The chapter Molecular Basis of Inheritance deals with one of the most fundamental aspects of biology – how genetic information is stored, expressed, and transmitted from one generation to the next. It focuses on the structure of DNA, the concept of genes, replication, transcription, translation, genetic code, regulation of gene expression, and modern applications like Human Genome Project and DNA fingerprinting.
Understanding this chapter provides the foundation of molecular genetics and helps explain how life operates at the molecular level.
Molecular Basis of Inheritance Class 12 Biology Notes | Complete Guide with MCQs & FAQs
Discovery of Genetic Material
DNA as Genetic Material
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Frederick Griffith (1928):
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Performed transformation experiment with Streptococcus pneumoniae.
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Discovered that the “transforming principle” could transfer genetic information.
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Oswald Avery, Colin MacLeod, and Maclyn McCarty (1944):
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Identified DNA as the transforming principle.
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Alfred Hershey and Martha Chase (1952):
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Proved conclusively through bacteriophage (T2 phage) experiment that DNA is the genetic material, not protein.
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RNA as Genetic Material
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Some viruses (like TMV, HIV) use RNA as the genetic material.
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RNA can act as genetic material and also as a catalyst (ribozymes).
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This supports the hypothesis of RNA world, where RNA was the first genetic material.
Structure of DNA
DNA as the Hereditary Material
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DNA (Deoxyribonucleic acid) carries the genetic blueprint.
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Composed of nucleotides (nitrogenous base + sugar + phosphate).
Double Helix Model (Watson and Crick, 1953)
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DNA is a double helix with two antiparallel strands.
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Sugar-phosphate backbone forms the outside.
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Nitrogenous bases (A, T, G, C) form complementary base pairs:
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A pairs with T (2 hydrogen bonds)
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G pairs with C (3 hydrogen bonds)
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Features of DNA Structure
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Right-handed double helix.
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One complete turn = 34 Å with 10 base pairs.
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Distance between two base pairs = 3.4 Å.
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Stability due to hydrogen bonds and base stacking.
Packaging of DNA
Length of DNA
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Human diploid cell has about 6.6 × 10⁹ base pairs.
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Stretched DNA length = ~2.2 meters per cell.
Packaging in Prokaryotes
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DNA is negatively charged and is held with positively charged proteins (Nucleoid-associated proteins).
Packaging in Eukaryotes
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DNA wraps around histone proteins to form nucleosomes.
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A nucleosome = DNA + histone octamer (H2A, H2B, H3, H4).
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Looks like “beads on string”.
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Higher order packaging forms chromatin:
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Euchromatin – loosely packed, transcriptionally active.
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Heterochromatin – densely packed, inactive.
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The Search for Genetic Code
Genetic Code
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It is a set of rules by which the sequence of nucleotides in mRNA is translated into amino acids.
Properties of Genetic Code
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Triplet codon – Three nucleotides form one codon.
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Unambiguous – One codon codes for one amino acid.
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Degenerate – More than one codon can code for the same amino acid.
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Universal – Same code is used in all living organisms.
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Start codon – AUG (codes for Methionine).
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Stop codons – UAA, UAG, UGA (terminate translation).
Experimental Proof
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Marshall Nirenberg and Har Gobind Khorana deciphered the genetic code using synthetic RNA.
Replication of DNA
Semi-Conservative Nature
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Watson and Crick proposed semi-conservative replication.
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Meselson and Stahl experiment (1958) proved it using isotopes of Nitrogen (¹⁵N and ¹⁴N).
Process of DNA Replication
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Origin of replication – Replication begins at a specific point.
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Helicase enzyme unwinds the double helix.
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Single-strand binding proteins stabilize unwound DNA.
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DNA polymerase synthesizes new strands:
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Leading strand synthesized continuously.
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Lagging strand synthesized discontinuously as Okazaki fragments.
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DNA ligase joins Okazaki fragments.
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Proofreading ensures fidelity.
Transcription
Definition
Transcription is the process of synthesizing RNA from DNA.
Steps of Transcription in Prokaryotes
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Initiation – RNA polymerase binds to promoter region.
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Elongation – RNA polymerase adds ribonucleotides complementary to DNA template.
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Termination – RNA polymerase detaches when it reaches terminator sequence.
Transcription in Eukaryotes
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Occurs inside nucleus.
