Transcription

DNA template strand DNA coding strand

RNA polymerase directs the sequential binding of ribonucleotides to the growing RNA chain in the 5’ to 3’ direction after the promoter. RNA polymerase forms phosphodiester bonds in the RNA chain Once the RNA polymerase machinery passes, the two DNA strands reform back together key notes The template strand is the strand being read the other strand is called the DNA coding strand and is not transcribed its called the coding strand as its sequence is the same as the RNA sequence that is produced, with the exception of U replacing T task before quiz: find out why these strands have those names.

enzyme RNA polymerase recognizes a promoter region, which lies “upstream” from the gene(usually repeating A and T) The RNA polymerase binds tightly to the promoter causing the DNA strands to pull apart within the promoter region • A and T have 2 bonds so easier to pull apart

• regions at the end of genes signal termination and are named “terminator sequence”, work with RNA polymerase to loosen the association between the mRNA strand and the DNA template -mRNA falls off the transcription complex and is modified in nucleus  RNA polymerase can bind to another promoter site on DNA

• Capping: Addition of a 5’ cap to start end  Protects it from digestion by nucleases and phosphatases as it exits the nucleus  Helps to initiate translation • Tailing: Addition of the poly-A tail by poly-A polymerase  200-300 adenine bases added to the end of mRNA transcript  Helps end translation • Introns are cut out by spliceosomes  Eukaryotic DNA has exons (coding regions) and introns (non-coding regions).  Introns are interspersed among the exons, therefore sections of the primary transcript do not code for part of the protein Won’t make proper amino acid sequence so the protein won’t fold properly

RNA just transcribed = primary transcript Modified primary transcript = mRNA

poly-A polymerase

topoisomerase

an enzyme because of the ase ending