Unit 9: DNA, Replication, Transcription, and Translation
The chemistry that makes base pairing specific, and the machinery that copies, reads, and translates it into protein.
10 topics · 36 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.
Why Base Pairing Is Chemically Specific
- How it worksHow do the number and position of hydrogen bonds in each base pair make only A-T and G-C fit together properly?
- Common mistakeWhy isn't base pairing held together by covalent bonds running between the two strands?
- Going furtherWhat happens to a DNA strand's hydrogen bonding when a mismatched base pair, like G paired with T, tries to form?
Your turn1 of 2
A short double-stranded DNA region is 45 base pairs long, with a fraction 0.619 of those pairs being A-T (2 hydrogen bonds each) and the remaining fraction being G-C (3 hydrogen bonds each). Find the total number of hydrogen bonds holding the region together.The Double Helix Structure
- How it worksHow do DNA's two strands run in opposite directions so their bases can fit together properly?
- Worked exampleFind how many base pairs fit in one full turn of the double helix if each turn spans 3.4 nanometers and each base pair rises 0.34 nanometers.
- Going furtherWhat happens to the width of the double helix if a purine tries to pair with another purine instead of a pyrimidine?
- Common mistakeWhy isn't DNA's major groove just an empty gap with no chemical role of its own?
Your turn1 of 2
Find how many base pairs fit in one full turn of the double helix if each turn spans 3.557 nanometers and each base pair rises 0.358 nanometers.Semiconservative Replication
- How it worksHow does separating the two original strands let each one serve as a template for building its complementary partner?
- Common mistakeWhy isn't DNA replication building one entirely new double helix while leaving the old one completely untouched?
- Going furtherWhat happens to one single original DNA strand's identity if you could track it across many rounds of replication?
Your turn1 of 2
DNA replication starts by separating the two original strands of the double helix before any new strand is built. A student assumes this separation step must just be a preliminary clearing of space, unrelated to how the new strands actually get built correctly. What role does each separated original strand actually play?DNA Polymerase and the Replication Fork
- How it worksHow does DNA polymerase read a template strand and add matching nucleotides only in the 5-prime to 3-prime direction?
- How it worksWalk me through how DNA polymerase proofreads its own work and removes a mismatched nucleotide it just added.
- Common mistakeWhy isn't DNA polymerase able to start building a brand new strand completely from scratch, with no primer at all?
- Going furtherWhat happens to a cell's mutation rate once its DNA polymerase's proofreading function gets disabled?
Your turn1 of 2
DNA polymerase can extend an existing strand rapidly, but it cannot begin building a brand new strand with no starting point at all. A student assumes such an efficient enzyme should be able to start from nothing just as easily as it extends. Why does DNA polymerase actually require a primer first?Leading and Lagging Strand Synthesis
- How it worksHow does the leading strand get synthesized continuously as the replication fork opens up ahead of it?
- How it worksWalk me through how the lagging strand gets built backward in short Okazaki fragments that are later stitched together.
- Worked exampleFind how many Okazaki fragments are needed to copy a 15,000-base lagging strand if each fragment averages 1,500 bases.
- Going furtherWhat happens to a cell's replication speed when DNA ligase, the enzyme that joins Okazaki fragments, stops working?
Your turn1 of 2
Find how many Okazaki fragments are needed to copy a 23321 base lagging strand if each fragment averages 1579 bases.Transcription: RNA Polymerase Reading a DNA Template
- How it worksHow does RNA polymerase find a gene's promoter sequence and know exactly where to start transcribing?
- How it worksWalk me through RNA polymerase unwinding DNA and building an RNA strand complementary to its template.
- Common mistakeWhy isn't RNA polymerase copying both strands of the DNA double helix into RNA at the same time?
- Going furtherWhat happens to transcription when a mutation disrupts a gene's promoter sequence?
Your turn1 of 2
RNA polymerase reliably starts transcribing at the correct location on a gene every time, rather than binding DNA at random and starting wherever it happens to land. A student wonders how it finds the right starting point so consistently. How does RNA polymerase know exactly where to start transcribing?RNA Processing: Splicing, Capping, and Polyadenylation
- How it worksHow does the spliceosome cut introns out and stitch exons together into a mature messenger RNA?
- Going furtherWhat happens to a protein's sequence when a mutation causes one exon to be skipped during splicing?
- Common mistakeWhy isn't every part of a gene's transcribed RNA actually used to code for protein?
Your turn1 of 2
A freshly transcribed RNA molecule contains both exons and introns, but only after processing does it become a mature messenger RNA ready to leave the nucleus. A student assumes this editing step must be optional cleanup, since the RNA already contains the gene's full sequence. Why is splicing actually essential rather than optional?The Genetic Code and Its Properties
- How it worksHow does grouping RNA bases into three-letter codons create enough combinations to cover every amino acid, with room to spare?
- Worked exampleFind how many possible three-letter codons can be built from an alphabet of 4 RNA bases.
- Common mistakeWhy isn't the genetic code assigning one single unique codon to every amino acid?
Your turn1 of 2
Find how many possible codons can be built from an alphabet of 5 RNA bases grouped into words of 3 bases each.Translation: The Ribosome Reading mRNA
- How it worksHow does a ribosome read messenger RNA three bases at a time and match each codon to the right amino acid?
- How it worksWalk me through the ribosome ratcheting through its A, P, and E sites as it adds one amino acid after another.
- How it worksHow does a transfer RNA's wobble position let one tRNA recognize more than one codon for the same amino acid?
- Common mistakeWhy isn't a stop codon ever read by a transfer RNA carrying an amino acid?
- Going furtherWhat happens to a protein when a mutation shifts the ribosome's reading frame by inserting one extra base?
Your turn1 of 2
A cell has far fewer distinct transfer RNA molecules than the 61 codons that specify amino acids, yet every one of those codons still gets read correctly. A student assumes this must mean the cell has simply not yet made every possible transfer RNA it needs. What actually lets fewer tRNAs cover all 61 codons?How Reverse Transcriptase Lets HIV Rewrite RNA into DNA
- How it worksHow does reverse transcriptase read an RNA template and build a complementary DNA strand from it?
- Going furtherWhat happens once HIV's newly made DNA copy gets inserted into the host cell's own genome?
- Ties togetherHow does reverse transcriptase reading a template strand reuse the exact same base-pairing chemistry a cell's own DNA polymerase depends on?
Your turn1 of 2
HIV stores its genetic information as RNA, yet it still ends up permanently written into a human cell's DNA. A student assumes RNA must somehow directly insert itself into the host's DNA without any intermediate step. How does reverse transcriptase actually accomplish this?