If one were to write out the structure of a polycistronic mRNA, it would be Shine-Dalgarno-AUG-STOP-Shine-Dalgarno-AUG-STOP-Shine-Dalgarno-AUG-STOP. Explain that as long as there are stop codons in the message, the polypeptides will be released and ribosomes reattached at the following Shine-Dalgarno sequence. Students have difficulty visualizing polycistronic messages. Shine Dalgarno sequence (binding to ribosome)ĪTG (start codon for protein transcription) Sigma binding consensus sequence/TATA box Change the order when giving the exercise to the class: The regions are given in the correct order here. Students confuse transcription, termination, and stop codons.Īsk students to diagram a generic gene and label the following regions in the correct sequence in the 5'-3' direction. Clarify, if necessary, the role of the sigma factor and rho proteins. There can be more than one consensus sequence in a genome as there are several sigma factors that recognize different sequences. The following drawing may clarify the text in the chapter. Use a diagram to illustrate rho-independent termination. In the same way consensus sequences, which indicate a promoter region where an RNA polymerase binds, contain several elements that are required for recognition. It is the combination of period followed by a space and an upper case which indicates the beginning of a sentence. Answer that some abbreviations are followed by a period. The couple AT/AU is much less stable than CG therefore promoter sequences will be rich in AT because it takes less energy to unzip DNA.Īsk the students, How do you recognize the beginning of a sentence? They may answer that they see a period. Review the complementarity of nitrogenous bases and the stability of base pairing as a function of number of hydrogen bonds. Use different colors to label the promoter and the terminator sequences. A Learning Objective merges required content with one or more of the seven Science Practices.Īsk students to draw a timeline of the steps needed for transcription and add all the different components as specific shapes. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP ® Biology course, an inquiry-based laboratory experience, instructional activities, and AP ® Exam questions. Information presented and the examples highlighted in the section support concepts outlined in Big Idea 3 of the AP ® Biology Curriculum Framework. Termination frees the mRNA and often occurs by the formation of an mRNA hairpin. Transcription continues until RNA polymerase reaches a stop or terminator sequence at the end of the gene. In prokaryotes, mRNA synthesis is initiated at a promoter sequence on the DNA template. How and when is transcription terminated?ĭuring transcription, the enzyme RNA polymerase moves along the DNA template, reading nucleotides in a 3′ to 5′ direction, with U pairing with A and C with G, and assembling the mRNA transcript in a 5′ to 3′ direction.What are the steps, in order, in prokaryotic transcription?.In this section, you will explore the following questions:

Promoter consensus sequence