Unit 8: Gene Expression: Transcription, RNA Processing, and Translation
How a gene's DNA sequence becomes a working protein, from copying it into RNA to reading it three bases at a time on a ribosome.
7 topics · 26 questions
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The Mechanism of Transcription
- How it worksHow does RNA polymerase find a promoter, unwind DNA, and build a complementary RNA strand?
- Worked exampleFind the mRNA sequence transcribed from the DNA template strand 3'-TACGGCATT-5'.
- Common mistakeWhy doesn't RNA polymerase need a primer to start transcription the way DNA polymerase needs one to start replication?
- Going furtherWhat happens to transcription when RNA polymerase reaches a terminator sequence at the end of a gene?
Your turn1 of 2
RNA polymerase transcribes a gene that is 2937 bases long, moving at a rate of 46 bases per second. About how many seconds does transcribing this gene take?RNA Processing: Splicing, Capping, and the Poly-A Tail
- How it worksHow does the spliceosome recognize and remove introns while joining exons together?
- Going furtherWhat happens to a protein if a mutation destroys the splice site between an intron and its neighboring exon?
- Common mistakeWhy doesn't every piece of a gene's transcribed RNA end up as part of the final protein-coding message?
Your turn1 of 2
A human gene's freshly transcribed pre-mRNA includes both exons and introns, but the final message exported to the cytoplasm is shorter and contains only exons. Why doesn't every piece of the transcribed RNA end up as part of the final protein-coding message?Alternative Splicing and One Gene, Many Proteins
- How it worksHow does including or skipping different exons during splicing generate multiple mRNA versions from one gene?
- Worked exampleTrace how a pre-mRNA with four exons gets spliced into two different mature mRNAs by skipping exon 3.
- Ties togetherHow does alternative splicing help explain why humans have far fewer genes than their total number of proteins?
- Worked exampleFind the number of possible mature mRNAs from a pre-mRNA with three optional exons that can each be included or skipped.
Your turn1 of 2
A pre-mRNA has 6 optional exons, each of which can independently be included in or skipped from the final mature mRNA during alternative splicing. How many different mature mRNA versions can this one gene potentially produce?The Genetic Code and Codons
- How it worksHow does each three-base codon in mRNA get matched to one specific amino acid?
- Worked exampleFind the amino acid sequence encoded by the mRNA strand AUG GCU UUA UAG.
- Common mistakeWhy doesn't the genetic code being redundant also make it ambiguous?
Your turn1 of 2
An mRNA's protein-coding region, not counting the stop codon, is 427 bases long. How many amino acids will the resulting polypeptide contain?Translation and the Ribosome
- How it worksHow does a ribosome move along mRNA reading one codon at a time to add amino acids to a growing chain?
- Worked exampleTrace the amino acid chain built by a ribosome translating the mRNA AUG UUU GGC UAA.
- Common mistakeWhy doesn't translation start at the very first base of an mRNA molecule?
- Worked exampleTrace the anticodons of the tRNAs needed to translate the mRNA sequence AUG GAA CGU UAA.
- Going furtherWhat happens to translation when a ribosome reaches a stop codon with no matching tRNA available to compete with it?
Your turn1 of 2
An mRNA molecule has a 5' untranslated region before its first AUG codon. Why doesn't the ribosome simply start translating at the very first base of the mRNA?tRNA Structure and the Wobble Hypothesis
- How it worksHow does loose base pairing at the third codon position let one tRNA recognize more than one codon?
- Worked exampleFind which codons a single tRNA with the anticodon 3'-AAG-5' can pair with under wobble rules.
- Going furtherWhat happens to protein synthesis speed when a rare codon has very few matching tRNAs available in the cell?
Your turn1 of 2
Wobble base pairing at the third codon position lets a single tRNA recognize about 2.31 synonymous codons on average. Roughly how many distinct tRNA molecules would a cell need to translate all 61 sense codons?How a DNA Mutation Changes the Final Protein
- Ties togetherHow does tracing a mutation from DNA through transcription and translation connect the genotype to the final phenotype?
- Worked exampleTrace how a single base substitution in the beta-globin gene changes glutamic acid to valine and produces sickle hemoglobin.
- Common mistakeWhy doesn't a mutation inside an intron usually change the protein a gene produces?
- Worked exampleFind the effect on hemoglobin of the DNA change that turns codon GAG into GTG at the sixth position of the beta-globin gene.
Your turn1 of 2
A single DNA base substitution in the beta-globin gene changes codon 6 from GAG to GTG, replacing glutamic acid with valine in the hemoglobin protein and causing sickle cell disease. Why does swapping just this one amino acid have such a large effect on hemoglobin's behavior?