MOLECULAR BASIS - Class 12 Maharashtra Board Science

Molecular Basis of Inheritance

The molecular basis of inheritance explains how genetic information is stored, transmitted, and expressed in living organisms. It focuses on the structure, function, and regulation of DNA, the genetic material, and the processes by which it is replicated and used to synthesize proteins. This topic covers the discovery of DNA as genetic material, its structure, replication, gene expression, and modern applications like DNA fingerprinting.

Discovery and Proof of DNA as Genetic Material

The identification of DNA as the genetic material involved a series of experiments by various scientists. Early work by Mendel established the…

Molecular Basis of Inheritance

The molecular basis of inheritance explains how genetic information is stored, transmitted, and expressed in living organisms. It focuses on the structure, function, and regulation of DNA, the genetic material, and the processes by which it is replicated and used to synthesize proteins. This topic covers the discovery of DNA as genetic material, its structure, replication, gene expression, and modern applications like DNA fingerprinting.

Discovery and Proof of DNA as Genetic Material

The identification of DNA as the genetic material involved a series of experiments by various scientists. Early work by Mendel established the concept of hereditary factors, but the chemical nature of these factors was unknown.

• Frederick Miescher (1869): Isolated "nuclein" from pus cells, later identified as nucleic acid.
• Griffith's Experiment (1928): Demonstrated the "transforming principle" in Streptococcus pneumoniae, showing that non-virulent bacteria could become virulent when mixed with heat-killed virulent bacteria.
• Avery, MacLeod, McCarty (1944): Identified DNA as the transforming principle by using enzymes to selectively destroy proteins, RNA, or DNA.
• Hershey and Chase (1952): Used radioactive labeling of DNA (phosphorus) and protein (sulfur) in bacteriophages to confirm that DNA, not protein, is the genetic material.

Structure and Properties of DNA

DNA is a double-helical molecule composed of two antiparallel strands. Its structure was elucidated by Watson and Crick, with key contributions from Rosalind Franklin and Maurice Wilkins.

• Double Helix: Two strands coiled around each other, with a sugar-phosphate backbone and nitrogenous bases facing inward.
• Antiparallel Strands: One strand runs 5' to 3', the other 3' to 5'.
• Base Pairing: Adenine pairs with thymine (2 hydrogen bonds), guanine pairs with cytosine (3 hydrogen bonds).
• Chargaff's Rule: Amount of purines (A+G) equals pyrimidines (T+C) in double-stranded DNA.
• Dimensions: Distance between two nucleotides is 3.4 Å; one complete turn of the helix contains 10 base pairs and measures 34 Å; diameter is 20 Å.
• Major and Minor Grooves: Alternating grooves along the helix.

DNA Packaging in Prokaryotes and Eukaryotes

DNA must be efficiently packed within the cell. The packaging differs between prokaryotes and eukaryotes.

• Prokaryotes: DNA is circular, supercoiled, and associated with non-histone proteins. Supercoiling reduces the length from hundreds of microns to a few microns.
• Eukaryotes: DNA is linear and wrapped around histone proteins to form nucleosomes (beads-on-a-string structure). Multiple levels of coiling lead to chromatin and, during cell division, chromosomes.
• Nucleosome: Consists of DNA wrapped around a histone octamer; linker DNA connects nucleosomes.
• Euchromatin vs. Heterochromatin:
  • Euchromatin: Loosely packed, transcriptionally active, contains genes.
  • Heterochromatin: Densely packed, transcriptionally inactive, gene-poor.

DNA Replication: Mechanism and Enzymes

DNA replication is the process by which DNA makes an exact copy of itself before cell division. It is semi-conservative, as shown by the Meselson-Stahl experiment.

• Semi-Conservative Replication: Each new DNA molecule consists of one parental and one newly synthesized strand.
• Key Steps:
  • Origin of Replication: Specific sites where replication begins (single in prokaryotes, multiple in eukaryotes).
  • Unwinding: Helicase unwinds the double helix; single-strand binding proteins stabilize separated strands.
  • Primer Synthesis: Primase synthesizes short RNA primers.
  • Elongation: DNA polymerase adds nucleotides in the 5' to 3' direction; leading strand synthesized continuously, lagging strand in Okazaki fragments.
  • Primer Removal and Ligation: DNA polymerase removes RNA primers; DNA ligase joins Okazaki fragments.
  • Topoisomerase/Gyrase: Relieves supercoiling ahead of the replication fork.

