Eukaryotic Gene Expression

• Organisms turn genes on and off which is called Gene Expression
  • This can be done in response to external and internal signals
    • These signals are based off of environmental factors
  • This is also be done in order to specialize cells
    • Certain cells need certain genes to preform their specific role

### Differential Gene Expression

• Human Cells can express about 20% of it's protein coded genes at any given time
• Most cells contain the same genome
  • Each cell type must use specific parts of this genome
    • This is called Differential gene expression
  • Exception would be cells of the immune system
• Due to the importance of gene expression when it has issues it can affect the organism significantly
• Process of Gene expression in a Eukaryotic cell
  • Chromatin (DNA unpacking) –>
  • RNA processing –>
  • Transport to cytoplasm –>
  • Translation –>
  • Protein processing –>
  • Transport to cellular destination–>
• This process can often be equated to transcription for Prokaryote cells

• The chromatin structure itself allows for the regulation of gene expression
  • This is partially due to the location of the promoter
• Chemical modifications to the histone proteins can affect the structure
  • This in turn can affect gene expression
  • Histone proteins are the proteins in which the DNA is wrapped
  • There are many types of modifications that can take place
    • Histone acetylation can tend to promote transcriptions by opening up the chromatin
    • Additional methyl groups tend to close up the chromatin and decrease transcription
  • DNA methylation occurs in most plants and animals as well as fungi
  • Methylated DNA will stay methalated through cell divisions
    • This accounts for genomic imprinting
    • These epigenetic markers can be inherited
      • There is continually more evidence for the importance of epigentics in gene expression ### Regulation of Transcription
• Chromatin changes are not permanent and can be reversed
• The next step of gene expression regulation is in the transcription factors
  • These either allow for or inhibit transcription
• These factors usually bind to proteins, but some of them bind to DNA
• High levels of transcription factors created for specific genes are associated with another protein thought creatively of as specific transcription factors
• Gene expression is dramatically increased or decreased by the binding of specific transcription factors
  • These are either activators or repressors
• There are many transcription factors
  • Repressors act in many different ways, but some bind directly to control element DNA blocking activator binding
  • Others interfere with the activator itself
• Coordinated control of genes can need to happen when multiple genes need to be expressed at the same time for something to function
  • These can often be signaled from the outside with something like a hormone
• The activation of receptors on the surface of the cell can release specific repressors and activators

### Mechanisms of Post-Transcriptional Regulation - Transcription is not the only thing that regulates gene expression - How much of the protein is created once the RNA is received is also a factor - RNA can be interpreted in different ways with different things being introns and others being exons - This allows for the creation of multiple proteins from the same strand of RNA - RNA splicing is critical since it allows a lot of information to be fit on a single strand of RNA - Around 75-100% of human genes with multiple exons undergo RNA splicing allowing for our genome to describe a lot of complexity without needing as many genes - Translation is another stage at which gene expression occurs - Some regulatory proteins can bock translation of an mRNA by preventing attachment to a ribisome - Length by which an mRNA is around is also crucial - This can vary greatly depending on the cell - Cells can mark proteins for destruction using something called ubiquitin