Gene Expression and Structure

• Gene expression uses information from a gene to synthesize a functional gene product.
• Functional gene products are often proteins, but can also be functional RNA molecules like tRNA or rRNA.
• Gene expression is a tightly regulated and dynamic process.
• It allows a cell to produce specific proteins when and where they are needed.
• Gene expression responds to intracellular and extracellular cues.

Gene Structure

• Genes have information needed to produce functional products.
• Genes include coding (exons) and non-coding regulatory regions (introns, promoters, enhancers).
• The promoter is a DNA sequence where RNA polymerase binds to initiate transcription.
• Enhancers are DNA sequences that increase the rate of transcription, even when located far from the promoter.
• A gene's structure and organization directly influence how they are expressed.

Transcription

• Transcription copies a gene's DNA sequence into an RNA molecule.
• RNA polymerase catalyzes transcription.
• RNA polymerase binds to the promoter region of the gene to start transcription.
• In eukaryotes, transcription occurs in the nucleus.
• The resulting RNA molecule, called pre-mRNA, undergoes processing steps to become mature mRNA.
• Processing steps include:
  • Capping, which is the addition of a modified guanine nucleotide to the 5' end.
  • Splicing, which is the removal of introns.
  • Polyadenylation, which is the addition of a poly(A) tail to the 3' end.
• Transcription factors are proteins that help regulate transcription.
• Transcription factors regulate transcription by binding to specific DNA sequences or interacting with RNA polymerase.

Translation

• Translation synthesizes a protein from an mRNA template.
• Translation occurs on ribosomes in the cytoplasm.
• tRNA molecules bring amino acids to the ribosome, matching them to the mRNA codons.
• Each codon (a sequence of three nucleotides) specifies a particular amino acid.
• Translation involves three main steps:
  • Initiation is the assembly of the ribosome and mRNA.
  • Elongation is the addition of amino acids to the growing polypeptide chain.
  • Termination is the release of the polypeptide chain.
• After translation, proteins may undergo post-translational modifications, such as folding, glycosylation, or phosphorylation.

Regulation of Gene Expression

• Gene expression can be regulated at various levels, including:
  • Transcriptional control.
  • RNA processing control.
  • Translational control.
  • Post-translational control.

Transcriptional Control

• Transcriptional control is a major mechanism for regulating gene expression.
• Transcriptional control involves the action of transcription factors.
• Activators enhance transcription.
• Repressors inhibit transcription.
• The binding of transcription factors to DNA can be influenced by signals from the environment, such as hormones and nutrients, along with developmental cues.
• In prokaryotes, genes are often organized into operons, which are regulated by a single promoter.
• The lac operon in E. coli is a classic example of transcriptional control, where the presence or absence of lactose affects gene expression.

RNA Processing Control

• RNA processing control involves the regulation of splicing, capping, and polyadenylation.
• Alternative splicing allows a single gene to produce multiple different mRNA transcripts and, therefore, different proteins.
• Alternative splicing increases the diversity of proteins that can be produced from a limited number of genes.

Translational Control

• Translational control regulates the rate at which mRNA molecules are translated into proteins.
• Mechanisms of translational control include:
  • mRNA stability (how long the mRNA molecule lasts).
  • Ribosome binding.
  • Regulatory RNAs, such as microRNAs.
• MicroRNAs (miRNAs) are small, non-coding RNA molecules that bind to mRNA molecules and inhibit their translation or promote their degradation.

Post-Translational Control

• Post-translational control regulates the activity and stability of proteins after they have been synthesized.
• Post-translational control can involve:
  • Protein folding.
  • Protein modification, such as phosphorylation, glycosylation, and ubiquitination.
  • Protein degradation.
• Ubiquitination is the process of attaching ubiquitin molecules to a protein, which can signal its degradation by the proteasome.

Epigenetics

• Epigenetics involves changes in gene expression that do not involve alterations to the DNA sequence itself.
• Epigenetic modifications can include:
  • DNA methylation, which is the addition of a methyl group to DNA.
  • Histone modification, which is chemical modifications to histone proteins.
• These modifications can alter the accessibility of DNA to transcription factors and RNA polymerase.
• Epigenetic changes can be influenced by environmental factors and development.
• Epigenetic changes can be passed down through cell divisions (mitosis) and, in some cases, across generations (meiosis).

Significance of Gene Expression

• Gene expression is essential for:
  • Cellular differentiation.
  • Development.
  • Adaptation to the environment.
• Dysregulation of gene expression can lead to:
  • Diseases, including cancer.
  • Developmental disorders.
• Understanding gene expression is crucial for:
  • Developing new therapies for diseases.
  • Understanding the complexity of biological systems.