Molecular Basis Of Inheritance Class 12 Notes

Key Definitions

DNA (Deoxyribonucleic Acid)

A nucleic acid containing the genetic instructions used in the development and functioning of all known living organisms and many viruses.

RNA (Ribonucleic Acid)

A polymeric molecule essential in various biological roles in coding, decoding, regulation, and expression of genes.

Gene

A segment of DNA or RNA that codes for a specific polypeptide or RNA molecule, functioning as the unit of heredity.

Genome

The complete set of genetic material of an organism, including all of its genes and non-coding sequences.

Replication

The biological process of producing two identical replicas of DNA from one original DNA molecule.

Transcription

The process by which genetic information from DNA is copied into an RNA sequence, primarily mRNA.

Translation

The process by which information encoded in mRNA is used to synthesize a polypeptide chain (protein) on ribosomes.

Codon

A sequence of three nucleotides in an mRNA molecule that specifies a particular amino acid or termination signal during protein synthesis.

Operon

A unit of genetic function consisting of a regulated cluster of genes with a common function, found primarily in prokaryotes (e.g., Lac Operon).

DNA Fingerprinting

A technique used to identify individuals by characteristics of their DNA, specifically based on variations in Variable Number Tandem Repeats (VNTRs).

DNA as the Genetic Material: Experimental Proofs

Early scientific investigations were crucial in establishing DNA as the carrier of genetic information over proteins.

1. Griffith's Transforming Principle (1928): Frederick Griffith experimented with Streptococcus pneumoniae bacteria. He observed that heat-killed virulent (S-strain) bacteria, when mixed with live non-virulent (R-strain) bacteria, caused the R-strain to transform into virulent S-strain, leading to the death of mice. He concluded that some 'transforming principle' from the heat-killed S-strain was taken up by the R-strain, conferring virulence. The chemical nature of this principle remained unknown at this stage.

1. Avery, MacLeod, and McCarty's Experiment (1944): Oswald Avery, Colin MacLeod, and Maclyn McCarty biochemically characterized Griffith's 'transforming principle'. They purified biochemicals (proteins, DNA, RNA, etc.) from heat-killed S-cells to see which one could transform live R-cells into S-cells. They found that DNA alone was able to transform R-cells. Treatment with DNase (DNA-digesting enzyme) inhibited transformation, while RNase and protease did not. This suggested that DNA was the hereditary material, but not all scientists were convinced.

1. Hershey-Chase Experiment (1952): Alfred Hershey and Martha Chase provided unequivocal proof that DNA is the genetic material. They worked with bacteriophages (viruses that infect bacteria). They labelled the DNA of some phages with radioactive phosphorus-32 (³²P, as DNA contains phosphorus but protein does not) and the protein coat of others with radioactive sulfur-35 (³⁵S, as protein contains sulfur but DNA does not). These labelled phages were allowed to infect E. coli bacteria. After infection, they agitated the mixture in a blender to separate viral coats from the bacterial cells and then centrifuged it. They observed that bacteria infected with ³²P-labelled phages were radioactive, while those infected with ³⁵S-labelled phages were not. The ³²P (DNA) was found inside the bacterial cells, indicating it entered the cell and was passed on to progeny phages. This conclusively proved that DNA, not protein, is the genetic material transferred from virus to bacteria and carries genetic information.

DNA Replication: The Semi-Conservative Model

1. Origin of Replication and Unwinding — Replication begins at specific sites called origins of replication (ori). The enzyme DNA helicase unwinds the DNA double helix, separating the two strands by breaking hydrogen bonds. Single-strand binding proteins (SSBPs) bind to the separated strands to prevent them from re-annealing. DNA topoisomerase (gyrase in prokaryotes) relieves supercoiling tension ahead of the replication fork.
2. Primer Synthesis — DNA polymerase cannot initiate synthesis on its own. An enzyme called primase synthesizes short RNA primers (5-10 nucleotides) which provide a free 3'-OH group to which DNA polymerase can add nucleotides.
3. Elongation (DNA Polymerization) — DNA polymerase III (in prokaryotes, δ in eukaryotes) adds deoxyribonucleotides to the 3'-OH end of the growing DNA strand, always synthesizing in the 5' → 3' direction. Leading strand: Synthesized continuously towards the replication fork as the DNA unwinds. Lagging strand: Synthesized discontinuously away from the replication fork in short fragments called Okazaki fragments. Each Okazaki fragment requires a new RNA primer.
4. Primer Removal and Ligation — DNA polymerase I (in prokaryotes, RNase H and DNA Pol α/δ in eukaryotes) removes the RNA primers and fills the gaps with DNA nucleotides. Finally, DNA ligase joins the Okazaki fragments and other DNA segments by forming phosphodiester bonds, sealing the nicks.
5. Proofreading and Termination — DNA polymerases have a proofreading ability (3' → 5' exonuclease activity) to correct errors during replication. Replication terminates when replication forks meet or at specific termination sequences.

