Molecular Basis of Inheritance

1. Chapter at a glance

• DNA is the predominant genetic material in organisms; it is a polymer of deoxyribonucleotides whose length (in base pairs) is characteristic of each organism.
• DNA structure is a right-handed double helix with two antiparallel polynucleotide strands held by complementary base pairing (A–T by two H-bonds, G–C by three H-bonds) and base stacking.
• DNA replicates semiconservatively: each parental strand acts as template for a new complementary strand, producing two identical daughter molecules.
• Genetic information flows from DNA → RNA → Protein (Central dogma); in some viruses the flow is reversed.
• Only one strand (template strand) of a DNA segment is transcribed into RNA; the process is defined by promoter, structural gene and terminator.
• Genetic code is triplet, degenerate, nearly universal, commaless and has specific start (AUG) and stop codons (UAA, UAG, UGA).
• tRNA acts as adapter molecule; ribosomes (with rRNA) catalyse peptide-bond formation during translation.
• Gene expression is regulated (e.g., lac operon) at transcriptional level; DNA fingerprinting exploits VNTR polymorphism for identification.

2. Definitions and laws

• “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material” (Watson and Crick, 1953) — basis of semiconservative replication.
• Central dogma: genetic information flows from DNA → RNA → Protein.
• Semiconservative DNA replication: after replication each DNA molecule has one parental strand and one newly synthesised strand.
• Genetic code features (exactly as stated): codon is triplet; 61 codons code for amino acids and 3 function as stop codons; code is degenerate; codon is read in contiguous fashion (no punctuations); code is nearly universal; AUG codes for methionine and acts as initiator; UAA, UAG, UGA are stop/terminator codons.
• Nucleosome: negatively charged DNA wrapped around positively charged histone octamer; typical nucleosome contains 200 bp of DNA.
• Euchromatin: loosely packed, transcriptionally active chromatin (stains light); heterochromatin: densely packed, transcriptionally inactive (stains dark).
• Transcription unit: segment of DNA defined by promoter (5′/upstream), structural gene and terminator (3′/downstream).
• Template strand: strand with 3′→5′ polarity that is transcribed; coding strand: strand with 5′→3′ polarity (same sequence as RNA except T for U).
• VNTR: variable number of tandem repeats; satellite DNA that shows high polymorphism and forms the basis of DNA fingerprinting.

3. Important diagrams and activities

• Figure 5.1 Polynucleotide chain — shows 5′-phosphate and 3′-OH ends and sugar-phosphate backbone.
• Figure 5.2/5.3 Double-stranded DNA / Double helix — antiparallel strands, base pairing, right-handed helix, 3.4 nm pitch, 0.34 nm per bp.
• Figure 5.4 Nucleosome and “beads-on-string” chromatin — DNA wrapped on histone octamer; EM appearance of chromatin.
• Figure 5.5 Hershey-Chase experiment — radioactive ³²P (DNA) enters bacteria while ³⁵S (protein) does not.
• Figure 5.6/5.7 Watson-Crick semiconservative model and Meselson-Stahl experiment — density-gradient separation proving semiconservative replication.
• Figure 5.8 Replicating fork — continuous (leading) and discontinuous (lagging) strands, DNA ligase joining Okazaki fragments.
• Figure 5.9 Transcription unit — promoter, structural gene, terminator and template/coding strands.
• Figure 5.10/5.11 Transcription in bacteria and eukaryotes — initiation, elongation, termination; splicing, capping, tailing of hnRNA.
• Figure 5.12 tRNA (clover-leaf/inverted-L) — anticodon loop and amino-acid acceptor end.
• Figure 5.13 Translation — ribosome, charged tRNAs, peptide-bond formation.
• Figure 5.14 lac operon — i gene (repressor), z, y, a genes and regulation by lactose/allolactose.
• Figure 5.16 DNA fingerprinting (VNTR/Southern blot autoradiogram) — band pattern unique to an individual (except identical twins).

4. Common misconceptions and exam pitfalls

• Assuming both DNA strands are transcribed — only the template strand (3′→5′) is transcribed; coding strand is not.
• Confusing stability: DNA is chemically more stable than RNA because of absence of 2′-OH and presence of thymine; RNA is labile and catalytic.
• Thinking genetic code has punctuation or is read non-contiguously — it is read in a contiguous triplet manner.
• Mixing up exons/introns — exons appear in mature mRNA; introns are removed by splicing.
• Believing replication or transcription initiates randomly — both require specific origin/promoter sequences.
• Forgetting that lac operon is an example of negative regulation by repressor and is inducible by lactose.
• Writing incorrect base-pair distances or helix dimensions (must use 0.34 nm/bp, 3.4 nm pitch, ~10 bp/turn).
• In DNA fingerprinting, stating that polymorphism occurs in coding sequences — it is mainly in non-coding repetitive DNA (VNTR).

5. Formula sheet

No numerical formulas are defined in the chapter. The only quantitative relations given are structural parameters (pitch = 3.4 nm, ~10 bp/turn, 0.34 nm per base pair) and genome sizes (e.g., E. coli 4.6 × 10⁶ bp, human haploid 3.3 × 10⁹ bp). These are factual values, not formulas.