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Three types of RNA polymerase:
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RNA Polymerase I – rRNA
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RNA Polymerase II – mRNA
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RNA Polymerase III – tRNA, 5S rRNA
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mRNA undergoes processing:
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Capping (5’ methyl guanosine cap).
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Tailing (poly-A tail at 3’ end).
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Splicing (removal of introns, joining of exons).
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Translation
Definition
Translation is the process of protein synthesis from mRNA template.
Components Required
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mRNA (template)
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tRNA (adaptor molecule proposed by Francis Crick)
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Ribosomes (site of protein synthesis)
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Amino acids, enzymes, GTP/ATP
Steps of Translation
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Initiation – Ribosome assembles around mRNA and start codon AUG.
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Elongation – tRNAs bring amino acids and peptide bonds form.
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Termination – Stop codon is reached, polypeptide released.
Regulation of Gene Expression
In Prokaryotes – Operon Model
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Lac Operon (Jacob and Monod) is the classical model.
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Components:
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Structural genes (lac Z, lac Y, lac A).
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Promoter and Operator.
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Regulatory gene (produces repressor).
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Mechanism:
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In absence of lactose → repressor binds to operator → genes not expressed.
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In presence of lactose → repressor inactivated → genes expressed.
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In Eukaryotes
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Gene expression is controlled at multiple levels:
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Transcriptional level
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Post-transcriptional level
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Translational level
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Post-translational level
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Human Genome Project (HGP)
Introduction
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Launched in 1990, completed in 2003.
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Aim: To sequence entire human DNA (~3 billion base pairs).
Objectives
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Identify all human genes.
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Study functions and interactions of genes.
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Develop new tools for data analysis.
Findings
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Humans have ~20,000–25,000 genes.
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Less than 2% DNA codes for proteins.
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Repetitive sequences are abundant.
Applications
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Advances in medical research.
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Identification of genetic disorders.
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Personalized medicine.
DNA Fingerprinting
Definition
DNA fingerprinting is a technique of identifying individuals based on unique patterns in their DNA.
Developed By
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Alec Jeffreys (1985).
Principle
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Uses Variable Number Tandem Repeats (VNTRs) or microsatellite sequences.
Steps
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DNA extraction.
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Restriction digestion.
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Separation by gel electrophoresis.
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Hybridization with DNA probes.
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Autoradiography to detect pattern.
Applications
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Forensic science.
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Paternity testing.
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Identification of criminals.
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Conservation of biodiversity.
Important Terms to Remember
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Nucleosome: DNA + histone core.
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Okazaki fragments: Short DNA fragments on lagging strand.
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Exons and Introns: Exons are coding sequences; introns are non-coding.
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Operon: Functional unit of gene regulation.
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VNTRs: Variable number tandem repeats used in DNA fingerprinting.
Practice Questions – Molecular Basis of Inheritance Class 12
Multiple Choice Questions (MCQs)
Q1. The transforming principle discovered by Griffith was later identified as:
a) Protein
b) DNA
c) RNA
d) Lipid
Answer: The transforming principle was later identified as DNA, which carries the genetic information.
Q2. The double helix model of DNA was proposed by:
a) Meselson and Stahl
b) Hershey and Chase
c) Watson and Crick
d) Nirenberg and Khorana
Answer: The double helix structure of DNA was proposed by Watson and Crick in 1953.
Q3. In DNA, adenine pairs with thymine through:
a) One hydrogen bond
b) Two hydrogen bonds
c) Three hydrogen bonds
d) Ionic bond
Answer: Adenine pairs with thymine through two hydrogen bonds.
Q4. The Okazaki fragments are formed during:
a) Leading strand synthesis
b) Lagging strand synthesis
c) Transcription
d) Translation
Answer: Okazaki fragments are formed during lagging strand synthesis in DNA replication.
Q5. The enzyme responsible for joining Okazaki fragments is:
a) DNA ligase
b) DNA helicase
c) RNA polymerase
d) DNA polymerase
Answer: The enzyme DNA ligase joins Okazaki fragments on the lagging strand.
Q6. The start codon for protein synthesis is:
a) UAA
b) AUG
c) UGA
d) UAG
Answer: The start codon is AUG, which codes for methionine.
Q7. Which of the following is a stop codon?
a) AUG
b) GUG
c) UAA
d) AAA
Answer: UAA is a stop codon, along with UAG and UGA.
Q8. The lac operon is switched on in the presence of:
a) Glucose
b) Lactose
c) Fructose
d) Maltose
Answer: The lac operon is switched on in the presence of lactose.
