Untitled

Crick’s Central Dogma of Molecular Biology

  • DNA
      - Found in the nucleus (in eukaryotic cells)

  • mRNA
      - Protein synthesis takes place in the cytoplasm
      - Ribosome

  • Central Dogma of Molecular Biology
      - Based on Francis Crick’s work
      - States the following sequences of information transfer:
        - DNA codes for RNA (process: transcription)
        - RNA codes for proteins (process: translation)

Transcription and Translation Overview

  • Transcription
      - The process of copying DNA to create mRNA
      - In eukaryotes, pre-mRNA is processed to form mRNA before translation

  • Translation
      - The process of converting mRNA into proteins
      - Occurs at the ribosomes

Extended Central Dogma

  1. DNA is transcribed to make RNA
       - Pre-mRNA is processed to make mRNA (eukaryotes only)

  2. mRNA is translated to make a protein
       - DNA transcribed by enzyme RNA polymerase
       - In eukaryotes, RNA must be processed before it becomes mature mRNA
       - Ribosomes translate mRNA into protein

Prokaryotes vs. Eukaryotes

  • Prokaryotes
      - Bacterial cells
      - mRNAs are essentially mature post-transcription
      - Can be translated immediately

  • Eukaryotes
      - More complex cells
      - mRNA must undergo further processing before translation

Key Concepts – Transcription

  • Gene Expression
      - Transcription of an individual gene can be switched on and off
      - The set of genes expressed differs between cell types
      - Expression results in the complete array of proteins within a cell

  • Roles of RNA Polymerase
      - Binds to a promoter region on the DNA strand, near the gene to be transcribed
      - Transcription takes place in the nucleus (eukaryotes) and cytosol (prokaryotes)

  • Gene Structure in Eukaryotes
      - Contains regions called exons (coding sequences) and introns (non-coding sequences)
      - Intron splicing occurs during RNA processing
      - Pre-mRNA receives a 5' cap and 3' poly(A) tail during processing

Similarities and Differences in Transcription

  • Similarity: RNA polymerase measures ribonucleotides to the 3' end of the growing RNA strand—similar to DNA replication.

  • Differences:
      - RNA polymerase does not require a primer to start transcription.
      - Unlike DNA polymerase, RNA polymerase unzips the DNA without the assistance of helicase.

Phases of Transcription

  • Three Phases: Initiation, Elongation, and Termination

  • Initiation in Prokaryotes:
      - Protein sigma binds to RNA polymerase, forming a holoenzyme
      - The holoenzyme binds to DNA
      - Holoenzyme: A multimeric protein with catalytic and regulatory subunits
      - Sigma helps RNA polymerase locate specific promoter sequences
      - Promoter: Located at the start of a gene, crucial for transcription initiation
      - Various sigma proteins guide RNA polymerase to different promoters

  • Initiation in Eukaryotes: Basal transcription factors bind to the DNA promoter first, creating bindings for RNA polymerase
      - Eukaryotic promoters generally contain a TATA box, located approximately 30 bases upstream of the start site

Mechanism of Transcription Initiation in Prokaryotes

  1. Forming the Holoenzyme: RNA polymerase and sigma form
       - Holoenzyme = Core enzyme (RNA polymerase) + sigma

  2. Sigma Binding: Sigma recognizes and binds to the promoter
       - Promoter contains:
         - –35 box
         - –10 box
         - +1 site

  3. Binding Retention: The sigma/RNA polymerase holoenzyme binds to the promoter region.

Termination of Transcription

  • In Prokaryotes: The termination signal forms a hairpin structure in the RNA sequence, causing RNA polymerase to dissociate from the mRNA transcript.

  • In Eukaryotes: A poly(A) signal in the DNA leads to enzyme binding that cuts the pre-mRNA transcript downstream of the signal.

  • RNA Polymerase Behavior: It continues transcription until it falls off the DNA template.

RNA Splicing in Eukaryotes

  • Pre-mRNA undergoes splicing to remove introns and join exons correctly.

  • Process involves snRNPs (small nuclear ribonucleoproteins):
      1. Bind to pre-mRNA
      2. Form a spliceosome
      3. Cut and join exons, release introns

Addition of a Cap and Tail (Eukaryotes Only)

  • Pre-mRNA gains a 5' cap and a poly(A) tail from different RNA processing enzymes:
      - 5' Cap:
        - Functions as recognition signal for ribosomes during translation initiation
      - Poly(A) Tail:
        - Consists of 100-250 adenine nucleotides
        - Protects mRNA from degradation and is also required for translation initiation.