TRANSCRIPTION Sections 5.2 & 5.3
Recall… Protein synthesis occurs in two distinct stages: Transcription, in the nucleus Product: mRNA Translation, in the cytoplasm Product: polypeptide
Transcription The process of copying DNA’s instructions for building a protein. The instructions are copied into a complementary mRNA molecule.
Overview Three stages: Initiation Elongation Termination
Stage Events Initiation RNA polymerase binds to the DNA and unwinds it Elongation RNA polymerase creates a complementary mRNA strand. It adds on ribonucleotides in the 5’ 3’ direction Termination Translation ceases, and the mRNA is released
Terminology The strand used as the template is the “template strand”
The strand not used as the template is the “coding strand.” The mRNA is identical to the coding strand except for one difference... uracil in RNA is complementary to adenine in DNA coding strand
The details of eukaryotic transcription Section 5.3 The details of eukaryotic transcription
Initiation RNA polymerase binds to a promoter region located upstream of the gene The promoter region is high in A-T base pairs Only two H-bonds; easy to break RNA polymerase unzips the double strands in order to expose the bases
Initiation Binding to promoter Unzipping the DNA
Elongation RNA polymerase builds an mRNA strand Incorporates ribonucleotides that are complementary to the template DNA Catalyzes formation of phosphodiester bonds A primer is not required
Elongation
Transcription (like replication) occurs in the 5’ 3’ direction (ribonucleotides are added on to the 3’ end of the nascent strand) The promoter does not get transcribed
Termination A terminator sequence is located at the end of the gene Eukaryotes vs. prokaryotes: different sequences RNA polymerase recognizes the terminator, and releases the mRNA strand
Termination
After transcription… The mRNA will exit the nucleus Transcription of mRNA protein will occur in the cytoplasm BUT Before it exits, the mRNA is modified
Post-transcriptional modifications “Primary transcript” – the newly-built mRNA Must be modified before it exits the nucleus. Why? remove non-coding regions protect the mRNA from cytoplasmic nucleases and phosphatases (enzymes that can degrade the RNA)
Modifications Splicing Add 5’ cap Add poly-A tail addition (“polyadenylation”)
Splicing removal of non-coding regions (introns) in the transcript spliceosomes (a) excise the introns, and (b) ligate the exons spliced introns are degraded in the nucleus
5’ cap is added poly-A tail is added 7-methyl guanosine 200 adenine nucleotides Catalyzed by poly-A polymerase
primary transcript (modification) mRNA TRANSCRIPT Following modification, the primary transcript is now called the “mRNA transcript” primary transcript (modification) mRNA TRANSCRIPT It is ready to leave the nucleus!!
Transcription: Quality control no proofreading mechanisms (unlike replication) more errors in transcription than replication but more copies of mRNA in a cell than DNA errors are less detrimental Backup mechanism: redundancy of the genetic code used in translation *more on this later on…
Homework Pg. 249 #1-9