6.2 Transcription - DNA to mRNA

UNIT 3: MOLECULAR GENETICS

Lesson 5: Protein Synthesis - Transcription

• Describe the three stages involved in transcription:

  • Initiation

  • Elongation

  • Termination

• Understand the role of enzymes and proteins in the molecular events of transcription

Central Dogma of Gene Expression

• Represents the flow of genetic information:

  • DNA → Replication → Transcription → RNA → Translation → Protein

• Important processes:

  • Transcription: Synthesis of mRNA from a DNA template

  • Translation: Synthesis of protein from an mRNA template

Overview of Gene Function

Transcription

• Occurs in the nucleus.

• The purpose is to produce a copy of a small section (gene) of DNA.

• The result is mRNA, which serves as a middleman for protein synthesis.

• Analogy:

  • Copying a recipe from a cookbook (DNA) to an index card (mRNA) for baking (protein synthesis).

DNA vs. RNA

Characteristics

• DNA (Deoxyribonucleic acid):

  • Nitrogenous bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C)

• RNA (Ribonucleic acid):

  • Nitrogenous bases: Adenine (A), Uracil (U), Guanine (G), Cytosine (C)

• Key Differences:

  • RNA contains uracil instead of thymine

Stages of Transcription

Initiation

• Process begins when RNA Polymerase binds to the promoter region of the DNA.

• The promoter is a sequence indicating where transcription starts and is often characterized by a TATA box.

  

• Benefits of the A-T rich promoter:

  • A and T share only 2 hydrogen bonds, making them easier to separate than C and G which share 3.

• RNA Polymerase unwinds the DNA strands and initiation occurs.

Elongation

• Transcribing the Gene:

  • Only one DNA strand (template strand or antisense strand) is transcribed into mRNA.

    

• RNA polymerase synthesizes mRNA in the 5' to 3' direction, incorporating RNA nucleotides, where T is replaced with U.

• Another RNA polymerase can bind to the promoter to initiate the synthesis of a second mRNA strand simultaneously.

Termination

• Transcription continues until the terminator sequence in the DNA is reached.

• RNA Polymerase detaches and the mRNA is released, allowing the DNA helix to re-form.

  

Post-transcriptional Modifications

Overview

• In eukaryotes, the primary transcript (pre-mRNA) must undergo modifications before leaving the nucleus.

Modifications Include:

1. Addition of a 5' cap

   • A modified guanine (7-methylguanosine) is added to the 5' end.

   • Functions:

     • Protects mRNA from degradation.

     • Aids in translation initiation.

       

2. Addition of a Poly-A tail

   • About 200 adenine nucleotides are added to the 3' end (Poly-A tail).

   • Function:

     • Also protects mRNA from degradation and helps in cytoplasmic transport.

       

3. Removal of Introns

   • Introns: Non-coding regions

   • Exons: Coding regions that will be translated into proteins.

   • Spliceosomes (complexes of snRNA and proteins) recognize and remove introns, joining exons to form mature mRNA.

     

Summary of Modifications

• The primary transcript is capped, tailed, and introns are excised

• Mature mRNA exits the nucleus through nuclear pores.

• Ready for translation into a protein.

Potential Errors

• Unlike DNA replication, there are no proofreading mechanisms during transcription, leading to a higher incidence of errors.