Gene Expression and Regulation

Promoters and Transcription Initiation

• Promoters are DNA sequences located upstream of the transcription start site.
  • RNA polymerase and transcription factors bind to promoters to initiate transcription.
  • The promoter region is often considered the "go" signal for transcription.
• Transcription initiation involves the interaction between:
  • Regulatory proteins
  • Regulatory genes
  • Transcription factors.
  • DNA sequence of the gene
  • Promoter region.

Regulation of Gene Expression

• The interactions between the DNA sequence, regulatory genes, and transcription factors determine whether a gene is transcribed.
• The specific combination of genes expressed determines the organism's phenotype.
  • Differential gene expression leads to different cell products and functions.
• Negative regulatory molecules can inhibit gene expression by blocking transcription.
  • These molecules prevent RNA polymerase from binding to the promoter.

Differential Gene Expression in Cells

• Cells within the same organism possess the same DNA.
• Only specific tissues contain activator proteins that activate regulatory genes.
• The phenotype of a cell or organism is determined by the combination of genes expressed.
• Example: Regulatory proteins found in the liver activate specific genes, enabling RNA polymerase to bind and initiate transcription.

Role of Small Molecules in Gene Regulation

• Small RNA fragments play a regulatory role in gene expression.
  • Encoded by DNA.
• Functions of small RNA fragments:
  • Break down messenger RNA (mRNA) in the cytoplasm by binding to it which prevents translation.
  • Block translation by binding to mRNA and preventing the ribosome from reading and completing translation.

Practice Question

• Question: Eye tissue cells contain regulatory proteins that interact with transcription factors to initiate transcription.
• Correct Answer: B, because the regulatory proteins are able to interact with a particular regulatory gene, and there is no inhibition occurring, allowing RNA polymerase to bind to the promoter and transcribe the gene. Because of the action of the tissue-specific regulatory factors interacting with transcription factors, transcription can happen.

Key Takeaways

• Promoters are upstream sequences where RNA polymerase and transcription factors bind to initiate transcription.
• The interaction between promoters and transcription factors determines phenotypic differences between tissues in an organism or between different organisms.
• Phenotypic differences in cells and organisms result from the combination of genes expressed.
• Negative regulatory molecules inhibit gene expression by binding to DNA and blocking transcription.
• Small RNA molecules regulate gene expression post-transcription by:
  • Blocking translation.
  • Breaking down messenger RNA.