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

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Protein Synthesis: Ch 17

one gene one protein
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)
beadle tatum
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.
central dogma of molecular biology
Central Dogma of Molecular Biology

How are genes connected to proteins?

triplet code
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)


The reading frame is critical for codons:

      • i.e. “The fat cat ate the wee rat”
        • Vs. “Thefatcatatet hew eer at”
  • mRNA is transcribed from DNA

Downstream = direction of transcription

steps for transcription
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)
steps for transcription1
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
steps for transcription2
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
transcription videos
Transcription videos
eukaryotic rna processing
Eukaryotic – RNA processing
  • Eukaryotes modify pre-mRNA before it leaves the cytoplasm
  • Pre -mRNA – the immediate product of RNA polymerase in eukaryotes
alteration of mrna ends
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
mrna processing
mRNA processing
rna splicing
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
alternative splicing
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
mrna splicing
mRNA splicing