Molecular Basis of Inheritance
Welcome to HSLC Guru. This page provides complete English-medium notes, summary, important questions and answers, MCQs, fill in the blanks, true/false statements and a glossary for Class 12 Biology Chapter 6 — Molecular Basis of Inheritance, prepared strictly according to the ASSEB (Assam State School Education Board) syllabus. The chapter explains how DNA stores, copies and expresses genetic information and introduces the experiments and discoveries that built modern molecular biology.
Summary
Deoxyribonucleic acid (DNA) is the universal carrier of genetic information in almost all living organisms. In 1953, James Watson and Francis Crick, using the X-ray diffraction images produced by Rosalind Franklin and Maurice Wilkins, proposed the famous double helix model of DNA. The molecule consists of two polynucleotide strands wound around a common axis. Each strand has a sugar-phosphate backbone, and the two strands are antiparallel, meaning one runs 5′ to 3′ while the other runs 3′ to 5′. The nitrogen bases project inward and pair specifically — adenine pairs with thymine through two hydrogen bonds (A=T) and guanine pairs with cytosine through three hydrogen bonds (G≡C), satisfying Chargaff’s rule. The helix is right-handed with a pitch of 3.4 nm and contains approximately 10 base pairs per turn, giving a distance of 0.34 nm between adjacent base pairs.
The search for genetic material spanned several decades. Frederick Griffith (1928) discovered the transformation principle using virulent (S) and non-virulent (R) strains of Streptococcus pneumoniae. Avery, MacLeod and McCarty (1944) chemically purified the transforming agent and proved it was DNA, not protein. The classic Hershey-Chase blender experiment (1952) using radioactive sulphur (³⁵S) for protein and radioactive phosphorus (³²P) for DNA in T2 bacteriophages confirmed unambiguously that DNA is the genetic material. Inside eukaryotic cells, the long DNA molecule is organised by packaging — DNA wraps around histone octamers to form nucleosomes, which coil to form chromatin fibres, which condense further into chromosomes.
DNA replication is semi-conservative, as proved by the elegant Meselson-Stahl experiment (1958) using ¹⁵N and ¹⁴N isotopes in E. coli. Each daughter DNA molecule has one parental strand and one newly synthesised strand. The enzyme DNA polymerase adds nucleotides in the 5′ → 3′ direction at the replication fork, producing a continuous leading strand and a discontinuous lagging strand made of Okazaki fragments. Transcription is the synthesis of RNA from a DNA template by RNA polymerase; it begins at a promoter and ends at a terminator. The genetic code, deciphered by Marshall Nirenberg, Har Gobind Khorana and others, consists of 64 triplet codons; it is universal, degenerate, non-overlapping and commaless, with AUG as the start codon (also coding for methionine) and UAA, UAG, UGA as the three stop codons.
Translation, the synthesis of a polypeptide on the ribosome, requires mRNA, tRNA, ribosomes and amino acids. Each amino acid is first charged onto its specific tRNA by aminoacyl-tRNA synthetase. Translation proceeds through three stages — initiation, elongation and termination. Gene expression in prokaryotes is regulated by operons. The Lac operon, proposed by François Jacob and Jacques Monod (1961), is a classic example of negative regulation in E. coli, where lactose acts as the inducer that inactivates the repressor. The Human Genome Project (1990–2003) aimed to sequence the entire human genome, identify all human genes and develop technology for genome analysis; it revealed about 30,000 genes in the human genome. DNA fingerprinting, developed by Sir Alec Jeffreys (1984), exploits hyper-variable repeated sequences called VNTRs (Variable Number Tandem Repeats) and is performed using techniques such as RFLP (Restriction Fragment Length Polymorphism); it is widely used in forensics, paternity testing and population studies.
1 Mark Questions
Q1. Who proposed the double helix model of DNA?
Answer: James Watson and Francis Crick proposed the double helix model of DNA in 1953.
Q2. What is the pitch of the DNA helix?
Answer: The pitch (length of one complete turn) of the DNA helix is 3.4 nm and contains about 10 base pairs.
Q3. Name the three stop codons of the genetic code.
Answer: The three stop (nonsense) codons are UAA, UAG and UGA.
Q4. Who discovered the phenomenon of transformation in bacteria?
Answer: Frederick Griffith, in 1928, discovered the transformation principle in Streptococcus pneumoniae.
Q5. What is a nucleosome?
Answer: A nucleosome is the basic unit of DNA packaging in eukaryotes, consisting of about 200 bp of DNA wrapped around an octamer of histone proteins.
Q6. What is meant by semi-conservative replication?
Answer: Semi-conservative replication means that each new DNA molecule contains one parental (old) strand and one newly synthesised strand.
Q7. Who developed the technique of DNA fingerprinting?
Answer: Sir Alec Jeffreys developed the technique of DNA fingerprinting in 1984.
