Molecular Basis of Inheritance - Quick Revision
DNA stores genetic information; RNA mostly transfers and expresses it. Both can act as genetic material, but DNA is more stable and is the better genetic material. RNA evolved first and DNA was derived from RNA (RNA world).
DNA structure (Watson and Crick, 1953)
- Built on X-ray data of Wilkins and Franklin and Chargaff's rule (A/T = G/C = 1).
- Two anti-parallel polynucleotide chains; sugar-phosphate backbone outside, bases inside.
- A=T (2 H-bonds), G(triple)C (3 H-bonds); a purine always pairs a pyrimidine.
- Right-handed helix; pitch 3.4 nm, ~10 bp/turn, 0.34 nm between bp.
- DNA first isolated by Meischer (1869, 'Nuclein'). Central dogma: DNA -> RNA -> Protein (reverse in some viruses).
Packaging
- Prokaryotes: DNA + positive proteins in a nucleoid (large loops).
- Eukaryotes: DNA wraps positively charged histone octamer (rich in lysine + arginine) to form a nucleosome (~200 bp), the repeating unit of chromatin (beads-on-string). Euchromatin = loose, light, active; heterochromatin = dense, dark, inactive.
Search for genetic material
- Griffith (1928): transforming principle (heat-killed S + live R -> mice die).
- Avery, MacLeod, McCarty: DNA alone transforms R to S (DNase blocks it).
- Hershey-Chase (1952): phages; 32P (DNA) enters bacteria, 35S (protein) does not -> DNA is genetic material.
Replication
- Semiconservative (Watson-Crick scheme); proved by Meselson-Stahl (1958) using 15N and CsCl density gradient; also Taylor on Vicia faba.
- Enzyme: DNA-dependent DNA polymerase (only 5'->3'); leading strand continuous, lagging discontinuous; fragments joined by DNA ligase. dNTPs provide energy.
Transcription
- Copies one strand of a segment into RNA (A pairs U). Unit = promoter + structural gene + terminator. Template strand (3'->5') is copied; coding strand same as RNA.
- Bacteria: single RNA polymerase + sigma (initiation) and rho (termination) factors; transcription and translation coupled.
- Eukaryotes: Pol I -> rRNA, Pol II -> hnRNA (mRNA), Pol III -> tRNA/5S rRNA/snRNA; hnRNA processed by capping (5' methyl guanosine), tailing (3' poly-A), splicing (introns out, exons joined).
Genetic code
- Triplet, degenerate, contiguous (no punctuation), nearly universal, non-overlapping. 64 codons; 61 code amino acids, 3 stop (UAA, UAG, UGA). AUG = Met + initiator. Gamow proposed triplet; Khorana, Nirenberg, Ochoa deciphered it.
- Insertion/deletion of 1-2 bases = frameshift; of 3 = adds/removes one amino acid, frame intact.
Translation
- Amino acids joined by peptide bonds on ribosomes. tRNA = adapter (anticodon + amino acid end); charging = aminoacylation (needs ATP). 23S rRNA = ribozyme. UTRs at both ends. Release factor at stop codon ends it.
Regulation: lac operon (Jacob & Monod)
- i (repressor), z (beta-galactosidase), y (permease), a (transacetylase). Lactose = inducer inactivates repressor -> transcription. Repressor binding = negative regulation.
HGP (1990-2003) and DNA fingerprinting
- ~3164.7 million bp, ~30,000 genes, <2% codes protein, 99.9% bases identical, ~1.4 million SNPs. Sequenced by Sanger method.
- DNA fingerprinting (Alec Jeffreys): polymorphism in repetitive VNTR (mini-satellite); identical only in monozygotic twins; used in forensics and paternity.