

Both phage DNA and protein entered the bacteria.

The bacterial DNA was transferred into the phage.

DNA polymerase builds a new strand only in the 5′ to 3′ direction. At a replication fork, which template strand is copied continuously?

The one read 3′ to 5′ in the direction the fork moves

The one read 5′ to 3′ in the direction the fork moves

Either template orientation, because DNA polymerase works in both directions

A template with no defined direction

A eukaryotic primary RNA transcript contains both exons and introns. Which change specifically occurs during splicing?

Introns are removed and exons are joined.

A methylated nucleotide is added to the 5′ end.

A tail of adenylate residues is added to the 3′ end.

The RNA is translated into a polypeptide.

When lactose is available to a bacterium, how does it help switch on the lac operon?

It binds to the repressor, preventing the repressor from blocking the operator.

It binds to RNA polymerase and makes the enzyme transcribe without a promoter.

It removes the structural genes from the DNA.

It acts as the repressor and permanently blocks transcription.

What are the most important topics in Class 12 Biology Molecular Basis of Inheritance for NEET?

Focus on DNA structure and base pairing, the Griffith and Hershey-Chase experiments, semiconservative replication, transcription and RNA processing, the genetic code, translation and the lac operon. Human Genome Project facts and DNA fingerprinting also appear often. The cheat sheet on this page covers all of them in order.

What is the difference between transcription and translation?

Transcription makes an RNA copy of one DNA strand and happens in the 5′ to 3′ direction using RNA polymerase. Translation reads the codons of mRNA on a ribosome, with tRNA bringing amino acids, to build a polypeptide. So transcription goes from DNA to RNA, and translation goes from RNA to protein.

Why is the genetic code called degenerate?

There are 64 codons but only about 20 amino acids, so several codons can specify the same amino acid. Of the 64, 61 code for amino acids and 3 (UAA, UAG, UGA) are stop codons. Degenerate does not mean ambiguous: each codon still gives only one amino acid.

Why does DNA replication give two daughter molecules that each carry an old strand?

Replication is semiconservative: each parental strand acts as a template for a new complementary strand. Every daughter DNA therefore has one old and one new strand, as the Meselson-Stahl experiment showed. You can also make your own flashcards or quiz for any chapter in Classly.

Molecular Basis of Inheritance

Cheat sheetClass 12 · Biology

⚑ Panic box

Base pairing — $A=T$ (2 H-bonds); $G\equiv C$ (3); strands are complementary and antiparallel.

Replication — semiconservative: each daughter DNA has one old and one new strand; synthesis runs $5'\to3'$ .

Gene expression — DNA $\to$ RNA $\to$ protein; in some viruses, RNA $\to$ DNA.

Code — triplet, degenerate, nearly universal; AUG starts and codes Met; UAA/UAG/UGA stop.

1. DNA: structure and packaging

Nucleotide — nitrogenous base + pentose sugar + phosphate · nucleoside — base + sugar

Bases — purines: A, G · pyrimidines: C, T, U · DNA has T; RNA has U and a 2′-OH group

Chain links — base–sugar: N-glycosidic · phosphate–sugar: phosphoester · nucleotides: 3′–5′ phosphodiester

Polarity — 5′ end has free phosphate · 3′ end has free OH · sugar–phosphate backbone; bases project inward

DNA feature	Key point	Recall check
Strands	Two, right-handed helix; antiparallel	One runs $5'\to3'$ , other $3'\to5'$
Pairing	$A=T$ ; $G\equiv C$	Purine pairs with pyrimidine
Chargaff rule	$A=T$ , $G=C$ ; total purines = pyrimidines	In double-stranded DNA
Dimensions	0.34 nm per bp; 3.4 nm per turn	About 10 bp per turn
Stability	H-bonds + stacked base pairs	Complementary strands can re-form

Packaging — prokaryotic DNA: protein-held loops in nucleoid · eukaryotic DNA: wraps around histone octamers → nucleosomes (~200 bp each) → chromatin fibre → condensed chromosome

Histones — positively charged, lysine- and arginine-rich proteins · Euchromatin — loosely packed, active · Heterochromatin — tightly packed, usually inactive

