Crick’s Central Dogma of Molecular Biology

  • Central Concept: Describes the flow of genetic information within a biological system.

  • Key Processes:
      - DNA: Found in the nucleus in eukaryotic cells.
      - mRNA: Messenger RNA is the intermediary that connects DNA and protein synthesis.
      - Protein Synthesis: Takes place in the cytoplasm, primarily at the ribosomes.

  • Explanation of Central Dogma:
      - DNA to RNA: The process of transcription, where DNA codes for RNA.
      - RNA to Protein: The process of translation, where RNA is translated into proteins.

Overview of Processes

Transcription and Translation

  • Polypeptide: The end product of gene expression.

  • Ribosome: The cellular machinery that facilitates translation.

  • Function of mRNA: Carries genetic information from DNA to ribosomes for protein synthesis.

  • Pre-mRNA: Initial RNA molecule that undergoes processing to become mature mRNA in eukaryotes.

  • RNA Processing: Modifies the pre-mRNA to mature mRNA prior to translation.
      - Eukaryotes need to process pre-mRNA; prokaryotes directly transcribe mRNA.

Extended Central Dogma Process

  1. Transcription:
       - DNA transcribed into RNA.
         - Eukaryotes produce pre-mRNA; must process to mRNA.

  2. Translation:
       - mRNA translated into proteins.

Key Concepts – Transcription

  • Gene Expression:
      - Refers to the process of transcription being regulated, allowing genes to be turned on and off.
      - Expression varies across different cell types, resulting in a unique set of proteins per cell type.

  • RNA Polymerase:
      - Binds to the promoter region of DNA to initiate transcription.

  • Promoter: A specific DNA sequence where RNA polymerase binds, starting RNA synthesis.

  • Difference in Gene Structures: In eukaryotes, genes consist of:
      - Exons: Coding regions that will be expressed in the protein.
      - Introns: Non-coding regions intervening between exons that are spliced out during RNA processing.

  • RNA Processing Steps:
      - Capping and Tailing: Adding a 5’ cap and a 3’ poly(A) tail for protection and recognition during translation.

  • Location of Transcription:
      - Eukaryotic: Occurs in the nucleus.
      - Prokaryotic: Occurs in the cytosol.

Similarities and Differences in Transcription vs. DNA Replication

  • Similarities:
      - RNA polymerase creates an RNA strand in the 5′ to 3′ direction.

  • Differences:
      - RNA polymerase does not need a primer to start transcription.
      - Unlike DNA replication, helicase is not required; RNA polymerase unwinds the DNA strand.

Initiation of Transcription

General Overview

  • Has three critical phases: initiation, elongation, termination.

  • Prokaryotic Initiation:
      - Sigma Factor: A protein that binds to RNA polymerase, forming a holoenzyme and guiding it to specific promoter sequences on DNA.

  • Eukaryotic Initiation:
      - Basal Transcription Factors: Proteins that initially bind to the DNA promoter before RNA polymerase.
      - TATA Box: A crucial promoter component located about 30 bases upstream of the transcription start site.

  • Holoenzyme Composition: Composed of a core enzyme (RNA polymerase) and additional regulatory subunits.

Initiation & Elongation in Prokaryotes

  • The promoter comprises specific regions including the -10 box and -35 box, where sigma protein assists in the stabilization of the polymerase to the promoter.

  • Process of Initiating Transcription:
      1. RNA polymerase forms a holoenzyme by associating with the sigma.
      2. Holoenzyme binds to the promoter region.
      3. Template strand of DNA is read by RNA polymerase to transcribe RNA.

Termination of Transcription

  • Prokaryotic Termination:
      - The process is signaled by the formation of a hairpin structure in RNA, leading to dissociation of RNA polymerase from the transcript.

  • Eukaryotic Termination:
      - Involves a poly(A) signal which causes an enzyme to attach and cleave the pre-mRNA.
      - RNA polymerase continues transcription until it falls off the DNA template.

RNA Splicing in Eukaryotes

  • Process of Splicing: SnRNP complexes bind to pre-mRNA and excise introns while ligating exons.

  • Formation of Spliceosome: A complex of small nuclear ribonucleoproteins (snRNPs) that facilitates the splicing process.

Adding a Cap and a Tail (Eukaryotes only)

  • 5’ Cap: Provides a binding site for ribosomes and is essential for translation initiation.

  • Poly(A) Tail:
      - Composed of 100-250 adenine nucleotides; protects mRNA from degradation.
      - Also aids in stabilization and initiation of translation in the cytoplasm.