Differences Between Introns and Exons

Key Definitions

• Gene: A section of DNA that contains information to construct a protein.
• Transcription: Process of using a DNA template to create a strand of RNA (in this case, messenger RNA).

Structure of a Gene

• A gene is composed of both introns (red) and exons (blue).
• Base Pairing: A pairs with T and C pairs with G in DNA.

Transcription Process

1. Initiation: RNA polymerase enzyme attaches to DNA and breaks hydrogen bonds between bases.
2. RNA Strand Growth:
   • As RNA polymerase moves along the DNA:
     • DNA G pairs with C in RNA.
     • DNA T pairs with A in RNA.
     • DNA A pairs with U in RNA.
     • DNA C pairs with G in RNA.
3. Formation of Pre-mRNA: Both introns and exons are transcribed, leading to the creation of pre-mRNA.
   • Pre-mRNA contains both introns and exons.

Processing Pre-mRNA

• Introns: Sections that do not code for a protein and are removed.
• Exons: Sections that code for proteins and are expressed.

Steps to Create Mature mRNA

1. Removal of Introns: Introns are discarded by proteins that splice them out.
2. Joining of Exons: Exons are spliced together to form a continuous sequence.
3. Result: The final product is mature mRNA which will move to a ribosome for translation.

Translation Process

• Location: Occurs at the ribosome.
• Codon Recognition:
  • Start codon: AUG (methionine).
  • Codons such as GCU (alanine) and UUU (phenylalanine) follow.
  • Ends when a stop codon (UGA) is reached.
• Outcome: A protein is synthesized based on the instructions from exons only, while introns are removed and recycled.

Summary

• Introns: Non-coding sections that are removed during mRNA processing.
• Exons: Coding sections that remain and are translated into proteins.

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