Central Dogma and Gene Expression

The central dogma describes the flow of genetic information from DNA to RNA to protein. Gene expression involves transcription and translation.

• Transcription: Synthesis of RNA from a DNA template. Only one DNA strand (template strand) is used.
  • Promoter: Region where RNA polymerase binds to initiate transcription (contains TATA box).
  • Structural Gene: Coding region transcribed into RNA.
  • Terminator: Sequence signaling the end of transcription.
  • RNA Processing (Eukaryotes): hnRNA undergoes capping (5' methyl guanosine), splicing (removal of introns), and polyadenylation (poly-A tail at 3' end) to form mature mRNA.
• Translation: Synthesis of protein from mRNA on ribosomes in the cytoplasm.
  • mRNA: Carries codons specifying amino acids.
  • tRNA: Adapter molecule with anticodon and amino acid attachment site.
  • Ribosome: Has A (aminoacyl), P (peptidyl), and E (exit) sites for tRNA binding.
  • Initiation: mRNA binds to small ribosomal subunit; initiator tRNA binds to start codon (AUG, methionine).
  • Elongation: tRNAs bring amino acids; peptide bonds form; ribosome translocates.
  • Termination: Stop codon (UAA, UAG, UGA) is reached; release factors promote polypeptide release.

Genetic Code: Features and Decoding

The genetic code is the set of rules by which nucleotide sequences are translated into amino acid sequences.

• Triplet Code: Each codon consists of three nucleotides.
• Non-Overlapping and Commaless: Codons are read sequentially without overlap or punctuation.
• Degeneracy: More than one codon can specify the same amino acid (e.g., leucine has six codons).
• Universality: The code is nearly universal across organisms.
• Start Codon: AUG (methionine) signals initiation.
• Stop Codons: UAA, UAG, UGA signal termination.
• Anticodon: Sequence on tRNA complementary to mRNA codon, determines amino acid attachment.

Mutation: Types and Effects

Mutations are sudden, heritable changes in the genetic material. They can alter protein structure and function.

• Point Mutation: Change or replacement of a single nucleotide (e.g., sickle cell anemia due to substitution in β-globin gene).
• Frameshift Mutation: Addition or deletion of a nucleotide shifts the reading frame, altering downstream amino acid sequence.
• Effects: Can result in nonfunctional proteins or altered traits.

Regulation of Gene Expression: Lac Operon

Gene expression in prokaryotes is regulated at the transcriptional level, exemplified by the lac operon in E. coli.

• Lac Operon Structure: Consists of regulatory gene (i), promoter, operator, and structural genes (lacZ, lacY, lacA).
• In Absence of Lactose: Repressor binds operator, blocking RNA polymerase; no transcription of structural genes.
• In Presence of Lactose: Lactose (inducer) binds repressor, inactivating it; RNA polymerase transcribes structural genes, producing enzymes for lactose metabolism.
• Negative Feedback: Operon is usually off; turned on only when lactose is present.

Human Genome Project and DNA Fingerprinting

The Human Genome Project (HGP) aimed to sequence the entire human genome and map all genes.

• Objectives: Sequence and map all human DNA, store and interpret data, develop bioinformatics tools, and address ethical, legal, and social issues.
• Findings: Human genome has about 3 billion base pairs and ~33,000 genes.
• DNA Fingerprinting: Technique to identify individuals based on unique patterns of variable number tandem repeats (VNTRs) in DNA.
  • Steps:
    1. Isolation of DNA from cells.
    2. Digestion with restriction enzymes.
    3. Separation by gel electrophoresis.
    4. Transfer to nylon/nitrocellulose membrane (Southern blotting).
    5. Hybridization with radioactive probes.
    6. Detection of unique banding patterns by autoradiography.
• Applications: Forensic science, paternity testing, identification of individuals.

Key Exam Takeaways

• DNA is the genetic material, proven by transformation and bacteriophage experiments.
• DNA structure: double helix, antiparallel, base pairing, Chargaff’s rule.
• Replication is semi-conservative, involves multiple enzymes, and is highly regulated.
• Central dogma: DNA → RNA → Protein; exceptions exist in retroviruses (reverse transcription).
• Genetic code: triplet, non-overlapping, degenerate, universal, with defined start/stop codons.
• Mutations can be point or frameshift, affecting protein synthesis.
• Gene expression is regulated by operons in prokaryotes; lac operon is inducible.
• Human Genome Project mapped the entire human genome; DNA fingerprinting uses VNTRs for identification.