Gene Expression and Regulation Notes

Gene Expression Overview

• Gene expression (or gene regulation) is the process where information flows from genes to proteins.
• It involves the control of which genes are transcribed (copied to RNA) and translated (converted to proteins).
• This process determines which genes are activated or not in each type of cell.

Operons in Prokaryotes

• Operon: A cluster of related genes with a shared promoter and operator, controlling transcription in prokaryotic cells.
  • Key Components of an Operon:
  • Genes: Group of genes that are transcribed together.
  • Promoter: The site where RNA polymerase attaches to begin transcription.
  • Operator: Located between the promoter and genes, determines whether RNA polymerase can bind to the promoter.

Example: Lac Operon in E. coli

• E. coli uses 3 enzymes to absorb and break down lactose, coded by 3 genes located together.
• Inducible Operon: Typically off, can be induced to turn on by the presence of lactose.
  • Repressor Protein:
  • Binds to the operator to block RNA polymerase from attaching.
  • Produced by a different gene elsewhere on the DNA.
  • Role of Lactose:
  • When lactose is present, it binds to and alters the repressor’s shape, preventing it from binding to the operator, resulting in the operator being “on.”

Repressible Operon

• Example: Tryptophan (Trp) Operon:
  • Typically active (genes are being transcribed).
  • Tryptophan (the end product) activates the repressor, allowing the bacteria to stop gene expression for producing tryptophan when it is already available in sufficient amounts.

Gene Expression in Eukaryotes

• Control of Transcription:
  • In eukaryotes, RNA polymerase needs help from several molecules:
  1. Activator Proteins: Bind to enhancer DNA sequences.
  2. DNA Bending Proteins: Bring activators closer to the promoter.
  3. Transcription Factor Proteins: Interact with activators and bind at the gene's promoter to assist in RNA polymerase attachment, initiating transcription.

Genetic Diseases and Gene Expression

• Genetic diseases may arise due to:
  • Alleles that do not produce the correct protein.
  • Correct alleles that are not properly activated or turned on.

X Inactivation in Females

• In female mammals, one of the two X chromosomes becomes inactive, forming a Barr body.
• The active X chromosome is the one that gets transcribed.
• Which X chromosome becomes inactive is random and occurs during fetal development.
• Example: Tortoiseshell and calico cats display this random inactivation of X chromosomes, leading to different fur coloration based on which X is active in each cell.