Unit 14: DNA Structure, Replication, and Repair
The double helix as an information-carrying molecule: how it's copied with almost no errors, and what fixes the errors that slip through anyway.
7 topics · 27 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
The Double Helix and Base Pairing
- How it worksHow do hydrogen bonds and base-stacking hold two antiparallel DNA strands together in a helix?
- Common mistakeWhy doesn't DNA leave the nucleus to go do its job somewhere else in the cell?
- Going furtherWhat happens to the double helix's stability when its GC content is much higher than its AT content?
Your turn1 of 2
A student learns that proteins are built from instructions encoded in DNA and assumes DNA itself must travel out of the nucleus to the ribosomes to directly participate in building those proteins. Why doesn't DNA leave the nucleus to go do its job somewhere else in the cell?Semiconservative Replication
- How it worksWalk me through how the Meselson-Stahl experiment used density labeling to prove replication is semiconservative.
- Common mistakeWhy isn't conservative replication, where the original double helix stays fully intact, what actually happens?
- Ties togetherHow does semiconservative replication explain why each daughter cell after mitosis carries one original and one newly made DNA strand?
Your turn1 of 2
Before the Meselson-Stahl experiment, some scientists proposed that DNA replication might be conservative, meaning the original double helix stays completely intact while an entirely new double helix is built alongside it. A student assumes this conservative model turned out to be correct, since it seems like the simplest way to make sure the original information is preserved. Why isn't conservative replication what actually happens?The Replication Fork: Helicase, Primase, and Polymerase
- How it worksHow does helicase unwind the double helix while single-strand binding proteins keep the two strands apart?
- How it worksHow does primase lay down a short RNA primer that DNA polymerase then extends?
- Worked exampleFind how long it takes a single replication fork to copy a 100,000 base pair region of DNA moving at 1,000 bases per second.
- Common mistakeWhy can't DNA polymerase start building a new strand completely from scratch without a primer?
- Going furtherWhat happens to replication when a mutation disables the helicase enzyme at the fork?
Your turn1 of 2
Find how long it takes a single replication fork to copy a 186248 base pair region of DNA moving at 1473 bases per second.Leading and Lagging Strands and Okazaki Fragments
- How it worksHow does DNA polymerase's requirement to build only 5-prime to 3-prime force the lagging strand into fragments?
- How it worksWalk me through how DNA ligase seals the gaps between Okazaki fragments into one continuous lagging strand.
- Common mistakeWhy isn't the lagging strand somehow a lower-quality or less accurate copy than the leading strand?
- Worked exampleFind how many Okazaki fragments form on the lagging strand of a 20,000 base pair replication bubble if each fragment averages 200 bases long.
Your turn1 of 2
Find how many Okazaki fragments form on the lagging strand of a 43607 base pair replication bubble if each fragment averages 262 bases long.Proofreading and DNA Polymerase Fidelity
- How it worksHow does DNA polymerase's proofreading function detect and remove a mismatched nucleotide right after adding it?
- Worked exampleFind the final error rate after proofreading if DNA polymerase's initial error rate is 1 in 100,000 and proofreading catches 99.9 percent of those mistakes.
- Common mistakeWhy isn't a DNA polymerase's built-in proofreading the same mechanism as the separate mismatch repair system that runs after replication?
Your turn1 of 2
Find the final error rate after proofreading if DNA polymerase's initial error rate is 14 in 100,000 and proofreading catches 96.43 percent of those mistakes.Telomeres and the End-Replication Problem
- How it worksHow does the lagging strand's need for a primer leave the very end of a chromosome unable to be fully copied?
- How it worksHow does telomerase add repetitive DNA sequence back onto a chromosome's end to counteract that loss?
- Common mistakeWhy isn't telomere shortening by itself the direct cause of aging in every cell type?
- Ties togetherHow does telomerase's reactivation in cancer cells connect back to the checkpoint failures covered in the cancer unit?
Your turn1 of 2
A cell's telomeres shorten with each round of division, and short telomeres are associated with cellular aging. A student assumes telomere shortening must be the direct cause of aging in every single cell type in the body. Why isn't telomere shortening by itself the direct cause of aging in every cell type?DNA Repair: Mismatch, Excision, and Double-Strand Break
- How it worksHow does mismatch repair scan newly replicated DNA for a base-pairing error the original proofreading missed?
- How it worksHow does nucleotide excision repair cut out and replace a bulky lesion like a UV-induced thymine dimer?
- How it worksHow does a double-strand break get repaired either by matching it to a homologous sequence or by rejoining the broken ends directly?
- Common mistakeWhy isn't every mutation the result of a repair system failing, given that most DNA damage does get fixed?
- Going furtherWhat happens to a cell's mutation rate when a double-strand break repair gene like BRCA2 is lost?
Your turn1 of 2
The vast majority of DNA damage that occurs in a cell over its lifetime is successfully repaired by one of several dedicated repair systems. A student assumes this must mean every mutation that does end up appearing must be caused by one of those repair systems failing outright. Why isn't every mutation the result of a repair system failing, given that most DNA damage does get fixed?