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Transcription. Central Dogma. DNA RNA Protein. Transcription. Translation. The Players. mRNA : messenger RNA, transports DNA “code” from nucleus to ribosome tRNA : transfer RNA, transfers amino acids from cytoplasm to ribosome Codons : sequence of 3 bases that code for one amino acid

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  1. Transcription

  2. Central Dogma DNA RNA Protein Transcription Translation

  3. The Players • mRNA: messenger RNA, transports DNA “code” from nucleus to ribosome • tRNA: transfer RNA, transfers amino acids from cytoplasm to ribosome • Codons: sequence of 3 bases that code for one amino acid • Start Codon: Codon that signifies the beginning of a polypeptide

  4. Players, continued • Stop Codon: signifies the end of a polypeptide • Amino acids: Building blocks of polypeptide • Ribosome: Organelle that provides site for protein synthesis (translation) • RNA polymerase: Enzyme that makes RNA using DNA template.

  5. Gene Promoter Sequence, TATA box Start Codon Coding Region Stop Codon Termination Sequence Sense Strand TATA TAT ATA TA TAC CGC ATA TCGGGT ATT GCC TAG AAC T ATAT ATA TAT AT ATG GCG TAT AGCCCA TAACGG ATC TT G A DNA Anti- Sense Strand A Closer Look at a Gene

  6. Transcription: Step 1: Initiation • RNA polymerase attaches to a section of the promoter region of DNA, called the TATA box • long sequence of repeating bases (usually A and T – only 2 H-bonds to break) • upstream of start codon

  7. Step 2: Elongation • RNA polymerase works 5’ to 3’, starting from start codon (promoter not copied) • Only copies 1 DNA strand, called the sense (or template) strand. • Other strand called the anti-sense strand. • Either strand of DNA can be sense strand. • RNA base sequence created is identical to anti-sense strand, except U is present instead of T

  8. Step 3: Termination • Stop codon is transcribed, RNA lets go • Termination sequence tells RNA polymerase to let go. • DNA zips back up

  9. RNA  mRNA (in eukaryotes) • Fig 8.13, p 262 • 5’ cap is added (made of GTP) • 3’ polyA tail is added (lots of Adenines added) • Give protection against catabolic enzymes in cytoplasm

  10. RNA  mRNA continued… • Introns are excised by spliceosome enzyme & RNA complex • Fig 8.11, p. 260 • Alternative splicing patterns can produce more than 1 type of protein. • Fig 8.12, p. 261

  11. Last, but not least • No proofreading of mRNA • BUT • More than 1 codon codes for one amino acid (except Start) • Many transcripts are made from the same gene and don’t last long in the cytoplasm • So, mistakes are limited

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