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.