Unit 15: Transcription, RNA Processing, and Translation
From a promoter recruiting RNA polymerase to a ribosome ratcheting out a finished protein, with the RNA editing steps in between.
8 topics · 32 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.
RNA Polymerase and the Promoter
- How it worksHow does a promoter sequence recruit RNA polymerase and position it to start transcription at the right spot?
- Common mistakeWhy isn't a promoter itself transcribed into the RNA it directs?
- Going furtherWhat happens to transcription when a mutation weakens a promoter's sequence?
Your turn1 of 2
A promoter sequence sits directly upstream of a gene and is essential for RNA polymerase to begin transcribing that gene. A student assumes the promoter sequence itself must therefore be transcribed into the resulting RNA as its first section. Why isn't a promoter itself transcribed into the RNA it directs?Transcription: Initiation, Elongation, and Termination
- How it worksWalk me through how RNA polymerase reads the template strand and builds a complementary RNA strand 5-prime to 3-prime.
- How it worksHow does RNA polymerase know where to stop transcribing once it reaches a terminator sequence?
- Worked exampleFind how long it takes RNA polymerase to transcribe a 3,000 base gene at an elongation rate of 50 nucleotides per second.
- Common mistakeWhy isn't RNA polymerase copying both strands of the DNA at the same time?
- Going furtherWhat happens to a gene's transcript when RNA polymerase encounters a terminator sequence that's been deleted?
Your turn1 of 2
Find how long it takes RNA polymerase to transcribe a 6511 base gene at an elongation rate of 65 nucleotides per second.Pre-mRNA Splicing and the Spliceosome
- How it worksHow does the spliceosome recognize intron boundaries and cut them out of a pre-mRNA?
- How it worksWalk me through how snRNPs assemble on an intron and loop it out before sealing the exons together.
- Common mistakeWhy isn't an intron simply junk sequence with no role in how a gene gets expressed?
- Worked exampleFind how many introns must be spliced out of a pre-mRNA built from 9 exons, if introns fall only between consecutive exons.
Your turn1 of 2
Find how many introns must be spliced out of a pre-mRNA built from 15 exons, if introns fall only between consecutive exons.Alternative Splicing and Protein Diversity
- How it worksHow does alternative splicing combine a gene's exons in different patterns to build different mRNAs?
- Common mistakeWhy isn't alternative splicing a mistake that the cell's editing machinery failed to catch?
- Ties togetherHow does alternative splicing let the same DNA sequence in every cell type produce completely different proteins in a neuron versus a muscle cell?
Your turn1 of 2
A single gene's pre-mRNA is spliced in several different patterns across different cell types, producing several distinct mature mRNAs and proteins from that one gene. A student assumes this variability must be a mistake the cell's splicing machinery failed to catch and correct. Why isn't alternative splicing a mistake that the cell's editing machinery failed to catch?mRNA Capping, Polyadenylation, and Export
- How it worksHow does a 5-prime cap protect an mRNA from degradation and help recruit the ribosome later?
- How it worksHow does a poly-A tail get added to the 3-prime end and help control how long the mRNA survives?
- Common mistakeWhy isn't an mRNA ready to be translated the instant transcription finishes?
- Going furtherWhat happens to protein production as an mRNA's poly-A tail gets shortened over time?
Your turn1 of 2
As soon as RNA polymerase finishes transcribing a gene, the resulting pre-mRNA already contains the complete sequence needed to encode a protein. A student assumes this transcript must therefore be immediately ready for translation the moment transcription ends. Why isn't an mRNA ready to be translated the instant transcription finishes?The Genetic Code and tRNA Aminoacylation
- How it worksHow does a tRNA's anticodon pair with an mRNA codon to bring in the correct amino acid?
- How it worksHow does an aminoacyl-tRNA synthetase attach the one correct amino acid to each specific tRNA?
- Common mistakeWhy isn't a separate tRNA needed for every one of the sixty-four possible codons?
- Worked exampleFind how many different amino acids the codon set AUU, AUC, and AUA specifies, given that all three code for isoleucine.
Your turn1 of 2
The genetic code has sixty-four possible three-letter codons but specifies only twenty amino acids (plus stop signals). A student assumes the cell must therefore need a separate, dedicated tRNA molecule for every one of the sixty-four possible codons. Why isn't a separate tRNA needed for every one of the sixty-four possible codons?The Ribosome and the Mechanics of Translation
- How it worksHow does a ribosome hold an mRNA and a charged tRNA in its A and P sites to catalyze peptide bond formation?
- How it worksWalk me through how the ribosome ratchets one codon at a time down the mRNA as tRNAs arrive and leave.
- Worked exampleFind how long it takes a ribosome to translate a 300-codon protein at a rate of 5 codons per second.
- Common mistakeWhy isn't the ribosome itself made only of protein, given that it's often described as a protein-making machine?
- Going furtherWhat happens to translation speed when a rare codon requiring a low-abundance tRNA appears in the mRNA?
Your turn1 of 2
Find how long it takes a ribosome to translate a 512-codon protein at a rate of 7 codons per second.Translation Initiation, Elongation, and Termination
- How it worksHow does the small ribosomal subunit find the start codon before the large subunit joins to begin elongation?
- How it worksWalk me through how a stop codon, which has no matching tRNA, causes a release factor to end translation.
- Common mistakeWhy isn't a stop codon translated into an amino acid the way every other codon is?
- Ties togetherHow does the ribosome releasing a finished protein at a stop codon connect back to that protein needing a signal sequence to reach its destination?
Your turn1 of 2
Every codon in an mRNA, in order, is matched to a tRNA carrying a specific amino acid during elongation — except when the ribosome reaches a stop codon. A student assumes a stop codon must simply be translated into some special, unusual amino acid that ends the protein chain. Why isn't a stop codon translated into an amino acid the way every other codon is?