Molecular Basis of Inheritance Class 12 Notes

• Chargaff's rule: In any species, A = T and G = C; therefore (A + G)/(T + C) = 1. Also, (A + T)/(G + C) varies between species but is constant within a species.
• Each turn of the helix spans 3.4 nm and contains 10 base pairs; the distance between adjacent base pairs is 0.34 nm.
• The diameter of the helix is 2 nm. The two strands are covalently bonded within each strand by phosphodiester bonds.
• Minor and major grooves in the helix serve as binding sites for regulatory proteins.

• Griffith's experiment (1928): Injected mice with live non-virulent (R) and heat-killed virulent (S) Streptococcus pneumoniae. Mice died — the R-strain was 'transformed' by some 'transforming principle' from the dead S-strain.
• Avery, MacLeod and McCarty (1944): Showed that the transforming principle was DNA — they purified components and treated with DNase, RNase and protease; only DNase destroyed transforming activity.
• Hershey-Chase experiment (1952): Used T2 bacteriophage labelled with ³⁵S (protein) and ³²P (DNA). After infection, only ³²P entered the bacteria and produced new phages, confirming DNA is the genetic material.
• Tobacco Mosaic Virus (TMV) — RNA is the genetic material in some viruses. In most organisms, DNA is the genetic material.

• DNA polymerase III catalyzes the addition of nucleotides in the 5'→3' direction, using the template strand (read 3'→5').
• The leading strand is synthesized continuously; the lagging strand is synthesized in short Okazaki fragments (each with an RNA primer) that are later joined by DNA ligase.
• DNA polymerase has proofreading (3'→5' exonuclease) activity, giving an error rate of about 10⁻⁹ per base pair per replication.
• Replication begins at the origin of replication (oriC in E. coli); eukaryotes have multiple origins per chromosome.

Post-Transcriptional Processing (Eukaryotes)

• 5' capping: A methylated guanine (m⁷G) is added to the 5' end — protects mRNA and helps ribosome binding.
• 3' polyadenylation: A poly-A tail (100–200 adenines) is added to the 3' end — stabilizes mRNA and aids export from the nucleus.
• Splicing: Introns (non-coding) are removed and exons (coding) are joined by the spliceosome. Alternative splicing allows one gene to code for multiple proteins.

• Degenerate (redundant): Most amino acids are coded by more than one codon (e.g. leucine has 6 codons). The third base ('wobble' position) often varies without changing the amino acid.
• Universal: The same codons code for the same amino acids in almost all organisms — from bacteria to humans (with minor exceptions in mitochondria).
• Non-overlapping and comma-free: The code is read sequentially from a fixed starting point without gaps or overlaps.

Lac Operon Model (Jacob and Monod, 1961)

The lac operon is a model for gene regulation in prokaryotes (E. coli) that controls the metabolism of lactose. It consists of regulatory genes (lacI), an operator (lacO), a promoter (lacP), and three structural genes (lacZ, lacY, lacA) that code for β-galactosidase, permease and transacetylase.

• In the absence of lactose: the repressor protein (from lacI) binds to the operator and blocks RNA polymerase → structural genes are NOT transcribed (operon is OFF).
• In the presence of lactose (inducer): lactose (allolactose) binds to the repressor, inactivating it → repressor cannot bind the operator → RNA polymerase transcribes lacZYA → lactose is metabolized (operon is ON).
• When glucose is present and lactose is absent: the operon is OFF regardless. When glucose is absent and lactose is present: the operon is ON at high levels (cAMP-CAP complex enhances transcription).

Human Genome Project (HGP)

• A mega-project launched in 1990 and completed in 2003, coordinated by the US Department of Energy and NIH (also called 'Moonshot of Biology').
• Sequenced approximately 3.2 × 10⁹ base pairs of the human genome.
• Key findings: humans have approximately 20,500 protein-coding genes (far fewer than the initially estimated 100,000); less than 2% of the genome codes for proteins.
• Applications: understanding genetic diseases, pharmacogenomics, forensic identification, evolutionary biology.
• Milestone technologies: chain-termination (Sanger) method, sequence tagged sites (STS), expressed sequence tags (ESTs).

DNA Fingerprinting

DNA fingerprinting (developed by Alec Jeffreys, 1984) is a technique to identify individuals based on their unique DNA profile. It uses VNTRs (Variable Number Tandem Repeats) or STRs (Short Tandem Repeats) — regions where the number of repeated sequences varies between individuals. These are non-coding, highly polymorphic regions.

• Process: DNA extracted → cut with restriction enzymes (RFLP method) → separated by gel electrophoresis → transferred to nylon membrane (Southern blotting) → hybridized with radioactive VNTR probes → autoradiography produces a unique banding pattern.
• Applications: forensic identification, paternity testing, immigration cases, identifying bodies in mass disasters, plant and animal breeding.
• VNTR analysis is so specific that it can distinguish between identical twins in some cases when somatic mutations differ.

Example: In the Hershey-Chase experiment, why was ³²P used to label DNA and ³⁵S to label protein? Could the isotopes be swapped? Explain.

