6.3 - Transcription and RNA Processing

Gene Expression

Definition: The process by which DNA directs the synthesis of proteins.

• Stages:

  1. Transcription: The synthesis of RNA using information from DNA.

  2. Translation: The synthesis of proteins using information from RNA.

• Universal Process: Occurs in all organisms.

Central Dogma of Molecular Biology

Pathway: DNA -> RNA -> Protein

• Transcription: DNA is transcribed into RNA.

• Translation: RNA is translated into protein.

Transcription

Definition: The synthesis of RNA using DNA as a template, allowing the genetic "message" to be transcribed from DNA to RNA.

• Location:

  1. Eukaryotes: Occurs in the nucleus.

  2. Prokaryotes: Occurs in the cytosol.

• Key RNA Molecules:

  1. Messenger RNA (mRNA): Carries genetic information from the DNA in the nucleus to the ribosomes in the cytoplasm.

  2. Ribosomal RNA (rRNA): Forms the core of the ribosome's structure and catalyzes protein synthesis.

  3. Transfer RNA (tRNA): Brings amino acids to the ribosome and matches them to the coded mRNA message.

The Genetic Code

Definition: The sequence of bases in DNA that codes for proteins.

• Triplet Code: The genetic code is read in groups of three nucleotides called codons.

• Template Strand: The DNA strand used to build complementary RNA bases in the 5’ to 3’ direction.

  • Base Pairing:

    • A pairs with U (in RNA)

    • T pairs with A

    • G pairs with C

    • C pairs with G

Transcription Process

Single-Strand Transcription:

• During transcription, only one DNA strand is used as a template to synthesize RNA.

• Template Strand: Also known as the noncoding or antisense strand.

Pre-mRNA Processing in Eukaryotes

Pre-mRNA:

• After the initial transcription of DNA to RNA, the RNA product is considered pre-mRNA in eukaryotes.

• Modifications Needed: Pre-mRNA undergoes several modifications before it is translated into protein.

RNA Processing in Eukaryotic Cells

Before mRNA leaves the nucleus in eukaryotic cells, it undergoes several modifications to become mature mRNA. These modifications ensure that the mRNA is stable, protected, and able to be properly translated into proteins.

Key Modifications

1. 5' Cap Addition:

   • What: A modified guanine nucleotide (GTP) is added to the 5' end of the pre-mRNA.

   • Purpose:

     • Helps the mature mRNA leave the nucleus.

     • Protects the mRNA from degradation in the cytosol.

     • Assists ribosomes in attaching to the 5' end of the mRNA for translation.

2. Poly-A Tail Addition:

   • What: A string of 50-250 adenine nucleotides is added to the 3' end of the pre-mRNA.

   • Purpose:

     • Similar to the 5' cap, it helps the mRNA exit the nucleus.

     • Provides protection against enzymatic degradation.

     • Facilitates ribosome binding for translation.

RNA Splicing

RNA Splicing:

• Process: Introns (non-coding regions) are removed from the pre-mRNA, and exons (coding regions) are joined together.

• Introns: Intervening sequences that do not code for amino acids.

• Exons: Expressed sequences that code for amino acids.

Why Splicing Occurs:

• Alternative Splicing: A single gene can produce multiple types of polypeptides, allowing for greater protein diversity. Different combinations of exons can be spliced together to create different mRNA molecules from the same pre-mRNA.

Final Steps

• Mature mRNA: Once the 5' cap, poly-A tail, and splicing are completed, the pre-mRNA becomes mature mRNA.

• Export: The mature mRNA is then transported out of the nucleus into the cytoplasm, where it will be translated into protein by the ribosomes.