In Depth Notes on DNA Transcriptions

DNA Transcriptions

Concepts of DNA and Genes
  • Gene Definition: A gene is a functional unit of heredity; it carries instructions for making proteins.
  • Analogy: DNA information encoding resembles how the alphabet forms words.
  • One Gene One Protein Hypothesis: Each gene corresponds to one protein; however, genes dictate protein's primary sequence, which determines its 1º, 2º, and 3º structures.
The Central Dogma of Molecular Biology
  • Definition: Central Dogma describes the flow of genetic information:
    • DNA → RNA → Protein
  • Exceptions: RNA viruses and non-coding RNAs (e.g., tRNA, rRNA) do not follow this flow.
Anatomy of a Gene
  • Key Regions:
    • Coding Sequence: Divided into exons (coding) and introns (non-coding).
    • Promoter Region: Upstream binding site for RNA polymerase to initiate transcription.
    • Terminator Sequence: Signals end of transcription.
    • Splicing Sites: Join exons after intron removal during RNA processing.
Mechanism of Gene Expression
  • Steps:
    1. Transcription: Copies DNA sequence into RNA.
    2. Translation: Converts RNA sequence into amino acid sequence.
  • In prokaryotes, mRNA transitions directly to translation; in eukaryotes, transcription occurs in the nucleus, followed by RNA processing.
Transcription Process
  • Stages:
    • Initiation: Begins at promoter with signaling from transcription start point.
    • Elongation: RNA polymerase synthesizes complementary RNA. Initiation progresses at 60 nucleotides/second.
    • Termination: Concludes transcription, releasing RNA transcript.
Transcription Initiation

  • In Prokaryotes:

    • Promoters contain conserved -10 & -35 sequences.
    • σ factor recognizes these sequences and assists RNA polymerase binding.

  • In Eukaryotes:

    • Initiates with TATA box ~25 nucleotides upstream.
    • The transcription initiation complex forms with RNA polymerase II and transcription factors.
Transcription Elongation
  • RNA polymerase travels 3’ to 5’ along DNA, adding RNA nucleotides and unwinding the DNA.
  • Nucleotide Addition: Added at 3’ end of the growing RNA strand.
Transcription Termination
  • Mechanisms:
    • Prokaryotes: Rho-independent (GC-rich hairpin forms causing dissociation) and Rho-dependent (Rho protein disrupts hybrid).
    • Eukaryotes: Mechanism remains unclear, without defined structures.
Differential Gene Expression
  • Variability in cell types relates to gene expression, with each differentiated cell expressing a unique subset of genes.
Protein Expression Types
  • Constitutive Expression: Continuous production regardless of conditions.
  • Inducible Expression: Activated by specific stimuli, often turned off and only on when necessary.
  • Repressible Expression: Typically active but can turn off in response to a metabolite.
Operons in Prokaryotes
  • Definition: Clusters of genes controlled by a single promoter, functioning together.
  • Components: Operator, repressor that interacts with operator to regulate transcription.
Lac Operon Activation
  • Involves a repressor that binds to the operator; when lactose (inducer) is present, it binds the repressor, allowing transcription to initiate.
Tryptophan Repression
  • Repressible regulation demonstrated by trp operon, where absence allows transcription and presence causes repressor activation.
RNA Structure and Modification
  • Introns and Exons: Non-coding regions (introns) removed during processing, while coding regions (exons) are expressed. Eukaryotes average 8.8 exons and 7.8 introns per gene.
  • Post-transcription Modifications: 5’ GTP cap and 3’ poly(A) tail added to mRNA for stability, splicing, and translation attachment.
Lifespan of mRNA
  • Varies between prokaryotes (short, <5 mins) and eukaryotes (minutes to hours or days), influenced by sequence and regulatory mechanisms.