Q8. What is a codon?
Answer: A codon is a sequence of three consecutive nucleotides on mRNA that codes for one specific amino acid or a stop signal.
Q9. Name the start codon used during translation.
Answer: AUG is the start codon. It also codes for the amino acid methionine.
Q10. Expand VNTR.
Answer: VNTR stands for Variable Number Tandem Repeats — short repeated DNA sequences that vary in copy number among individuals and form the basis of DNA fingerprinting.
Q11. What is meant by “central dogma” of molecular biology?
Answer: Proposed by Francis Crick, the central dogma states that genetic information flows from DNA to RNA (transcription) and from RNA to protein (translation): DNA → RNA → Protein.
Q12. What are Okazaki fragments?
Answer: Okazaki fragments are short DNA segments synthesised discontinuously on the lagging strand during DNA replication, later joined by DNA ligase.
2–3 Marks Questions
Q1. Describe the salient features of the Watson-Crick model of DNA.
Answer: The Watson-Crick model proposes that DNA is a right-handed double helix made of two polynucleotide strands. The strands are antiparallel — one runs 5′ to 3′ and the other 3′ to 5′. The sugar-phosphate backbone lies on the outside while the nitrogen bases project inward and pair specifically: A with T (two hydrogen bonds) and G with C (three hydrogen bonds). One complete turn of the helix is 3.4 nm long and contains 10 base pairs, giving a distance of 0.34 nm between adjacent base pairs. The diameter of the helix is uniform at 2 nm.
Q2. Briefly describe the Hershey-Chase experiment.
Answer: Alfred Hershey and Martha Chase (1952) used T2 bacteriophages grown in radioactive media — some phages had ³²P-labelled DNA and others had ³⁵S-labelled protein coats. They allowed the phages to infect E. coli, then separated the empty viral coats from the bacteria using a kitchen blender and a centrifuge. They found ³²P (DNA) inside the bacterial cells while ³⁵S (protein) remained outside. This proved that DNA, not protein, is the genetic material that enters the host cell and produces new phages.
Q3. What are the salient features of the genetic code?
Answer: The genetic code has the following features: (i) it is a triplet code — three nucleotides specify one amino acid; (ii) there are 64 codons in total; (iii) it is universal — almost the same in all organisms; (iv) it is degenerate — many amino acids are coded by more than one codon; (v) it is non-overlapping and commaless — read continuously in one frame; (vi) AUG is the initiator codon and also codes for methionine; and (vii) UAA, UAG, UGA are stop or nonsense codons.
Q4. Differentiate between transcription and translation.
Answer: Transcription is the synthesis of RNA (mRNA) from a DNA template, catalysed by RNA polymerase, occurring in the nucleus of eukaryotes. Translation is the synthesis of a polypeptide from the mRNA template, occurring on ribosomes in the cytoplasm and requiring tRNA, amino acids and various factors. Transcription uses ribonucleotides while translation links amino acids through peptide bonds.
Q5. What is the role of tRNA in translation?
Answer: Transfer RNA (tRNA) is the adapter molecule of translation. Each tRNA has an anticodon that base-pairs with a complementary codon on the mRNA, and an amino acid acceptor end (CCA-3′) that carries the specific amino acid. Aminoacyl-tRNA synthetases charge each tRNA with the correct amino acid (charging or activation). Inside the ribosome, tRNAs deliver amino acids in the order specified by the mRNA, allowing peptide bond formation.
Q6. Briefly explain DNA packaging in eukaryotic cells.
Answer: The very long DNA molecule is packaged in stages. First, about 200 bp of DNA wraps around an octamer of histone proteins (H2A, H2B, H3, H4 — two of each) to form a nucleosome, the fundamental unit. Nucleosomes connected by linker DNA appear as “beads on a string” and coil up to form a 30 nm chromatin fibre. The fibre is further looped and condensed by non-histone scaffold proteins to form the highly compact chromosome visible during cell division.
5–7 Marks Questions
Q1. Describe the Meselson-Stahl experiment that proved DNA replication is semi-conservative.
Answer: Matthew Meselson and Franklin Stahl (1958) grew Escherichia coli for many generations in a medium containing ¹⁵NH₄Cl as the sole nitrogen source, so all DNA contained the heavy isotope ¹⁵N. They then transferred the bacteria to a normal ¹⁴N medium and extracted DNA samples after each generation. The DNA was analysed by caesium chloride (CsCl) density gradient centrifugation. After one generation in ¹⁴N, all DNA showed an intermediate density (a hybrid of ¹⁵N and ¹⁴N), ruling out the conservative model. After two generations, two bands appeared — one of intermediate density and one of light density — exactly as predicted by the semi-conservative model. The experiment thus established that each daughter DNA molecule contains one old (parental) strand and one newly synthesised strand.