2. Genetic material: evidence and properties

Evidence	Experimental pattern	Conclusion
Griffith (1928)	Heat-killed virulent S + live non-virulent R → mice die; live S recovered	R cells transformed; factor not identified
Avery, MacLeod, McCarty	Protease/RNase do not stop transformation; DNase does	DNA is transforming principle
Hershey–Chase (1952)	$^{32}P$ labels phage DNA; $^{35}S$ labels protein; only DNA label enters bacteria	DNA, not protein, is inherited by infected bacteria

Hershey–Chase logic — infect E. coli with labelled phages → blend off coats → centrifuge → radioactivity with bacterial pellet only for DNA-labelled phages

Genetic material must — replicate · remain sufficiently stable · allow mutation · express inherited traits

DNA vs RNA — DNA is less reactive and more stable (deoxyribose; T instead of U) → better information storage · RNA is reactive, less stable, and can act as messenger, adapter, structure or catalyst

RNA world — RNA likely preceded DNA: it could store information and catalyse reactions; more stable DNA later became the main information store

3. Replication

Semiconservative replication — each parental strand templates a complementary new strand; each daughter double helix contains one parental + one newly made strand

Meselson–Stahl — grow E. coli in $^{15}N$ → transfer to $^{14}N$ → separate DNA by CsCl density gradient: generation 1, all hybrid; generation 2, half hybrid and half light. This supports semiconservative copying.

Replication feature	What to recall	Condition / consequence
DNA polymerase	DNA-dependent; adds deoxynucleotides	Synthesises only $5'\to3'$
Substrates	Deoxyribonucleoside triphosphates	Building blocks and energy source
Replication fork	Local opening of long DNA helix	Whole molecule does not unwind at once
Leading strand	Continuous synthesis	Template runs $3'\to5'$
Lagging strand	Discontinuous fragments; DNA ligase joins them	Template runs $5'\to3'$
Origin	Specific DNA site where copying begins	Polymerase cannot initiate randomly

Timing — eukaryotic DNA replication occurs in S phase · replication errors can cause mutations

4. Transcription and RNA processing

Transcription — copying one DNA segment, using one strand, into RNA · RNA is made $5'\to3'$ ; A pairs with U

Transcription unit	Position / role
Promoter	Upstream; RNA polymerase binding; sets strand choice and direction
Structural gene	DNA region transcribed into RNA
Terminator	Downstream; signals transcription end

Template strand — read $3'\to5'$ ; RNA is complementary · Coding strand — $5'\to3'$ ; same sequence as RNA except DNA has T where RNA has U

Transcription in bacteria — RNA polymerase binds promoter (initiation) → extends RNA (elongation) → reaches terminator and releases RNA (termination) · $\sigma$ helps initiation; $\rho$ helps termination

Eukaryotic polymerase	Main transcripts
RNA polymerase I	28S, 18S, 5.8S rRNA
RNA polymerase II	hnRNA, the mRNA precursor
RNA polymerase III	tRNA, 5S rRNA, snRNA

hnRNA processing — splice out introns and join exons · add 5′ methyl-guanosine cap · add 3′ poly-A tail (about 200–300 A residues) → mature mRNA leaves nucleus

RNA roles — mRNA carries codons · tRNA brings amino acids and reads codons · rRNA forms ribosome structure and catalyses peptide-bond formation

Gene terms — Cistron: DNA segment coding for a polypeptide · Exons: retained in mature RNA · Introns: removed during processing · bacterial genes often polycistronic; eukaryotic structural genes usually monocistronic and split

5. Genetic code and translation

Genetic code — triplet (3 bases/codon) · 64 codons total: 61 amino-acid codons + 3 stops · degenerate (several codons may specify one amino acid) · contiguous, no punctuation · nearly universal

Start / stop — AUG: methionine and initiation · UAA, UAG, UGA: termination; no tRNA reads stop codons

Mutation pattern — single-base substitution can change an amino acid (β-globin Glu → Val in sickle-cell anaemia) · insertion/deletion of 1 or 2 bases shifts reading frame · insertion/deletion of 3 or a multiple changes whole codons but preserves downstream frame

