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Transcription. May, 2013. The Central Dogma. DNA. Transcription. R NA. Translation. Protein. Transcription. Process in which cells make a copy of their genetic instructions DNA into RNA. Revisiting RNA. Uracil. Steps. Initiation. Elongation. Termination. Modification. Initiation.

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  1. Transcription May, 2013

  2. The Central Dogma DNA Transcription RNA Translation Protein

  3. Transcription • Process in which cells make a copy of their genetic instructions DNA into RNA

  4. Revisiting RNA Uracil

  5. Steps Initiation Elongation Termination Modification

  6. Initiation • RNA polymerase II (RNAP2) and a number of general transcription factors (TFs) bind to a promoter on the DNA • Promoter is located upstream of the gene • Promoter is rich in A and T (TATA box) • RNAP2 unwinds the DNA double helix and transcribes the template strand in a 5’ to 3’ direction

  7. RNA Polymerases

  8. Elongation • RNAP2 elongates the single-stranded mRNA as soon as the TFs bind to the promoter • Promoter does not get transcribed • RNAP2 does not need a primer • RNAP2 uses the template strand for mRNA synthesis; the other strand is the coding strand • RNA sequence is complementary to the template strand and identical to the coding strand, except T are replaced by U

  9. Termination • mRNA is synthesized until the end of the gene • RNAP2 stops when it recognizes a terminator sequence • mRNA dissociates from the DNA template strand • RNAP2 can bind to another promoter region to transcribe another gene • mRNA = primary transcript

  10. Modification • mRNA needs a 5’ cap and a poly-A tail to leave the nucleus • 5’ cap = protects mRNA from enzyme digestion as it enters the cytoplasm; also initiates translation • Consists of a 7-methyl guanosine • Poly-A tail = approx. 200 adenines added to the 3’ end by poly-A polymerase to protect it from degradation

  11. Splicing • mRNA contains exons (coding regions) and introns (noncoding regions) • Introns needs to be removed for translation and proper folding • Spliceosomes cut the introns and join the exons together to form a continuous mRNA molecule • mRNA with 5’cap, poly-A tail and exons = mRNA transcript; ready to be exported out of the nucleus to be translated into protein

  12. Errors • Unlike DNA replication, mRNA has no quality control and is more error-prone • DNAP error rate: 1 in 107 nucleotides • RNAP error rate: 1 in 104 nucleotides • However, errors in RNA transcripts are not detrimental because hundreds of transcripts are produced to compensate

  13. Summary Transcription: http://www.youtube.com/watch?v=icZjgZozkB8&feature=related Modification: http://www.youtube.com/watch?v=YjWuVrzvZYA&feature=related Splicing: http://www.youtube.com/watch?v=2gT1eAcK7T8&feature=related

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