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Transcription. Protein Synthesis: Ch 17. One Gene – One protein.

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  1. Transcription Protein Synthesis: Ch 17

  2. One Gene – One protein • In 1902, Archibald Garrod described the inherited disorder alkaptonuria as an "inborn error of metabolism." He proposed that a gene mutation causes a specific defect in the biochemical pathway for eliminating liquid wastes. The phenotype of the disease — dark urine — is a reflection of this error. • (from DNA from the Beginning)

  3. Beadle & Tatum • Investigated bread mold • Found that mutated bread mold could not produce essential nutrients,and would die. • However, if the mutated mold was provided with a specific supplement, the mold could grow. • Each mutant was defective for a particular gene, so they concluded that the function of the gene was to determine the production of a particular enzyme.

  4. Central Dogma of Molecular Biology How are genes connected to proteins?

  5. Triplet code Codon –a triplet - made of 3 consecutive nucleotides – found on mRNA each codon codes for a specific amino acid 20 amino acids – 64 codons (3 are STOP codons) Codons are read in the 5  3 direction along mRNA Determined from triplet on DNA (complementary)

  6. The reading frame is critical for codons: • i.e. “The fat cat ate the wee rat” • Vs. “Thefatcatatet hew eer at”

  7. Transcription • mRNA is transcribed from DNA Downstream = direction of transcription

  8. Steps for Transcription • Initiation • RNA polymerase binds to promoter • Prokaryotes – direct binding • Eukaryotes- transcription factors bind first, then RNA polymerase • The promoter is a specific sequence of DNA where the RNA polymerase attaches (includes a TATA box)

  9. Steps for Transcription • Elongation • RNA polymerase untwists DNA, read from 3’-5’, elongates mRNA from 5’-3’ • RNA nucleotides have ribose instead of deoxyribose, and uracil instead of thymine • NTPs are used, so cleavage of two phosphates powers transcription

  10. Steps for Transcription • Termination • When terminator sequence in DNA is reached, the transcribed terminator (in RNA) signals the end • Prokaryotes – stops at end of termination signal • Eukaryotes –transcribes a polyadenylation sequence that codes for the polyadenylation signal (AAUAAA) • The pre mRNA is cut about 10-35 nucleotides past AAUAAA sequence

  11. Transcription videos • http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter3/animation__mrna_synthesis__transcription___quiz_1_.html • http://www.dnalc.org/resources/3d/12-transcription-basic.html

  12. Eukaryotic – RNA processing • Eukaryotes modify pre-mRNA before it leaves the cytoplasm • Pre -mRNA – the immediate product of RNA polymerase in eukaryotes

  13. Alteration of mRNA ends • GTP cap – A modified guanosine triphosphate is added to the 5’ end • (also known as a “methyl cap”, 7-methylguanylate cap) • Poly (A) tail – 50-250 adenine nucleotides added to 3’ end • Functions for both: • Helps mRNA leave nucleus • Protects mRNA from degradation • Help ribosomes attach to 5’ end, once mRNA in cytoplasm

  14. mRNA processing • http://www.youtube.com/watch?v=YjWuVrzvZYA

  15. RNA splicing • Pre mRNA has • Introns – intervening sequences • Exons – expressed sequences • The introns are removed and the exons are spliced together • Spliceosome – does the cutting & splicing • Made of snNRP- small nuclear ribonucleoproteins + larger proteins

  16. Alternative splicing • A single gene can code for more than one kind of polypeptide • Depends on what segments are treated as exons during RNA splicing

  17. mRNA splicing • http://www.hhmi.org/biointeractive/mrna-splicing • http://www.youtube.com/watch?v=aVgwr0QpYNE

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