Molecular Basis of Inheritance — Revision Notes

⚡ 30-Second Revision

DNA Structure: — Double helix, antiparallel, A=T (2 H-bonds), G≡C (3 H-bonds), $3.4,\text{nm}$ pitch, $2,\text{nm}$ diameter.
Chargaff's Rules: — A=T, G=C, A+G = T+C.
DNA Replication: — Semi-conservative (Meselson & Stahl), 5'→3' synthesis.
Key Enzymes: — Helicase (unwinds), Primase (RNA primer), DNA Pol III (synthesis), DNA Pol I (primer removal, gap fill), Ligase (joins fragments).
Central Dogma: — DNA → RNA → Protein (exceptions: reverse transcription).
Transcription: — DNA to RNA. RNA Pol (prokaryotes), RNA Pol I, II, III (eukaryotes).
Eukaryotic mRNA Processing: — Capping (5'-7mG), Tailing (3'-polyA), Splicing (intron removal, exon ligation).
Genetic Code: — Triplet, Degenerate, Unambiguous, Universal, Non-overlapping, Comma-less. Start: AUG (Met). Stop: UAA, UAG, UGA.
Translation: — mRNA to protein. Ribosomes, tRNA (adaptor, anticodon, amino acid).
Lac Operon: — Inducible. Repressor (from *i* gene) binds operator. Allolactose (inducer) inactivates repressor. Glucose causes catabolite repression (low cAMP, low CAP binding).
HGP: — Goals, 3.16 billion bp, ~20-25k genes, <2% coding, >50% repetitive DNA, SNPs.
DNA Fingerprinting: — VNTRs, Restriction enzymes, Gel electrophoresis, Southern blotting, Probes, Autoradiography.

2-Minute Revision

The Molecular Basis of Inheritance revolves around DNA, the genetic blueprint, and its expression. DNA's double helix structure, with specific base pairing (A-T, G-C), allows for its faithful replication, a semi-conservative process proven by Meselson and Stahl.

Key enzymes like helicase, DNA polymerase, and ligase orchestrate this copying. The Central Dogma outlines information flow: DNA is transcribed into RNA, which is then translated into protein. Transcription, catalyzed by RNA polymerase, creates mRNA from a DNA template.

In eukaryotes, this mRNA undergoes crucial processing: 5' capping, 3' polyadenylation, and splicing to remove non-coding introns. The genetic code, a triplet and degenerate but unambiguous set of rules, dictates which amino acid each mRNA codon specifies, with AUG as the start and UAA/UAG/UGA as stop signals.

Translation occurs on ribosomes, where tRNA molecules act as adaptors, bringing specific amino acids to the mRNA codons. Gene expression is tightly regulated, as exemplified by the Lac Operon, an inducible system controlled by a repressor and an inducer (allolactose), and also influenced by glucose levels.

Landmark projects like the Human Genome Project have elucidated the entire human genetic sequence, while DNA fingerprinting utilizes unique repetitive DNA sequences (VNTRs) for individual identification.

5-Minute Revision

The journey of genetic information begins with DNA, a double-helical molecule whose structure, elucidated by Watson and Crick, explains its function. It's composed of nucleotides, with adenine (A) pairing with thymine (T) via two hydrogen bonds, and guanine (G) with cytosine (C) via three.

Chargaff's rules (A=T, G=C) are fundamental here. DNA replication is the process of making identical copies, essential for cell division. It's semi-conservative, meaning each new DNA molecule has one original and one new strand, a fact elegantly demonstrated by Meselson and Stahl.

This process involves unwinding by helicase, primer synthesis by primase, DNA synthesis by DNA polymerase (continuously on the leading strand, discontinuously in Okazaki fragments on the lagging strand), and fragment joining by DNA ligase.

The Central Dogma describes how this information flows: DNA to RNA (transcription), and RNA to protein (translation). Transcription, mediated by RNA polymerase, synthesizes an RNA molecule from a DNA template.

In eukaryotes, the initial RNA transcript (hnRNA) undergoes post-transcriptional modifications: a 7-methylguanosine cap at the 5' end, a poly-A tail at the 3' end, and splicing to remove non-coding introns, leaving only exons to form mature mRNA.

The genetic code, a triplet code, is the language that translates mRNA into protein. It's degenerate (multiple codons for one amino acid) but unambiguous (one codon for one amino acid), universal, non-overlapping, and comma-less.

AUG is the start codon, and UAA, UAG, UGA are stop codons.

Translation, or protein synthesis, occurs on ribosomes. It involves tRNA molecules, which act as adaptors, each carrying a specific amino acid and possessing an anticodon that pairs with an mRNA codon.

The process has three stages: initiation (ribosome assembles, initiator tRNA binds AUG), elongation (amino acids are added sequentially, peptide bonds form), and termination (stop codon reached, release factors bind, polypeptide released).

Gene regulation, crucial for cellular function, is exemplified by the Lac Operon in *E. coli*. This inducible operon is normally 'off' due to a repressor binding to the operator. Lactose, converted to allolactose, acts as an inducer, inactivating the repressor and allowing transcription.

