Protein Synthesis - Transcription Notes

Protein Synthesis - Transcription

• One gene – one polypeptide.
• Transcription is the first step.
• Central dogma: DNA --> RNA --> protein.

One Gene – One Polypeptide Hypothesis

• Each gene is unique.
• Codes for a single polypeptide.

DNA to Proteins

• Two-step process:
  • Transcription: DNA transcribed into complementary RNA in the nucleus.
  • Translation: Assembly of amino acids into a polypeptide using RNA.

DNA vs. RNA

• DNA:
  • Double Helix
  • Deoxyribose sugar
  • Adenine pairs with Thymine (A-T)
  • Stays in the nucleus
• RNA:
  • Single strand
  • Ribose sugar
  • Uracil replaces Thymine!
  • Leaves the nucleus to make proteins

Three Types of RNA

• mRNA:
  • Messenger RNA.
  • Carries code for proteins from DNA.
  • Carries the codon.
• tRNA:
  • Transfer RNA.
  • Attaches specific Amino Acids to the protein chain by matching the mRNA codon with the anticodon.
• rRNA:
  • Ribosomal RNA.
  • Where Protein synthesis occurs.

Transcription Steps

• Initiation:
  • RNA polymerase binds to the promoter.
  • No primer needed.
  • Template strand is copied.
  • Coding strand has the same sequence as RNA (except T instead of U).
• Elongation:
  • DNA splits, and RNA polymerase adds matching bases to form a new RNA strand.
  • RNA made in the 5’→3’ direction, using the 3’→5’ DNA strand as a template.
• Termination:
  • RNA polymerase reaches the termination site.
  • RNA polymerase leaves the DNA.
  • RNA strand is released.
  • DNA rewinds.

Post-Transcriptional Modifications

• Pre-mRNA is modified before leaving the nucleus.
• Poly(A) tail: 50-250 adenine nucleotides added to the 3’ end, protecting mRNA from enzymes.
• 5’ cap: Seven guanine’s added to the start, which ribosomes recognize for attachment.
• Introns (non-coding) are removed by spliceosome, and exons (coding) are connected.

Alternate Splicing

• Certain Exons may be left out of the final mRNA strand.
• Enables different mRNAs from a single DNA gene sequence.
• Explains how humans produce approximately 100,000 proteins from about 20,000 genes.

Summary of Transcription

• DNA unravels, and mRNA is synthesized using RNA polymerase.
• Complementary mRNA is produced.