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05 transcription

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05 transcription

  1. 1. Transcription Four major steps to transcription: 1. Initiation 2. Elongation 3. Termination 4. Posttranscriptional Modification
  2. 2. Parts of a Gene promoter - DNA sequences which indicate the location of a gene Promoters are located upstream from the DNA region that contains the information to be transcribed into mRNA gene promoter transcription region termination sequence
  3. 3. Gene Orienation upstream negative numbers +1 downstream positive numbers start transcription RNA polymerase – transcription enzyme (synthesizes mRNA in 5’  3’ direction) Requires upstream, promoter region to determine where to start mRNA transcription
  4. 4. 1. Transcription Initiation Promoter regions are often sequences of A’s and T’s 2 H-bonds between A&T easier to break than 3 H-bonds between G&C many eukaryotic genes have a TATA box dsDNA (double stranded DNA) needs to be opened for mRNA to be synthesized. RNA polymerase opens the dsDNA.
  5. 5. transcription factors numerous protein factors are involved in starting transcription Some of these proteins help control how often genes are transcribed
  6. 6. 2. Transcription Elongation RNA polymerase synthesizes mRNA in the 5’  3’ direction; very similar to DNA synthesis no primer is necessary template strand - only one strand of the DNA is transcribed
  7. 7. 2. Transcription Elongation DNA 5’ A T T A C G A T C T G C A C A A G A T C C T 3’ 3’ T A A T G C T A G A C G T G T T C T A G G A 5’ mRNA 5’ A U U A C G A U C U G C A C A A G A U C C U 3’ DNA SENSE STRAND ANTISENSE STRAND The template strand is the antisense strand
  8. 8. 3. Transcription Termination RNA polymerase stops transcribing once it reaches the termination sequence enzyme dissociates with DNA strand and binds to another promoter sequence Termination sequences differ between prokaryotes and eukaryotes
  9. 9. Transcription Art in biology: Electron micrograph of a gene being transcribed into many pieces of mRNA at simultaneously. Called a transcription tree.
  10. 10. Transcription
  11. 11. Transcription Animation Transcription Animation
  12. 12. 4. Posttranscriptional Modification Modification of the mRNA transcript is only conducted in eukaryotic organisms. Why? This modification occurs before the transcript leaves the nucleus through the nuclear pores.
  13. 13. 4. Posttranscriptional Modification primary transcript – initial eukaryotic mRNA transcript, before modification Three types of modifications: 1. 5’ cap 2. poly-A tail 3. intron excision
  14. 14. 4. Posttranscriptional Modification The 5’ cap is the addition of a 7-methylguanosine molecule. Its primary function is to protect the mRNA strand from degradation.
  15. 15. 4. Posttranscriptional Modification The poly-A tail is an addition of ~ 200 adenine ribonucleotides to the end of the RNA. It also helps prevent mRNA degradation.
  16. 16. primary transcript 5’ 3’ 5’ cap added by capping enzyme complex mG 3’ poly-A tail added by poly-A polymerase mG AAAAAAA
  17. 17. 4. Posttranscriptional Modification Eukaryotic genes are longer than prokaryotic genes most of this extra DNA signals when and how often genes should be transcribed  regulatory DNA Primary transcript is longer than necessary exons – RNA sequences that will be expressed; helps makes the protein introns – interfering RNA sequences; need to be removed before translation
  18. 18. intron exon intron exon intron exon intron mG AAAAAAA introns removed by spliceosome proteins exon mG exon exon AAAAAAA mRNA transcript
  19. 19. 4. Posttranscriptional Modification Spliceosomes are a series of small nuclear ribonucleoproteins (snRNP) that work together to remove introns. snRNPs recognize specific sequences on the introns cuts out intron sequences splices exon sequences together
  20. 20. 4. Posttranscriptional Modification

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