Glucose also negatively regulates the operon (catabolite repression).

Finally, modern molecular biology has yielded powerful technologies. The Human Genome Project (HGP) sequenced the entire human genome, revealing ~20,000-25,000 genes, with less than 2% coding for proteins and a large portion being repetitive DNA. DNA Fingerprinting, based on the unique patterns of Variable Number Tandem Repeats (VNTRs) in an individual's DNA, is used in forensics and paternity testing. Understanding these concepts and their interconnections is vital for NEET.

Prelims Revision Notes

1
DNA Structure: — Watson & Crick model: double helix, antiparallel strands (5'->3' and 3'->5'). Sugar-phosphate backbone, nitrogenous bases inside. A pairs with T (2 H-bonds), G pairs with C (3 H-bonds). Helix pitch $3.4,\text{nm}$, $10,\text{bp}$ per turn, diameter $2,\text{nm}$.
2
Chargaff's Rules: — In dsDNA, A=T and G=C. Also, A+G = T+C. (A+T)/(G+C) ratio varies among species.
3
Packaging of DNA: — Prokaryotes: nucleoid, supercoiling. Eukaryotes: DNA wrapped around histone octamers (nucleosome), forms chromatin, further condensed into chromosomes.
4
Search for Genetic Material:

* Griffith (1928): Transformation in *Streptococcus pneumoniae*. 'Transforming principle'. * Avery, MacLeod, McCarty (1944): DNA is the transforming principle (DNase inhibited transformation). * Hershey & Chase (1952): DNA is genetic material (using $^{32}\text{P}$ for DNA, $^{35}\text{S}$ for protein in bacteriophage).

1
DNA Replication: — Semi-conservative (Meselson & Stahl experiment using $^{15}\text{N}$ and $^{14}\text{N}$). Occurs in S-phase. Enzymes:

* Helicase: Unwinds DNA. * SSB proteins: Stabilize separated strands. * Primase: Synthesizes RNA primer. * DNA Polymerase III (prokaryotes): Main elongating enzyme, 5'→3' synthesis, proofreading. * DNA Polymerase I (prokaryotes): Removes RNA primers, fills gaps. * DNA Ligase: Joins Okazaki fragments (lagging strand). * Topoisomerase/Gyrase: Relieves supercoiling.

1
Central Dogma: — DNA → RNA → Protein. (Reverse transcription: RNA → DNA, e.g., retroviruses).
2
Transcription: — DNA to RNA. Only one strand (template/antisense) used. RNA Polymerase binds promoter, synthesizes RNA, terminates at terminator sequence.

* Prokaryotes: Single RNA Pol for all RNA types. No splicing. * Eukaryotes: RNA Pol I (rRNA), RNA Pol II (mRNA, snRNA), RNA Pol III (tRNA, 5S rRNA). Pre-mRNA processing: 5' capping (7-methylguanosine), 3' polyadenylation (poly-A tail), Splicing (intron removal, exon ligation by spliceosome).

1
Genetic Code: — Triplet codons. 64 codons (61 for amino acids, 3 stop). Start codon: AUG (Methionine). Stop codons: UAA, UAG, UGA. Properties: Degenerate, Unambiguous, Universal, Non-overlapping, Comma-less.
2
Translation: — mRNA to protein on ribosomes.

* tRNA: Adaptor molecule, carries specific amino acid, has anticodon. * Ribosomes: Sites of protein synthesis (rRNA + proteins). Peptidyl transferase (rRNA enzyme) forms peptide bonds. * Steps: Aminoacylation of tRNA, Initiation, Elongation, Termination.

1
Gene Regulation (Lac Operon): — Inducible system in *E. coli*.

* Structural genes: *lacZ* ($\beta$-galactosidase), *lacY* (permease), *lacA* (transacetylase). * **Regulator (*i*) gene:** Produces repressor. Repressor binds operator in absence of lactose, blocking transcription. * Inducer: Allolactose (isomer of lactose) binds repressor, inactivating it, allowing transcription. * Catabolite repression: Glucose presence inhibits Lac Operon (low cAMP, low CAP binding).

1
Human Genome Project (HGP): — Goals, methods (BACs, YACs, sequencing), salient features (3.16 billion bp, ~20-25k genes, <2% coding, >50% repetitive, SNPs, Chromosome 1 most genes, Y least).
2
DNA Fingerprinting: — Principle: VNTRs (Variable Number Tandem Repeats) are highly polymorphic. Steps: DNA isolation, Restriction digestion, Gel electrophoresis, Southern blotting, Hybridization with labeled VNTR probe, Autoradiography. Applications: Forensics, paternity testing.

Vyyuha Quick Recall

For the properties of the Genetic Code: 'T-D-U-N-C'

Triplet: Three bases per codon.
Degenerate: Multiple codons for one amino acid.
Unambiguous: One codon for only one amino acid.
Non-overlapping: Codons read sequentially, no shared bases.
Comma-less: No intervening nucleotides between codons.