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Genetics CH9 ProK
exam 4 review/gene expression
| Question | Answer |
|---|---|
| What is transcription? | the polymerization of ribonucleotides, guided by complementary base pairing, produces an RNA transcript of a gene. |
| In transcription: the template for the RNA transcript is | one strand of that portion of the DNA double helix that constitutes the gene. |
| What are the 3 stages of transcription? | Initiation, Elongation, Termination |
| In ProKs, how is transcription initiated? | Mostly by promoters; where RNA polymerase binds to promoters |
| What are promoters? | specialized DNA sequences near the transcription start site |
| What role does sigma factor play here? | is only involved in initiation --> The sigma subunit reduced RNA polymerase’s general affinity for DNA and simultaneously increases the enzyme’s affinity for the promoter. |
| Which site does RNA polymerase bind on DNA? | binds to promoters |
| In what direction is RNA made? | 5’ to 3’ direction |
| Is RNA made in an antiparallel direction? | yes because it forms a DNA/RNA hybrid that has the RNA strand complementary to the template strand. |
| What is required for transcription elongation to begin? | elongation begins once the enzyme has left the promoter (so that other RNA polymerase molecules can move in to initiate transcription.) |
| What are the two ways transcription is terminated? | Transcription is terminated by intrinsic terminators or extrinsic terminators. |
| What are intrinsic terminators? | cause the RNA polymerase core enzyme to terminate transcription on its own |
| What are extrinsic terminators? | require additional proteins – particularly a polypeptide called Rho – to bring about termination in transcription |
| What is rho? | a polypeptide” required for extrinsic termination. |
| Can Transcription and Translation happen at the same time in prokaryotes? | Yes, Bc there is no nucleus, the moment RNA polymerase makes an RNA transcript, ribosomes can bind and start translating it immediately |
| Why can prokaryotes translate RNA immediately after it is made? | Bc transcription extends mRNAs in the same 5′ → 3′ direction as the ribosome moves, allowing ribosomes to start translation on “a partial mRNA that the RNA polymerase is in the process of transcribing |
| What is special about prokaryotic mRNA structure? | Many prok mRNAs are polycistronic. They contain the information of several genes, each of which can be translated independently starting at its own ribosome binding site. |
| How does translation initiation start in prokaryotes? | In prokaryotes, this signal is the ribosome binding site… the Shine–Dalgarno box adjacent to an initiating AUG codon. |
| How does the ribosome bind to the mRNA? | The 30S subunit binds first, using its 16S rRNA to recognize the ribosome binding site. |
| What is the Shine–Dalgarno box? | A six-nucleotide sequence – usually 5′ … AGGAGG … 3′ – named the Shine Dalgarno box.” It positions the ribosome correctly for translation initiation. |
| What is the initiation codon in prokaryotes? | The triplet 5′ AUG 3′ acts as the initiation codon.” |
| What amino acid does the first AUG code for in prokaryotes? | The initiator tRNA carries N-formylmethionine (fMet), |
| What is N-formylmethione? | a unique methionine to prokaryotic cells, whose amino end is blocked by a formyl group |
| What does N-formylmethionine do? | starts off the peptide (unique to prokaryotic) any subsequent AUgs after the first one just code for regular methionine |
| How does transcription start in prokaryotes? | RNA polymerase binds to promoters. The σ (sigma) subunit “increases the enzyme’s affinity for the promoter.” This forms a closed promoter complex, then an open promoter complex as DNA unwinds. |
| How does transcription stop in prokaryotes? | Intrinsic terminators and Extrinsic terminators |
| How is prokaryotic transcription different from eukaryotic transcription? | Prokaryotes: No nuclear membrane; transcription & translation can be coupled. mRNA is ready for translation immediately (no processing). |
| What is meant by transcription and translation being coupled in prokaryotes? | simultaneous process is called coupling when translation and transcription happen at the same time in Prokaryotes bc they lack a nuclear membrane. |
| What is polycistronic mRNA and what cell types have it? | Only prokaryotes have this - contains the information for several different proteins encoded in one mRNA molecule, each with its own ribosome binding site. |
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