Translation component	Role	Distinguish it by
mRNA	Codon template	Codons are read in triplets
tRNA	Adapter; carries amino acid	Anticodon pairs with codon; amino-acid acceptor end
Ribosome	Binds mRNA; positions tRNAs	rRNA catalyses peptide-bond formation; bacterial 23S rRNA is a ribozyme
Release factor	Ends translation	Recognises stop codon; releases polypeptide

Translation sequence — charge each tRNA with its amino acid using ATP → small ribosomal subunit binds mRNA at AUG with initiator tRNA → matching charged tRNAs enter as ribosome advances codon by codon → peptide bonds form → release factor recognises stop codon and frees polypeptide

Translational unit — mRNA sequence from start to stop codon · UTRs — untranslated regions before AUG and after stop; support efficient translation

6. Regulation: lac operon

Gene expression can be regulated — transcription · RNA processing · mRNA transport · translation (eukaryotes)

Operon — coordinated bacterial gene unit: shared promoter and regulatory control for related structural genes · transcription initiation is the main prokaryotic control point

Lactose condition	Repressor / operator	Outcome
No lactose	Active repressor binds operator	Transcription blocked
Lactose or allolactose present	Inducer binds and inactivates repressor	RNA polymerase can transcribe operon
Lactose used up	Repressor becomes active again	Operon switches off

lac operon parts — i: makes repressor · z: β-galactosidase, splits lactose · y: permease, increases β-galactoside entry · a: transacetylase

Inducer — lactose/allolactose, not glucose or galactose · repressor control is negative regulation; low baseline expression allows lactose entry

7. Human Genome Project

HGP — international genome-sequencing project launched in 1990; completed in 2003 · enabled by DNA sequencing, cloning and bioinformatics

Goals — identify human genes · sequence ~3 billion base pairs · store and analyse data · develop tools and transfer technology · address ethical, legal and social issues (ELSI)

Two approaches — EST: identify genes expressed as RNA · sequence annotation: sequence the whole genome, then assign functions to regions

Workflow — isolate DNA → make random fragments → clone/amplify in host using BAC or YAC vectors → sequence fragments → align overlaps computationally → annotate and assign to chromosomes

Genome finding (chapter figures)	Recall value
Genome size	3164.7 million bp
Protein-coding portion	Less than 2%
Human similarity	99.9% of bases shared among people
Repeated DNA	Large fraction; often non-coding, useful for studying genome structure and variation
Gene count	About 30,000 in the chapter’s account; over half had unknown function
SNPs	About 1.4 million identified variable single-base sites

SNP — single-nucleotide polymorphism · Bioinformatics — computational storage, retrieval and analysis of biological data

8. DNA fingerprinting

DNA polymorphism — inherited sequence variation in a population; conventionally, a variant at a locus occurs above 1% frequency

Basis — highly variable repetitive DNA (satellite DNA, including minisatellite VNTRs) differs in repeat number → DNA fragments differ in length → individual banding patterns

VNTR — tandem repeats with copy number varying among chromosomes; chapter range: 0.1–20 kb · pattern generally differs between people, except identical twins

Method — isolate DNA → cut with restriction enzymes → separate fragments by electrophoresis → blot onto nylon/nitrocellulose → hybridise labelled VNTR probe → detect bands by autoradiography

Uses — forensic identification · paternity disputes · population and genetic diversity studies · DNA from different tissues of one individual shares the same polymorphism pattern

9. Traps

~~DNA polymerase works in either direction~~ → DNA synthesis is only $5'\to3'$ .
~~Both DNA strands are transcribed~~ → one strand per transcription unit; two RNAs could be complementary and encode different products.
~~Every codon has a matching tRNA~~ → stop codons have no tRNA; release factors terminate translation.
~~All mutations shift the reading frame~~ → only insertions/deletions not in multiples of three shift it.
~~Lactose permanently keeps lac operon on~~ → lactose depletion restores active repressor and shuts transcription down.

⚑ Panic box

Base pairing — $A=T$ (2 H-bonds); $G\equiv C$ (3); strands are complementary and antiparallel.

Replication — semiconservative: each daughter DNA has one old and one new strand; synthesis runs $5'\to3'$.

Gene expression — DNA $\to$ RNA $\to$ protein; in some viruses, RNA $\to$ DNA.

Code — triplet, degenerate, nearly universal; AUG starts and codes Met; UAA/UAG/UGA stop.

