Gene Regulation in Eukaryotes Notes

Gene Regulation in Eukaryotes

Promoters in Eukaryotic Genes

• Genes coding for proteins have specific regions known as promoters which include:

  • Transcriptional start site: where RNA synthesis begins
  • TATA box: a conserved sequence typically found about 25-30 bases upstream from the start site, designated as ‘-25’.
  • Regulatory elements: sequences that modulate gene expression, located around -50 to -100 from the start site.

• Core Promoter: The core promoter consists of the TATA box and the transcriptional start site. Genes with only a core promoter generally have a low (basal) level of transcription.

TATA Box

• The TATA box is crucial for the initiation of transcription:
  • Serves as a binding site for general transcription factors and RNA polymerase.
  • The position of the TATA box is vital as it influences where RNA synthesis begins.
  • TATAAA is a common sequence found in the TATA box.

Regulatory Elements

• Regulatory elements are essential for controlling gene expression:
  • Function to increase transcription levels when transcription factors (activators) bind to them.
  • These elements can exist as binding sites for specific transcription factors that are crucial for gene expression regulation.

Coordinate Gene Expression

• Coordinate regulation allows genes coding for related functions or that operate in the same pathway to be expressed simultaneously:
  • This occurs even when these genes are located on different chromosomes, which is unlike prokaryotic operons.
  • Genes that share similar regulatory elements can be activated by common stimuli, allowing for coordinated action.

Enhancers

• Enhancers are DNA sequences that can be located far from the transcription start site (even thousands of bases away) and play a key role in increasing transcription levels:
  • Activator proteins, a type of transcription factor, bind to enhancers.
  • The interaction between enhancers and promoters is facilitated by DNA bending, forming a complex that includes general transcription factors and RNA polymerase II.
  • Enhancers are unique to eukaryotes and provide an additional layer of transcriptional control.

Tissue-Specific Gene Expression

• Every cell in an organism contains the same DNA but produces different RNA and proteins based on:
  • Unique combinations of regulatory elements present in the enhancers specific to each tissue type.
  • Specific activator proteins bind to these regulatory elements, leading to varying patterns of gene expression across cell types.

Regulation of Chromatin Structure

• The structure of chromatin (the packaged form of DNA) plays a significant role in gene expression:

  • Euchromatin: Less condensed, associated with active genes (genes ON).
  • Heterochromatin: More condensed, associated with silent genes (genes OFF).

• The chromatin configuration can be modified to facilitate or repress transcription:

  • Histone Code: Modifications such as acetylation (which unwinds DNA and activates genes) and methylation (which can repress gene expression) change how tightly DNA is wound around histones.

Levels of Gene Regulation

• In eukaryotic cells, gene expression regulation can occur at various levels, including:

  • Transcriptional control: Initiation of RNA synthesis
  • RNA processing control: Modifications to RNA before translation
  • Translation control: Regulation of mRNA translation into protein
  • Protein activity control: Modifications affecting protein function post-translation

• The complexity of these regulatory mechanisms contributes to the diverse functionality of eukaryotic organisms.