Lecture 23: Review of Genetic Code and Transcription
(Old Lecture 22)
The third examination will only cover topics from Lecture 23 through the end of prokaryotic transcription. Review questions that are applicable to the third examination are identified with a double asterisk (**)
**1. What is the special significance of the sigma-subunit of prokaryotic RNA polymerase?
**2. How does the nucleotide sequence of the sense strand of DNA differ from the nucleotide sequence of the corresponding messenger RNA?
**3. What is the nature of a promoter sequence and why is it important?
**4. What is the nature of the transcription termination signal in E. coli?
**5. How does the RNA polymerase core enzyme differ from the RNA polymerase holoenzyme. Answer both in terms of structure and in terms of function.
6. Describe three distinctly different ways in which eukaryotic messenger RNA is modified after the gene is transcribed.
**7. The genetic code is organized into units called codons.
A. What constitutes a codon?B. How many different codons are possible, based on the structural organization of individual codons?
C. How many different codons are there that specify amino acids in the most common version of the genetic code?
D. What is the function of the codons that do not code for amino acids?
E. Compare the number of amino acid-coding codons with the number of amino acids that are coded for and explain how cells deal with the discrepancy in the two numbers.
**8. What effect on the coded protein do you expect from each of the following:
A. Deletion of one nucleotide from near the 5'-end of the coding sequence?B. Deletion of one nucleotide from near the 3'-end of the coding sequence?
C. Deletion of three nucleotides from near the middle of the coding sequence?
D. Inserting one nucleotide near the 5'-end of the coding sequence.
E. Deleting one nucleotide near the 5' end of the coding sequence and inserting one nucleotide 9 nucleotides downstream from the deletion.
9. Identify the three types of RNA polymerase found in eukaryotic cells and briefly describe the function of each.
**10. Describe the differences that exist between prokaryotic and eukaryotic cells with regard to the temporal and spatial relationship between transcription and translation.
11. What is an enhancer and what are the special properties that distinguish it from cis-acting sequences that are directly a part of the promoter.
12. What are transcription factors and how are they beleived to function?
**13. What role is played by the antisense strand of the double stranded DNA during the process of transcription?
**14. What is the nature of the prokaryotic stop transcription signal?
**15. Is the genetic code universal? Justify your answer.
16. Parts of this question will require speculative answers based on reasonable extrapolation from information that has been given.
**A. In a random DNA sequence read three bases at a time, how frequently would you expect to encounter a stop codon?**B. How would you determine whether a known prokaryotic DNA sequence that is 1000 base pairs in length contains a protein-coding gene. (Assume for purposes of this question that any functional protein would be at least 100 amino acids in length. Also, keep in mind the possibility that the protein could be coded for in any of the three possible reading frames, and that the coding could be in a 5' to 3' direction on either of the two strands of DNA, since you do not know whether the sequence you have was read from the sense or the antisense strand.).
**C. What characteristic features would you expect to be contained in a complete coding sequence for a protein?
**D. What additional features would you expect to see upstream from the coding sequence for a protein?
**E. What additional features would you expect to see downstream from the coding sequence for a protien?
F. What added complication might you encounter with eukaryotic DNA?
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