1. TRANSCRIPTION

2. Initiation  Transcription factors bind to the promoter region  RNA polymerase binds to the promoter region  The enzyme’s active site only recognizes the promoter region  The promoter region contains many Adenine and Thymine bases and is upstream from the gene to be translated

3. Initiation  A’s and T’s have __ H-bonds, C’s and G’s have __ H- bonds  _______ take less energy to break than ______ which makes it an ideal spot for the promoter region  RNA polymerase moves past the promoter region until it reaches the start sequence and unwinds the DNA  For transcription, DNA is only unwound when being transcribed, then it rewinds  The part of DNA that is unwound is a transcription bubble

4. Elongation  Just like DNA, RNA is built in the 5’ to 3’ direction  A primer is not required by the RNA polymerase, as was the case for DNA polymerase in DNA replication  The promoter is not transcribed  The strand of the DNA that is transcribed is called the template strand

5. Elongation  The complimentary strand of the DNA that is not transcribed is called the coding strand  This strand in identical to the mRNA except that it has Thymine instead of Uracil  The DNA is transcribed into RNA

6. Termination  There are two different ways Transcription can be terminated 1. Rho-dependant  This method of termination relies on the appearance of a Rho protein to ‘boot off’ the RNA polymerase and release the mRNA

7. Termination 2. Rho-independent  This method does not rely on a Rho protein to terminate transcription. In this case, the RNA polymerase reaches the termination sequence.  The termination sequence is high in A’s and T’s just like the promoter region.  Both types of termination cause the mRNA transcript to fold back on itself and disassociate with the DNA template strand, freeing up RNA polymerase to transcribe another gene

8. Posttranscriptional Modifications  In eukaryotic cells, the primary transcript (unprocessed RNA) needs to undergo capping, tailing and base excision, before it can leave the nucleus  Capping is the addition of a 5’ cap to the start of the transcript, basically it is a modified guanine nucleotide.  Capping has two functions:  to protect the mRNA from digestive nucleases and phosphatases as it exits the nucleus and enters the cytoplasm  to initiate translation

9. Posttranscriptional Modifications  Tailing: about 200 adenine ribonucleotides, are added to the RNA. The poly-A-tail, is added to the 3’ end of the transcript by an enzyme called poly-A-polymerase  The entire mRNA transcript consists of two regions – a coding region called exons, and a non-coding region called introns (which are interspread among the exons)  The introns are removed by spliceosomes so that their translation is prevented (because they don’t code for anything!)  The introns stay inside the nucleus and are degraded into recycled nucleotides

10. Posttranscriptional Modifications  The “primed” mRNA transcript then moves out of the nucleus and into the cytoplasm where it will be translated  There is no “quality control” mechanism for the mRNA strand  An incorrectly-made mRNA transcript will amount to a defective protein  However, so long as the original DNA template strand is correct, the multiple copies of the mRNA transcripts will more than likely compensate for any mistake in one mRNA strand

11. Summary!  Initiation  Transcription factors bind to promoter region followed by RNA polymerase  Elongation  mRNA is synthesized throughout the coding region  Termination  The terminating sequence is reached and the mRNA and RNA polymerase are released  http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a2.html http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a2.html