transcription - amazon s3 rna processing only occurs in eukaryotes . rna processing "...
TRANSCRIPT
TRANSCRIPTION DNA à mRNA
Central Dogma
Animation
DNA: The Secret of Life (from PBS) http://www.youtube.com/watch?v=41_Ne5mS2ls&list=PL2B2BD56E908DA696&index=3 Transcription http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter3/animation__mrna_synthesis__transcription___quiz_1_.html
Translation http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter3/animation__how_translation_works.html
Gene
¨ The segment of DNA that is transcribed into RNA ¨ Also called a transcription unit
Stages of Transcription
¨ Initiation ¨ Elongation ¨ Termination
Promoter
¨ Nucleotide sequence ¨ Located upstream of the gene ¨ Where RNA polymerase (RNAP) first binds to starts
transcribing ¨ Composed of an AT rich region
¤ Why is this region rich in A & T? Hint: DNA is double stranded
Transcription Initiation
¨ RNAP recognize and bind to promoter ¤ Key element in
promoter is TATA box in eukaryote.
¨ DNA unwinds at the front of RNAP
Defining the strands
¨ Coding strand = sense strand = DNA strand that is not being copied (contains same sequence as new RNA except TàU)
¨ Noncoding strand = antisense strand = template ¨ Transcript = newly synthesized RNA
Practice
¨ In this diagram identify the following: ¤ Transcript ¤ Template strand ¤ Coding strand ¤ Sense strand ¤ Antisense strand
¨ Label the 5’ and 3’ end on each strand
Transcription Elongation
¨ RNAP unwinds DNA exposing 10-20 bases
¨ Uses template strand to add complementary RNA nucleotides
¨ Direction of transcription: downstream from 5’à3’ of the transcript
Transcription Elongation
¨ RNA transcript separates from template as RNAP continues elongation
¨ DNA helix reforms behind RNAP
¨ Many RNAP can transcribe simultaneously
Transcription Elongation
Transcription Termination
¨ Termination sequence: signals end of transcription ¨ Prokaryote: transcription ends immediately at signal ¨ Eukaryote:
¤ RNAP continues for hundreds of nucleotides past termination signal
¤ at 10-35 nucleotide past signal pre-mRNA is released
Transcription Termination
RNA Processing
¨ Precursor mRNA = primary transcript: Pre-mRNA need additional modification before it become mRNA
¨ RNA processing only occurs in eukaryotes
RNA Processing
¨ Post-transcriptional modification: ¤ Capping: 5’ cap ¤ Polyadenylation: Poly A tail
¨ Splicing: intron excision
Post-Transcriptional Modification: Capping
¨ Soon after transcription 5’ cap is added ¨ 5’ cap
¤ Modified guanine added to 5’ end
¨ Function: ¤ Protect mRNA from degradation ¤ Signals ribosome attachment
Post-Transcriptional Modification: Polyadenylation
¨ PolyA tail ¤ 50-250 adenine added to 3’ end ¤ Added by poly-A polymerase
¨ Function ¤ Protect mRNA from degradation ¤ Facilitate export of mRNA from nucleus
RNA Processing: Splicing
¨ Splicing: removal of introns ¨ Introns: intervening (noncoding) sequences,
interspersed between exons ¤ Removed by splicesome
¨ Exons: coding sequences, expressed
Introns are removed Exons are joined together
3D animation: http://www.dnalc.org/view/16933-3D-Animation-of-DNA-to-RNA-to-Protein.html
Splicing Video
¨ General intro to splicing: http://www.dnalc.org/view/16939-RNA-Splicing.html Splicing mechanism: ¨ 3D animation -
http://www.dnalc.org/view/16938-3D-Animation-of-RNA-Splicing.html ¨ http://highered.mcgraw-hill.com/olc/dl/120077/bio30.swf
Intron Function
¨ Regulatory role ¨ Alternative splicing: single gene encode more than
one kind of polypeptide depending on which segments are treated as exons
Prokaryotic versus Eukaryotic protein synthesis ¨ Video:
http://highered.mcgraw-hill.com/olc/dl/120077/bio25.swf
HW Questions
¨ Which enzymes in DNA replication mimic the function of RNAP in initiation and elongation?
¨ Why would promoters have a higher abundance of A-T than G-Cs?
¨ Why is post-transcriptional modifications not necessary in prokaryotes?
¨ Summarize in a chart, all the differences between prokaryotes and eukaryotes in transcription