Lecture: Transcription

Introduction to Transcription

• Definition: Transcription is the first step in using DNA's genetic code to synthesize proteins.
• Location: Takes place in the nucleus but proteins are synthesized outside the nucleus (ribosomes in cytoplasm or rough ER).
• Purpose: To carry genetic instructions from DNA to ribosomes using mRNA.

Process of Transcription

• RNA Polymerase: An enzyme that initiates transcription by attaching to a gene (segment of DNA).
  • Gene: Contains the code to build a specific protein.
  • Protein: Made of a sequence of amino acids in a specific order.

Role of Codons

• Codon: A group of three consecutive nitrogenous bases in a gene.
  • Codes for a specific amino acid.

Transcription Steps

1. Unwinding DNA: RNA polymerase unwinds and separates DNA into two strands.
   • Template Strand: The side of DNA transcribed by mRNA.
   • Non-template Strand: Not transcribed.
2. Base Pairing Rules:
   • RNA polymerase reads the template strand and assembles free nucleotides into complementary RNA.
   • Base pairing follows:
     • Thymine (DNA) pairs with Adenine (RNA).
     • Cytosine pairs with Guanine.
     • Adenine pairs with Uracil (instead of Thymine).
3. Completion: The resulting mRNA is a copy of the non-template DNA strand with uracil replacing thymine.

Function of mRNA

• Messenger RNA (mRNA): Carries the transcribed genetic code out of the nucleus to ribosomes for protein synthesis.
• Nuclear Pore: Allows mRNA to exit the nucleus.

Summary

• Transcription: Copying the genetic code for protein synthesis into mRNA.
• Gene: DNA segment containing protein-building instructions.
• Codon: Codes for an amino acid.
• RNA Polymerase Function: Unwinds DNA, assembles mRNA following base pair rules.
• Result: mRNA is a replica of the DNA non-template strand with uracil instead of thymine.

Next Steps

• Translation: The process of building proteins at the ribosome (covered separately).