NCERT Solutions for Class 12 Biology Chapter 5 Molecular Basis of Inheritance
NCERT Solutions for Class 12 Biology Chapter 5 Molecular Basis of Inheritance provides detailed notes and answers on the structure of DNA/RNA, central dogma, experiments proving DNA as genetic material, DNA replication, transcription, translation, genetic code, Lac Operon model, Human Genome Project (HGP), and DNA Fingerprinting.
Overview of Topics & Subtopics (Click to Jump)
1. Structure of DNA, RNA & Search for Genetic Material
Short Note:
• DNA Structure: Double helix model proposed by Watson and Crick (1953) based on X-ray diffraction data of Rosalind Franklin & Maurice Wilkins. Nitrogenous bases are Purines (Adenine, Guanine) and Pyrimidines (Cytosine, Thymine). Distance between base pairs is 0.34 nm with a pitch of 3.4 nm per turn.
• Packaging of DNA Helix: In eukaryotes, negatively charged DNA wraps around positively charged histone octamer to form a Nucleosome.
• Search for Genetic Material:
1. Griffith Experiment (1928): Transforming principle in Streptococcus pneumoniae.
2. Avery, MacLeod, McCarty (1944): Proved DNA is the transforming principle using DNase, RNase, and Protease enzymes.
3. Hershey & Chase Experiment (1952): Unequivocal proof using radioactive isotopes (35S for protein, 32P for DNA) in Bacteriophages.
2. DNA Replication, Transcription & Genetic Code
Short Note:
• Semiconservative DNA Replication: Experimentally proved by Meselson and Stahl (1958) using 15N isotope in E. coli. Main enzyme is DNA-dependent DNA Polymerase (5' to 3' direction synthesis leading to leading and lagging strands with Okazaki fragments joined by DNA Ligase).
• Transcription: Process of copying genetic information from DNA to RNA. A transcription unit consists of Promoter, Structural Gene, and Terminator.
- Eukaryotic RNA Processing: Splicing (removal of introns), Capping (addition of methyl guanosine triphosphate at 5' end), and Tailing (addition of poly-A tail at 3' end).
• Genetic Code: Triplet codon (64 codons: 61 code for amino acids, 3 are stop codons - UAA, UAG, UGA). AUG acts as initiator codon (codes for Methionine). Unambiguous, degenerate, and universal.
3. Translation, Lac Operon, HGP & DNA Fingerprinting
Short Note:
• Translation: Synthesis of polypeptide chain from mRNA sequence on ribosomes (tRNA acts as adapter molecule with anticodon loop).
• Regulation of Gene Expression (Lac Operon): Elucidated by François Jacob and Jacques Monod. Polycistronic structural genes (z, y, a) code for β-galactosidase, Permease, and Transacetylase. In presence of lactose (inducer), repressor protein gets inactivated allowing transcription.
• Human Genome Project (HGP): Mega project (1990-2003). Human genome contains ~3 x 109 bp and ~30,000 genes. Methodologies used were ESTs (Expressed Sequence Tags) and Sequence Annotation.
• DNA Fingerprinting: Developed by Alec Jeffreys. Based on Variable Number of Tandem Repeats (VNTRs) which show high degree of polymorphism.
Intext & Conceptual Questions
| Feature | DNA (Deoxyribonucleic Acid) | RNA (Ribonucleic Acid) |
|---|---|---|
| Sugar Unit | Deoxyribose sugar. | Ribose sugar. |
| Nitrogenous Bases | Adenine, Guanine, Cytosine, and Thymine. | Adenine, Guanine, Cytosine, and Uracil. |
| Structure | Double-stranded helix (usually). | Single-stranded molecule (usually). |
| Stability | Chemically less reactive and structurally more stable. | Chemically reactive due to 2'-OH group, unstable and mutates fast. |
| Role | Acts as primary genetic material in most organisms. | Acts mainly as messenger, adapter, and catalyst (ribozyme). |
Hershey and Chase worked with bacteriophages (viruses that infect bacteria) in 1952:
1. They grew some viruses on a medium containing radioactive Phosphorus (32P) to label viral DNA and another set on radioactive Sulfur (35S) to label viral Protein.
2. These radioactive phages were allowed to infect E. coli bacteria.
3. After infection, viral coats were removed by blending and separated from bacterial cells by centrifugation.
4. Bacteria infected with viruses having 32P were found radioactive, whereas bacteria infected with 35S showed no radioactivity.
This proved that DNA, and not protein, passed from the virus into the bacteria as genetic material.
Textbook Chapter End Exercise Questions
The Lac Operon consists of one regulatory gene (i gene) and three structural genes (z, y, and a):
• In the Absence of Inducer (Lactose): The repressor protein produced by the i gene binds tightly to the operator region of the operon and prevents RNA polymerase from transcribing the structural genes (Operon is OFF).
• In the Presence of Inducer (Lactose/Allolactose): Inducer binds to the repressor protein, inactivating it. The inactive repressor cannot bind to operator. RNA Polymerase binds to promoter and transcribes genes z, y, and a to produce β-galactosidase, permease, and transacetylase respectively (Operon is ON).
1. Triplet Codon: 61 codons code for amino acids while 3 codons (UAA, UAG, UGA) function as stop codons.
2. Unambiguous & Specific: One codon codes for only one specific amino acid.
3. Degenerate: Some amino acids are coded by more than one codon.
4. Contiguous Reading: Read without any punctuation marks in a continuous fashion.
5. Universal: A codon codes for the same amino acid across organisms (e.g., UUU codes for Phenylalanine from bacteria to human).
6. Dual Function: AUG acts as initiator codon and also codes for Methionine.
The step-by-step procedure of DNA Fingerprinting is as follows:
1. Isolation and extraction of DNA from blood, hair root, or skin cell samples.
2. Digestion of DNA by restriction endonucleases into smaller fragments.
3. Separation of DNA fragments by Gel Electrophoresis.
4. Southern Blotting: Transferring separated DNA fragments to synthetic membranes (Nitrocellulose/Nylon).
5. Hybridization using labeled VNTR probe.
6. Detection of hybridized DNA fragments by Autoradiography showing characteristic dark band patterns.
Read More & Chapter Resources
These short notes are designed for quick revision. Read the main points once, then use the related material below to practise the same chapter. Revising a concept and checking it with questions can make your preparation more focused.