Q9. RNA polymerase II in eukaryotes transcribes:
a) rRNA
b) tRNA
c) mRNA
d) snRNA
Answer: RNA polymerase II transcribes mRNA in eukaryotes.
Q10. Which scientist gave the operon model of gene regulation?
a) Hershey and Chase
b) Jacob and Monod
c) Watson and Crick
d) Franklin and Wilkins
Answer: The operon model was proposed by Jacob and Monod.
Q11. The length of DNA in a diploid human cell is approximately:
a) 2.2 meters
b) 22 cm
c) 22 meters
d) 0.2 meters
Answer: The total DNA length in a diploid human cell is about 2.2 meters.
Q12. Which type of RNA carries amino acids to the ribosome?
a) rRNA
b) mRNA
c) tRNA
d) snRNA
Answer: tRNA carries amino acids to ribosomes during translation.
Q13. Which of the following is NOT a property of the genetic code?
a) Triplet nature
b) Ambiguous
c) Universal
d) Degenerate
Answer: The genetic code is not ambiguous; it is unambiguous, meaning one codon codes for only one amino acid.
Q14. DNA replication is:
a) Conservative
b) Semi-conservative
c) Dispersive
d) Random
Answer: DNA replication is semi-conservative, as proved by Meselson and Stahl.
Q15. DNA fingerprinting is based on:
a) Introns
b) Exons
c) VNTRs
d) mRNA
Answer: DNA fingerprinting is based on Variable Number Tandem Repeats (VNTRs).
Q16. The total number of amino acids coded by the genetic code is:
a) 10
b) 20
c) 64
d) 4
Answer: The genetic code specifies 20 amino acids using 64 codons.
Q17. Who discovered the transforming principle?
a) Griffith
b) Hershey
c) Franklin
d) Nirenberg
Answer: The transforming principle was first discovered by Griffith in 1928.
Q18. Which base is not present in RNA?
a) Adenine
b) Uracil
c) Thymine
d) Cytosine
Answer: Thymine is absent in RNA; uracil replaces it.
Q19. The Human Genome Project revealed that only about ______% of DNA codes for proteins.
a) 10%
b) 2%
c) 20%
d) 50%
Answer: The Human Genome Project showed that only about 2% of DNA codes for proteins.
Q20. The process of removing introns and joining exons is called:
a) Capping
b) Tailing
c) Splicing
d) Ligation
Answer: The process is called splicing, which occurs during mRNA processing.
Short Answer Questions Molecular Basis of Inheritance Class 12
Q1. What is the difference between Euchromatin and Heterochromatin?
Answer: Euchromatin is a loosely packed form of chromatin that is transcriptionally active, whereas heterochromatin is densely packed and transcriptionally inactive.
Q2. What is the role of DNA ligase in replication?
Answer: DNA ligase seals the gaps between Okazaki fragments on the lagging strand, forming a continuous DNA strand.
Q3. Why is the genetic code called degenerate?
Answer: The genetic code is called degenerate because more than one codon can code for the same amino acid, allowing redundancy.
Q4. Define VNTRs.
Answer: VNTRs, or Variable Number Tandem Repeats, are short sequences of DNA repeated in tandem. They vary among individuals and form the basis of DNA fingerprinting.
Q5. What is an Operon?
Answer: An operon is a functional unit of DNA containing structural genes, an operator, and a promoter, which together regulate gene expression in prokaryotes.
Long Answer Questions
Q1. Explain Meselson and Stahl’s experiment that proved semi-conservative replication of DNA.
Answer: Meselson and Stahl grew E. coli in a medium containing heavy isotope of nitrogen (¹⁵N). The DNA incorporated this isotope and became heavy. They then shifted the bacteria to a medium containing ¹⁴N. After one generation, the DNA was intermediate in density, and after two generations, both light and intermediate DNA molecules were found. This showed that each new DNA molecule had one parental strand and one newly synthesized strand, proving the semi-conservative mode of replication.
Q2. Describe the structure of a nucleosome.
Answer: A nucleosome is the fundamental unit of DNA packaging in eukaryotes. It consists of a histone octamer made up of two molecules each of H2A, H2B, H3, and H4. Around this histone core, 146 base pairs of DNA are wrapped in nearly two turns. The nucleosomes appear as “beads on a string” under an electron microscope, helping in compact packaging of long DNA molecules.
