TRANSCRIPTION. DNA à mrna

Transcription

1 TRANSCRIPTION DNA à mrna

2 Central Dogma

3 Animation DNA: The Secret of Life (from PBS) v=41_ne5ms2ls&list=pl2b2bd56e908da696&index=3 Transcription chapter3/animation mrna_synthesis transcription quiz_1_.html Translation chapter3/animation how_translation_works.html

4 Gene The segment of DNA that is transcribed into RNA Also called a transcription unit

5 Stages of Transcription Initiation Elongation Termination

6 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

7 Transcription Initiation RNAP recognize and bind to promoter Key element in promoter is TATA box in eukaryote. DNA unwinds at the front of RNAP

8 Defining the strands Coding strand = sense strand = DNA strand that is not being copied (contains same sequence as new RNA except Tà U) Noncoding strand = antisense strand = template Transcript = newly synthesized RNA

9 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

10 Transcription Elongation RNAP unwinds DNA exposing bases Uses template strand to add complementary RNA nucleotides Direction of transcription: downstream from 5 à 3 of the transcript

11 Transcription Elongation RNA transcript separates from template as RNAP continues elongation DNA helix reforms behind RNAP Many RNAP can transcribe simultaneously

12 Transcription Elongation

13 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 nucleotide past signal pre-mrna is released

14 Transcription Termination

15 RNA Processing Precursor mrna = primary transcript: Pre-mRNA need additional modification before it become mrna RNA processing only occurs in eukaryotes

16 RNA Processing Post-transcriptional modification: Capping: 5 cap Polyadenylation: Poly A tail Splicing: intron excision

17 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

18 Post-Transcriptional Modification: Polyadenylation PolyA tail adenine added to 3 end Added by poly-a polymerase Function Protect mrna from degradation Facilitate export of mrna from nucleus

19 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:

20 Splicing Video General intro to splicing: Splicing mechanism: 3D animation -

21 Intron Function Regulatory role Alternative splicing: single gene encode more than one kind of polypeptide depending on which segments are treated as exons

22 Prokaryotic versus Eukaryotic protein synthesis Video: bio25.swf

23 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