Q2. Explain the process of transcription in prokaryotes.
Answer: Transcription is the process of synthesising RNA from a DNA template using RNA polymerase. In prokaryotes such as E. coli, a single RNA polymerase carries out transcription of all RNA types and proceeds in three steps:
(i) Initiation: RNA polymerase, with the help of a sigma (σ) factor, binds to the promoter region upstream of the gene and unwinds a short stretch of DNA.
(ii) Elongation: The polymerase moves along the template strand (3′ → 5′) and synthesises an RNA strand in the 5′ → 3′ direction, adding ribonucleotides complementary to the template (A pairs with U, T with A, G with C). The non-template strand has the same sequence as the RNA except for U replacing T and is called the coding/sense strand.
(iii) Termination: When the polymerase encounters the terminator sequence, the RNA-DNA hybrid is destabilised and the newly formed RNA is released. In prokaryotes, mRNA does not require post-transcriptional processing and translation often begins even before transcription is complete.
Q3. Describe the mechanism of translation in detail.
Answer: Translation is the process by which the sequence of nucleotides in mRNA is decoded to form a specific polypeptide on the ribosome. The major steps are:
(a) Charging of tRNA (aminoacylation): Each amino acid is attached to its specific tRNA by aminoacyl-tRNA synthetase using ATP, forming aminoacyl-tRNA.
(b) Initiation: The smaller ribosomal subunit binds mRNA at the start codon AUG. The initiator tRNA carrying methionine (formyl-methionine in prokaryotes) base-pairs with AUG. The larger subunit then joins to form a complete ribosome with the initiator tRNA at the P-site.
(c) Elongation: The next aminoacyl-tRNA enters the A-site according to the codon. A peptide bond is formed by the enzyme peptidyl transferase, transferring the amino acid from P-site tRNA to A-site tRNA. The ribosome then translocates by one codon, moving the deacylated tRNA to the E-site (exit) and the peptidyl-tRNA to the P-site. This cycle repeats.
(d) Termination: When a stop codon (UAA/UAG/UGA) reaches the A-site, release factors bind, the polypeptide is released, and the ribosomal subunits dissociate from the mRNA.
Q4. Explain the structure and regulation of the Lac operon.
Answer: The Lac operon, proposed by François Jacob and Jacques Monod in 1961, regulates the metabolism of lactose in E. coli. It consists of:
(i) Regulator gene (i): located upstream and codes for the repressor protein.
(ii) Promoter (p): binding site for RNA polymerase.
(iii) Operator (o): the binding site of the repressor.
(iv) Structural genes — z, y, a: code for β-galactosidase (hydrolyses lactose), permease (lactose uptake) and transacetylase respectively.
Regulation: In the absence of lactose, the repressor binds to the operator and prevents RNA polymerase from transcribing the structural genes — the operon is OFF. In the presence of lactose, a small amount of lactose is converted to allolactose, which acts as the inducer. The inducer binds the repressor, changes its shape and prevents it from binding to the operator. RNA polymerase then transcribes the structural genes — the operon is ON. This is an example of negative regulation by induction.
Q5. Write a brief account of the Human Genome Project (HGP) and DNA fingerprinting.
Answer: The Human Genome Project was an international scientific effort launched in 1990 and declared complete in 2003. Its main goals were: (i) to identify all the genes in human DNA; (ii) to determine the sequence of about 3 billion base pairs that make up the human genome; (iii) to store this information in databases; (iv) to develop tools for data analysis; and (v) to address the related ethical, legal and social issues. Major findings included that the human genome has about 30,000 genes (much fewer than expected), that less than 2% of the genome codes for proteins and that there is large repetitive non-coding DNA. The HGP has greatly advanced medicine, biotechnology and forensic science.
DNA fingerprinting, developed by Sir Alec Jeffreys in 1984, is a technique used to identify individuals from their unique DNA pattern. It uses hyper-variable regions called VNTRs (Variable Number Tandem Repeats), which vary in copy number from person to person. The DNA is cut by restriction enzymes, separated by gel electrophoresis, transferred onto a membrane (Southern blotting) and detected by labelled probes — a technique known as RFLP (Restriction Fragment Length Polymorphism) analysis. The unique band pattern obtained is the DNA fingerprint, used in forensic investigations, paternity disputes, identification of bodies and population/evolutionary studies.
Multiple Choice Questions (MCQs)
Q1. The double helix model of DNA was proposed by:
(a) Mendel and Morgan
(b) Watson and Crick
(c) Hershey and Chase
(d) Meselson and Stahl
Answer: (b) Watson and Crick.
Q2. One complete turn of the DNA double helix contains approximately:
(a) 5 base pairs
(b) 10 base pairs
(c) 20 base pairs
(d) 34 base pairs
Answer: (b) 10 base pairs.