1. DNA: structure and packaging

Nucleotide — nitrogenous base + pentose sugar + phosphate · nucleoside — base + sugar

Bases — purines: A, G · pyrimidines: C, T, U · DNA has T; RNA has U and a 2′-OH group

Chain links — base–sugar: N-glycosidic · phosphate–sugar: phosphoester · nucleotides: 3′–5′ phosphodiester

Polarity — 5′ end has free phosphate · 3′ end has free OH · sugar–phosphate backbone; bases project inward

DNA feature	Key point	Recall check
Strands	Two, right-handed helix; antiparallel	One runs $5'\to3'$, other $3'\to5'$
Pairing	$A=T$; $G\equiv C$	Purine pairs with pyrimidine
Chargaff rule	$A=T$, $G=C$; total purines = pyrimidines	In double-stranded DNA
Dimensions	0.34 nm per bp; 3.4 nm per turn	About 10 bp per turn
Stability	H-bonds + stacked base pairs	Complementary strands can re-form

Packaging — prokaryotic DNA: protein-held loops in nucleoid · eukaryotic DNA: wraps around histone octamers → nucleosomes (~200 bp each) → chromatin fibre → condensed chromosome

Histones — positively charged, lysine- and arginine-rich proteins · Euchromatin — loosely packed, active · Heterochromatin — tightly packed, usually inactive

2. Genetic material: evidence and properties

Evidence	Experimental pattern	Conclusion
Griffith (1928)	Heat-killed virulent S + live non-virulent R → mice die; live S recovered	R cells transformed; factor not identified
Avery, MacLeod, McCarty	Protease/RNase do not stop transformation; DNase does	DNA is transforming principle
Hershey–Chase (1952)	$^{32}P$ labels phage DNA; $^{35}S$ labels protein; only DNA label enters bacteria	DNA, not protein, is inherited by infected bacteria

Hershey–Chase logic — infect E. coli with labelled phages → blend off coats → centrifuge → radioactivity with bacterial pellet only for DNA-labelled phages

Genetic material must — replicate · remain sufficiently stable · allow mutation · express inherited traits

RNA world — RNA likely preceded DNA: it could store information and catalyse reactions; more stable DNA later became the main information store

3. Replication

Semiconservative replication — each parental strand templates a complementary new strand; each daughter double helix contains one parental + one newly made strand

Replication feature	What to recall	Condition / consequence
DNA polymerase	DNA-dependent; adds deoxynucleotides	Synthesises only $5'\to3'$
Substrates	Deoxyribonucleoside triphosphates	Building blocks and energy source
Replication fork	Local opening of long DNA helix	Whole molecule does not unwind at once
Leading strand	Continuous synthesis	Template runs $3'\to5'$
Lagging strand	Discontinuous fragments; DNA ligase joins them	Template runs $5'\to3'$
Origin	Specific DNA site where copying begins	Polymerase cannot initiate randomly

Timing — eukaryotic DNA replication occurs in S phase · replication errors can cause mutations

4. Transcription and RNA processing

Transcription — copying one DNA segment, using one strand, into RNA · RNA is made $5'\to3'$; A pairs with U

Transcription unit	Position / role
Promoter	Upstream; RNA polymerase binding; sets strand choice and direction
Structural gene	DNA region transcribed into RNA
Terminator	Downstream; signals transcription end

Template strand — read $3'\to5'$; RNA is complementary · Coding strand — $5'\to3'$; same sequence as RNA except DNA has T where RNA has U

Eukaryotic polymerase	Main transcripts
RNA polymerase I	28S, 18S, 5.8S rRNA
RNA polymerase II	hnRNA, the mRNA precursor
RNA polymerase III	tRNA, 5S rRNA, snRNA

hnRNA processing — splice out introns and join exons · add 5′ methyl-guanosine cap · add 3′ poly-A tail (about 200–300 A residues) → mature mRNA leaves nucleus

RNA roles — mRNA carries codons · tRNA brings amino acids and reads codons · rRNA forms ribosome structure and catalyses peptide-bond formation

5. Genetic code and translation

Start / stop — AUG: methionine and initiation · UAA, UAG, UGA: termination; no tRNA reads stop codons

