1. Transcription Chapter 25

2. Objectives  Understand the process of transcription  Recognize the role of RNA Polymerase  Recognize the significance of promoter and terminator regions of DNA  Explain how transcribed RNA is modified prior to exiting the nucleus. Understand the significance of this process

3. Transcription  Consists of three stages  Initiation: attachment of RNA Polymerase to the promotor region on DNA  Elongation: building of the mRNA from the 3’ end of the DNA  Termination: release of RNA polymerase and mRNA following transcription of the terminator region of the DNA

5. Initiation  Genes on the DNA begin with a promoter region -- a sequence of A & T (TATA box)  Transcription initiation complex: transcription factors & RNA polymerase are bound to the promoter region on the DNA

7. Elongation Termination  Once initiation is complete the 2 strands of the DNA unwind to be copied into mRNA  RNA polymerase builds a mRNA strand complimentary to the DNA  Once the RNA Polymerase (Transcriptase) is done copying, the DNA strands zips back up to form their double helix  When the RNA Polymerase reaches the terminator region of the DNA, it lets go and releases the mRNA  The transcribed termination sequence on the mRNA is AAUAAA

8.  Transcribed mRNA (pre-mRNA) must be modified before leaving the nucleus  There are special “dividers” in the mRNA that need to be cut out. They are not part of the genetic information, just organizers. Modification of mRNA

9.  Transcribed mRNA is too long and is shortened before it leaves the nucleus by a special cutting process  Exons are segments of the pre-mRNA that contain genetic information that will be create proteins  Introns are just divisions between the EXONS Further Modifications

10. How is this done?  Spliceosomes remove introns while connecting exons together  Ribozymes also help catalyze the removal of introns from mRNA

11. Why bother with introns?  Introns may regulate gene activity and the passage of mRNA into the cytoplasm  Genes may play roles in multiple proteins, introns may enable a gene to be diverse in function  May increase recombination of genetic material (easier to cut and paste)