Q3. The Hershey-Chase experiment used:
(a) ¹⁴C and ³H
(b) ³²P and ³⁵S
(c) ¹⁵N and ¹⁴N
(d) ³²P and ¹⁵N
Answer: (b) ³²P and ³⁵S.
Q4. The basic unit of DNA packaging in eukaryotes is the:
(a) Chromatin
(b) Chromosome
(c) Nucleosome
(d) Nucleolus
Answer: (c) Nucleosome.
Q5. DNA replication is:
(a) Conservative
(b) Semi-conservative
(c) Dispersive
(d) Random
Answer: (b) Semi-conservative.
Q6. The number of codons in the genetic code is:
(a) 16
(b) 20
(c) 61
(d) 64
Answer: (d) 64.
Q7. AUG is the codon for:
(a) Stop signal
(b) Methionine and start signal
(c) Tryptophan
(d) Cysteine
Answer: (b) Methionine and start signal.
Q8. The Lac operon was proposed by:
(a) Watson and Crick
(b) Beadle and Tatum
(c) Jacob and Monod
(d) Nirenberg and Khorana
Answer: (c) Jacob and Monod.
Q9. The Human Genome Project was completed in:
(a) 1990
(b) 1995
(c) 2000
(d) 2003
Answer: (d) 2003.
Q10. DNA fingerprinting is based on:
(a) SNPs
(b) VNTRs
(c) Operons
(d) Codons
Answer: (b) VNTRs.
Fill in the Blanks
Q1. The two strands of DNA are held together by ________ bonds between complementary bases.
Answer: hydrogen.
Q2. The enzyme that synthesises RNA from a DNA template is ________.
Answer: RNA polymerase.
Q3. The Meselson-Stahl experiment used the heavy isotope ________.
Answer: ¹⁵N (nitrogen-15).
Q4. The Human Genome Project revealed that the human genome contains about ________ genes.
Answer: 30,000.
Q5. ________ is the inducer of the Lac operon in E. coli.
Answer: Allolactose (a derivative of lactose).
True or False
Q1. Adenine pairs with cytosine in DNA.
Answer: False. Adenine pairs with thymine in DNA.
Q2. The genetic code is degenerate.
Answer: True. Most amino acids are coded by more than one codon.
Q3. Avery, MacLeod and McCarty proved that DNA is the genetic material.
Answer: True. They identified DNA as the transforming principle in 1944.
Q4. DNA polymerase can synthesise DNA in the 3′ to 5′ direction.
Answer: False. DNA polymerase synthesises DNA only in the 5′ to 3′ direction.
Q5. DNA fingerprinting was developed by Alec Jeffreys.
Answer: True. He developed the technique in 1984.
Glossary
| Term | Meaning |
|---|---|
| DNA | Deoxyribonucleic acid; double-stranded molecule that stores genetic information. |
| Double Helix | The two-stranded helical structure of DNA proposed by Watson and Crick (1953). |
| Antiparallel | Property of DNA strands where one runs 5′ to 3′ and the other 3′ to 5′. |
| Chargaff’s Rule | In DNA, A = T and G = C in molar amounts. |
| Transformation | Genetic change caused by uptake of foreign DNA, discovered by Griffith (1928). |
| Bacteriophage | A virus that infects bacteria; used by Hershey and Chase to prove DNA is genetic material. |
| Nucleosome | Basic unit of chromatin — about 200 bp DNA wrapped around a histone octamer. |
| Replication | Process by which DNA makes an identical copy of itself; semi-conservative. |
| Okazaki Fragments | Short DNA fragments synthesised on the lagging strand during replication. |
| Transcription | Synthesis of RNA from a DNA template by RNA polymerase. |
| Promoter | DNA sequence where RNA polymerase binds to begin transcription. |
| Terminator | DNA sequence that signals the end of transcription. |
| Codon | A triplet of nucleotides on mRNA coding for one amino acid. |
| Anticodon | Triplet on tRNA that pairs with the mRNA codon. |
| Genetic Code | Set of rules by which codons specify amino acids; universal and degenerate. |
| Translation | Synthesis of polypeptide on the ribosome using mRNA as template. |
| Charging of tRNA | Attachment of an amino acid to its specific tRNA by aminoacyl-tRNA synthetase. |
| Lac Operon | Cluster of genes regulating lactose metabolism in E. coli; proposed by Jacob and Monod. |
| Inducer | Molecule that activates an operon by inactivating the repressor (e.g., allolactose). |
| Human Genome Project | International project (1990–2003) that sequenced the entire human genome. |
| VNTR | Variable Number Tandem Repeats — basis of DNA fingerprinting. |
| RFLP | Restriction Fragment Length Polymorphism — technique used in DNA fingerprinting. |
| DNA Fingerprinting | Technique to identify individuals using unique DNA patterns; developed by Alec Jeffreys. |