Key Features of the Genetic Code

• Triplet nature: Each amino acid is specified by a sequence of three nucleotides, called a codon.
• Degeneracy (Redundancy): More than one codon can code for the same amino acid (e.g., UUU and UUC both code for Phenylalanine).
• Unambiguous and Specific: One codon codes for only one specific amino acid.
• Non-overlapping: The code is read in a contiguous sequence of three bases without any overlap. (e.g., AUG CUC is read as AUG, then CUC, not GCU, etc.)
• Comma-less: There are no intervening nucleotides or pauses between codons.
• Universal: The genetic code is largely the same for all organisms, from bacteria to humans (with minor exceptions in mitochondria and some protozoans).
• Start Codon: AUG (Methionine) acts as the initiator codon in most organisms.
• Stop/Termination Codons: UAA, UAG, UGA do not code for any amino acid and signal the termination of protein synthesis.

Transcription vs. Translation

Regulation of Gene Expression: The Lac Operon

Gene expression is a multi-step process, and its regulation is crucial for an organism's survival and adaptability. In prokaryotes, gene regulation often occurs at the transcriptional level through systems like operons. The Lac Operon (Lactose Operon) in E. coli is a classic example, elucidated by Jacob and Monod. It controls the metabolism of lactose.

Components of the Lac Operon:

1. Structural Genes:

• _lacZ_: Codes for β-galactosidase, which hydrolyzes lactose into glucose and galactose.
• _lacY_: Codes for permease, which increases the permeability of the cell to lactose.
• _lacA_: Codes for transacetylase, whose function is not fully clear in lactose metabolism.

1. Operator (O) Region: A short DNA sequence adjacent to the promoter that acts as a binding site for the repressor protein.
2. Promoter (P) Region: The binding site for RNA polymerase.
3. Regulator (i) Gene: Located upstream of the operon, it codes for the repressor protein.

Regulation Mechanism:

• Absence of Lactose (Repressed State): The repressor protein synthesized by the _i_ gene is active. It binds to the operator region (O), physically blocking RNA polymerase from transcribing the structural genes (_lacZ, lacY, lacA_). Thus, enzymes for lactose metabolism are not produced when lactose is absent.
• Presence of Lactose (Induced State): Lactose acts as an inducer. A small amount of lactose is converted to allolactose, which binds to the repressor protein. This binding causes a conformational change in the repressor, inactivating it. The inactive repressor cannot bind to the operator. RNA polymerase can then freely bind to the promoter and transcribe the structural genes. This leads to the synthesis of β-galactosidase, permease, and transacetylase, allowing the cell to utilize lactose. This is an example of inducible regulation where the presence of a substrate induces gene expression.

Glucose Effect: Even in the presence of lactose, if glucose is available, E. coli prefers glucose. High glucose levels lead to low cAMP, which prevents cAMP-CAP complex formation, an activator that enhances RNA polymerase binding. Thus, the Lac Operon is also under positive control.

Exam Tip: Mastering Molecular Basis

For this chapter, focus on understanding the mechanisms rather than just memorizing facts. Be prepared to explain:

1. Experimental Proofs: Detailed accounts of Griffith's, Avery-MacLeod-McCarty's, and Hershey-Chase experiments, including their conclusions and significance.
2. DNA Replication: The semi-conservative nature, key enzymes (helicase, primase, DNA Pol I & III, ligase, topoisomerase), and their specific roles. Be ready to draw a labelled diagram of the replication fork.
3. Genetic Code: All characteristics (triplet, degenerate, universal, etc.) are frequently asked.
4. Lac Operon: Its structure (promoter, operator, structural genes, regulator gene) and mechanism of regulation in both the presence and absence of lactose. A clear diagram is often required.
5. Transcription and Translation: The steps involved and the roles of mRNA, tRNA, and ribosomes. Distinguish clearly between prokaryotic and eukaryotic processes where applicable.
6. Human Genome Project (HGP) & DNA Fingerprinting: Know the salient features and applications of each. Practice short answer questions on VNTRs and RFLP in DNA fingerprinting.

Practice Questions with Solutions

• Q: Why is DNA replication considered semi-conservative? A: Because each new DNA molecule formed consists of one original (parental) strand and one newly synthesized strand.
• Q: List any three features of the genetic code. A: Triplet nature, degenerate, unambiguous, non-overlapping, universal (any three).
• Q: What is the role of DNA ligase in DNA replication? A: DNA ligase joins the Okazaki fragments on the lagging strand by forming phosphodiester bonds, sealing the nicks.
• Q: What would happen to the lac operon if the regulator gene mutated such that it could not synthesize the repressor protein? A: The lac operon would be constitutively expressed (always on), meaning the structural genes would be transcribed even in the absence of lactose, as there would be no repressor to bind to the operator.

Frequently Asked Questions