In which direction does proofreading exonuclease act, and why?
How do bacteria vs eukaryotes tell the new strand from the old?
What does a glycosylase do in BER?
Why can DNA be repaired but single-stranded RNA usually can't?
Recall Feynman: explain to a 12-year-old
Imagine copying a huge book by hand. Layer 1: you carefully choose each letter (right letters fit). Layer 2: as you write, you instantly notice and erase a typo with a built-in eraser — that's proofreading. Layer 3: afterward, an editor reads your copy and compares it to the original to catch leftover typos — that's mismatch repair. The editor knows which page is the original because the original has a tiny stamp (a methyl mark) and your fresh copy doesn't. And since the book has a matching mirror-page next to every page, even if one page tears, you can rebuild it from its partner. That partner trick is why DNA is so repairable.
DNA Replication — proofreading happens during this process.
DNA Polymerase — carries both 5′→3′ synthesis and 3′→5′ exonuclease.
DNA Structure - Antiparallel Strands — why directions and templating matter.
Mutation and Cancer — what surviving errors and broken repair genes cause.
DNA Methylation — strand-discrimination signal in bacteria.
Okazaki Fragments — their nicks flag the new strand in eukaryotes.
What three mechanisms together give DNA its ~10⁻⁹ error rate?
Base selection by polymerase, proofreading (3′→5′ exonuclease), and post-replication mismatch repair.
In which direction does the proofreading exonuclease act and why?
3′→5′ — opposite to synthesis — because the just-added wrong base sits at the new strand's 3′ end, so the enzyme must back over it.
Why do independent fidelity layers give such low error rates?
Their error fractions multiply (e.g. 10⁻⁵ × 10⁻² × 10⁻² = 10⁻⁹), so mediocre steps combine into near-perfection.
What is the core problem mismatch repair must solve, beyond spotting the mismatch?
Strand discrimination — knowing which strand (new, wrong) to fix versus which (old, correct) to keep as template.
How do bacteria identify the newly made strand for MMR?
The old strand is methylated (Dam methylase at GATC); the new strand is transiently unmethylated, so MMR targets the unmethylated strand.
How do eukaryotes identify the new strand for MMR?
By nicks/discontinuities (e.g. between Okazaki fragments) present in the new strand.
What does Nucleotide Excision Repair (NER) fix and how?
Bulky helix-distorting lesions like UV thymine dimers; it cuts the strand on both sides of the lesion, removes a patch, then resynthesises and ligates.
What does Base Excision Repair (BER) fix and what enzyme starts it?
Single damaged/altered bases (e.g. uracil from deaminated cytosine); a DNA glycosylase removes just the faulty base.
What is the universal logic of all DNA repair?
Recognise the damage → excise the bad strand portion → resynthesise from the good complementary strand → ligate.
Why is double-strandedness essential for repair?
One strand is a backup copy of the other, so the correct sequence can be rebuilt; single-stranded molecules lack this template.
Distinguish proofreading from mismatch repair.
Proofreading: by polymerase, during synthesis, at the 3′ end (3′→5′ exo). MMR: separate enzymes, after synthesis, requiring strand discrimination.
What disease results from defective mismatch repair genes?
Lynch syndrome (hereditary nonpolyposis colorectal cancer) and increased cancer risk generally.
Dekho, DNA copy karte waqt polymerase bilkul perfect nahi hota — par final galti rate sirf 1 in a billion (10⁻⁹) hoti hai. Yeh magic teen layers ki wajah se hota hai. Pehla: base selection — polymerase sirf sahi Watson-Crick pair (A-T, G-C) add karta hai. Dusra: proofreading — synthesis ke time hi, agar galat base add ho gaya, polymerase ka apna 3′→5′ exonuclease us galat base ko turant erase kar deta hai. Yaad rakho: building 5′→3′ hoti hai, isliye galti hamesha naye strand ke 3′ end pe hoti hai, isliye proofreading ulti direction (3′→5′) mein back-up karta hai.
Teesra layer hai mismatch repair (MMR) — yeh replication ke baad alag enzymes karte hain. Sabse tricky cheez: kaunsa strand galat hai? Purana (template) strand sahi hai, naya galat. Bacteria yeh methylation se pehchante hain (purana strand methylated, naya abhi tak nahi). Eukaryotes nicks (gaps) se pehchante hain. Phir naye strand ka galat patch nikaal kar, purane strand ko template bana ke sahi se dobara bana dete hain.
Iske alawa NER (bulky damage jaise UV thymine dimers fix karta hai) aur BER (ek single bad base, jaise uracil, glycosylase se nikaalta hai) bhi hote hain. Har repair ka same funda hai: pehchaano → galat strand kaato → achhe strand se dobara banao → ligate karo. Yeh sab isliye possible hai kyunki DNA double-stranded hai — ek strand hamesha doosre ka backup copy hota hai.
Yeh matter kyun karta hai? Agar yeh systems fail ho jayen (jaise MMR genes kharab ho — Lynch syndrome), to mutations jama hote hain aur cancer ho sakta hai. Toh fidelity layers sirf biology theory nahi, real medicine hai.