Q3. Explain the Lac Operon with the help of a diagram.
Answer: The lac operon in E. coli regulates lactose metabolism. It consists of structural genes (lacZ, lacY, lacA), a promoter, an operator, and a regulator gene. In the absence of lactose, the repressor binds to the operator, preventing transcription. When lactose is present, it acts as an inducer by binding to the repressor, inactivating it. This allows RNA polymerase to transcribe the structural genes, leading to the synthesis of enzymes required for lactose metabolism.
Q4. What are the objectives and findings of the Human Genome Project?
Answer: The Human Genome Project aimed to sequence all 3 billion base pairs of human DNA and identify all genes. It revealed that humans have around 20,000–25,000 genes, and only 2% of DNA codes for proteins. The rest consists of non-coding sequences, many of which are repetitive. The project also advanced medical research by helping identify genetic disorders and promoting the concept of personalized medicine.
Q5. Describe the process of transcription in eukaryotes.
Answer: In eukaryotes, transcription occurs inside the nucleus and involves three RNA polymerases. RNA polymerase II transcribes mRNA. The process begins when RNA polymerase binds to the promoter region, initiating transcription. During elongation, nucleotides are added complementary to the DNA template strand. Termination occurs at specific sequences. The pre-mRNA undergoes processing that includes capping at the 5’ end, addition of a poly-A tail at the 3’ end, and splicing to remove introns. The mature mRNA then moves to the cytoplasm for translation.
Frequently Asked Questions (FAQs)
Molecular Basis of Inheritance Class 12
Q1. What is the molecular basis of inheritance?
Answer: The molecular basis of inheritance refers to the study of how DNA and RNA store, transmit, and express genetic information. It includes processes like replication, transcription, translation, genetic code, and gene regulation that together explain how traits are passed from one generation to another.
Q2. Who proved that DNA is the genetic material?
Answer: DNA as the genetic material was proved by Hershey and Chase in 1952 using bacteriophage experiments. Their work confirmed that DNA, and not protein, carries genetic information in viruses.
Q3. Why is DNA replication called semi-conservative?
Answer: DNA replication is called semi-conservative because each newly formed DNA molecule contains one parental strand and one newly synthesized strand. This was experimentally proved by Meselson and Stahl in 1958.
Q4. What is the importance of the lac operon model?
Answer: The lac operon model proposed by Jacob and Monod explains gene regulation in prokaryotes. It shows how genes can be switched on or off depending on the presence or absence of a substrate like lactose, thus conserving energy and resources.
Q5. What is the difference between transcription and translation?
Answer: Transcription is the process of synthesizing RNA from a DNA template, whereas translation is the process of synthesizing proteins using the information carried by mRNA. Transcription occurs in the nucleus (in eukaryotes), while translation occurs in the cytoplasm on ribosomes.
Q6. What is the role of tRNA in protein synthesis?
Answer: Transfer RNA (tRNA) acts as an adaptor molecule during protein synthesis. It carries specific amino acids to the ribosome and pairs its anticodon with the codon on mRNA, ensuring the correct sequence of amino acids in the growing polypeptide chain.
Q7. What are introns and exons?
Answer: In eukaryotic genes, exons are coding sequences that express proteins, while introns are non-coding sequences that are removed during RNA splicing. Only exons remain in the mature mRNA for translation.
Q8. How many stop codons are there in the genetic code?
Answer: There are three stop codons in the genetic code – UAA, UAG, and UGA. They do not code for any amino acid but signal the termination of protein synthesis.
Q9. What did the Human Genome Project reveal about human DNA?
Answer: The Human Genome Project revealed that humans have around 20,000–25,000 genes, and only about 2% of the DNA actually codes for proteins. The rest consists of non-coding and repetitive sequences, many of which have regulatory or unknown functions.
Q10. What is DNA fingerprinting and where is it used?
Answer: DNA fingerprinting is a technique to identify individuals based on their unique DNA sequence patterns, particularly VNTRs. It is widely used in forensic science, paternity testing, criminal investigations, and biodiversity conservation.
Conclusion
The Molecular Basis of Inheritance explains the central dogma of molecular biology – DNA makes RNA, and RNA makes protein. It provides insight into how genetic information is replicated, expressed, and regulated. The discoveries like the genetic code, lac operon, Human Genome Project, and DNA fingerprinting have revolutionized biology, medicine, and biotechnology.
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