Translation component	Role	Distinguish it by
mRNA	Codon template	Codons are read in triplets
tRNA	Adapter; carries amino acid	Anticodon pairs with codon; amino-acid acceptor end
Ribosome	Binds mRNA; positions tRNAs	rRNA catalyses peptide-bond formation; bacterial 23S rRNA is a ribozyme
Release factor	Ends translation	Recognises stop codon; releases polypeptide

Translational unit — mRNA sequence from start to stop codon · UTRs — untranslated regions before AUG and after stop; support efficient translation

6. Regulation: lac operon

Gene expression can be regulated — transcription · RNA processing · mRNA transport · translation (eukaryotes)

Operon — coordinated bacterial gene unit: shared promoter and regulatory control for related structural genes · transcription initiation is the main prokaryotic control point

Lactose condition	Repressor / operator	Outcome
No lactose	Active repressor binds operator	Transcription blocked
Lactose or allolactose present	Inducer binds and inactivates repressor	RNA polymerase can transcribe operon
Lactose used up	Repressor becomes active again	Operon switches off

lac operon parts — i: makes repressor · z: β-galactosidase, splits lactose · y: permease, increases β-galactoside entry · a: transacetylase

Inducer — lactose/allolactose, not glucose or galactose · repressor control is negative regulation; low baseline expression allows lactose entry

7. Human Genome Project

HGP — international genome-sequencing project launched in 1990; completed in 2003 · enabled by DNA sequencing, cloning and bioinformatics

Goals — identify human genes · sequence ~3 billion base pairs · store and analyse data · develop tools and transfer technology · address ethical, legal and social issues (ELSI)

Two approaches — EST: identify genes expressed as RNA · sequence annotation: sequence the whole genome, then assign functions to regions

Genome finding (chapter figures)	Recall value
Genome size	3164.7 million bp
Protein-coding portion	Less than 2%
Human similarity	99.9% of bases shared among people
Repeated DNA	Large fraction; often non-coding, useful for studying genome structure and variation
Gene count	About 30,000 in the chapter’s account; over half had unknown function
SNPs	About 1.4 million identified variable single-base sites

SNP — single-nucleotide polymorphism · Bioinformatics — computational storage, retrieval and analysis of biological data

8. DNA fingerprinting

DNA polymorphism — inherited sequence variation in a population; conventionally, a variant at a locus occurs above 1% frequency

Basis — highly variable repetitive DNA (satellite DNA, including minisatellite VNTRs) differs in repeat number → DNA fragments differ in length → individual banding patterns

VNTR — tandem repeats with copy number varying among chromosomes; chapter range: 0.1–20 kb · pattern generally differs between people, except identical twins

Uses — forensic identification · paternity disputes · population and genetic diversity studies · DNA from different tissues of one individual shares the same polymorphism pattern

9. Traps

~~DNA polymerase works in either direction~~ → DNA synthesis is only $5'\to3'$.
~~Both DNA strands are transcribed~~ → one strand per transcription unit; two RNAs could be complementary and encode different products.
~~Every codon has a matching tRNA~~ → stop codons have no tRNA; release factors terminate translation.
~~All mutations shift the reading frame~~ → only insertions/deletions not in multiples of three shift it.
~~Lactose permanently keeps lac operon on~~ → lactose depletion restores active repressor and shuts transcription down.

Molecular Basis of InheritanceClass 12 Biology, Chapter 5

Cheat sheet

Molecular Basis of Inheritance

1. DNA: structure and packaging

2. Genetic material: evidence and properties

3. Replication

4. Transcription and RNA processing

5. Genetic code and translation

6. Regulation: lac operon

7. Human Genome Project

8. DNA fingerprinting

9. Traps

1. DNA: structure and packaging

2. Genetic material: evidence and properties

3. Replication

4. Transcription and RNA processing

5. Genetic code and translation

6. Regulation: lac operon

7. Human Genome Project

8. DNA fingerprinting

9. Traps

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Questions about Molecular Basis of Inheritance

1. DNA: structure and packaging

2. Genetic material: evidence and properties

3. Replication

4. Transcription and RNA processing

5. Genetic code and translation

6. Regulation: lac operon

7. Human Genome Project

8. DNA fingerprinting

9. Traps