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