Solution: ³²P labels DNA because DNA contains phosphorus in its sugar-phosphate backbone but protein does not. ³⁵S labels protein because protein contains sulfur in amino acids like cysteine and methionine but DNA does not. The isotopes cannot be swapped — ³⁵P would label both DNA and protein (since both contain phosphorus in some amino acids), and ³²S would label only protein but is less specific. The key was choosing isotopes unique to each molecule.

Example: Explain why the lac operon is ON only when lactose is present AND glucose is absent. What would happen if both were present?

Solution: The lac operon requires two conditions: (1) Lactose must be present to inactivate the repressor (allolactose binds the repressor → operator is free → RNA polymerase can bind). (2) Glucose must be absent so that cAMP levels rise, and the cAMP-CAP complex binds to the promoter to enhance transcription. If both glucose and lactose are present, the repressor is removed, but without cAMP-CAP activation, transcription occurs only at very low (basal) levels. E. coli preferentially uses glucose (a more efficient energy source) via catabolite repression.

Exercises (NCERT)

1. 1
   Group the following as nitrogenous bases and nucleosides: adenosine, cytidine, thymine, guanosine, uracil, cytosine.
2. 2
   If a double-stranded DNA has 20% cytosine, what is the percentage of adenine?
3. 3
   What is the difference between a nucleotide and a nucleoside?
4. 4
   Describe the Watson-Crick model of DNA.
5. 5
   Who proved that DNA is the genetic material? Describe their experiment.
6. 6
   What is semi-conservative replication? How was it proved?
7. 7
   Why is DNA a better hereditary material than RNA?
8. 8
   Describe the process of transcription in prokaryotes.
9. 9
   What is the difference between the template strand and the coding strand?
10. 10
    Describe the salient features of the genetic code.
11. 11
    What is the central dogma?
12. 12
    Describe the process of translation.
13. 13
    What is the lac operon? How is it regulated?
14. 14
    Explain the role of the lac operon in E. coli.
15. 15
    What is the Human Genome Project? Mention its goals.
16. 16
    What is DNA fingerprinting? Mention its applications.
17. 17
    What is the difference between repetitive and satellite DNA?
18. 18
    What are the salient features of the human genome?

Answers at a glance

1. 1
   Nitrogenous bases: thymine, uracil, cytosine. Nucleosides: adenosine, cytidine, guanosine.
2. 2
   Adenine is 30%. (C = G = 20%, so A + T = 60%, hence A = T = 30%.)
3. 3
   A nucleoside has a base + sugar; a nucleotide has a base + sugar + phosphate group.
4. 4
   The Watson-Crick model: DNA is a double helix of two antiparallel polynucleotide chains, with bases on the inside paired by hydrogen bonds (A=T, G≡C), sugar-phosphate backbones on the outside, and one complete turn every 3.4 nm with 10 base pairs.
5. 5
   Avery, MacLeod and McCarty (building on Griffith's transformation experiment) showed that DNA is the transforming principle and hence the genetic material.
6. 6
   Semi-conservative replication produces two DNA molecules, each with one old and one new strand; proved by the Meselson-Stahl experiment using 15N.
7. 7
   DNA is more stable, has thymine instead of uracil, and its double-stranded structure allows faithful replication and repair; RNA is less stable and catalytic.
8. 8
   In prokaryotes, RNA polymerase with the sigma factor binds the promoter and synthesises mRNA using the template strand until the terminator.
9. 9
   The template strand (3'→5') is copied; the coding strand (5'→3') has the same sequence as the mRNA except T is replaced by U.
10. 10
    The genetic code is triplet, degenerate, non-overlapping, comma-less, unambiguous and nearly universal; AUG is the start codon and UAA/UAG/UGA are stop codons.
11. 11
    The central dogma: DNA → RNA → protein.
12. 12
    Translation: the ribosome reads mRNA codons; tRNAs bring amino acids; peptide bonds form as the ribosome moves along the mRNA.
13. 13
    The lac operon is a set of genes (z, y, a) with a promoter and operator; it is regulated by the repressor (i gene) and induced by lactose/allolactose.
14. 14
    In E. coli, the lac operon is switched on in the presence of lactose when glucose is scarce.
15. 15
    The Human Genome Project (1990–2003) mapped and sequenced the entire human genome; goals included identifying all genes and storing the information in databases.
16. 16
    DNA fingerprinting identifies individuals from variable repetitive DNA; it is used in forensics, paternity testing and identification.
17. 17
    Repetitive DNA is present in many copies; satellite DNA shows a high degree of polymorphism and is used in DNA fingerprinting.
18. 18
    The human genome has about 3.1 billion base pairs, ~30,000 genes, and only about 2% codes for proteins.

MCQ Questions

2 Mark Questions

1. 1
   Differentiate between a nucleotide and a nucleoside.
2. 2
   State Chargaff's rule.
3. 3
   Why is DNA a better hereditary material than RNA?
4. 4
   What are stop codons? Give examples.
5. 5
   What is the central dogma?

3 Mark Questions

1. 1
   Describe the Watson-Crick model of DNA.
2. 2
   Describe the process of transcription in prokaryotes.
3. 3
   Explain the salient features of the genetic code.
4. 4
   Describe the process of translation.
5. 5
   What is DNA fingerprinting? Mention its applications.

5 Mark Questions

1. 1
   Describe the process of DNA replication.
2. 2
   Explain the lac operon and its regulation.
3. 3
   Describe the Human Genome Project and its salient features.