Biology I for Non-Majors

1. Biology I for Non-Majors Gene Expression

2. Expression of Genes • Gene regulation is how a cell controls which genes, out of the many genes in its genome, are “turned on” (expressed) • Nearly all the cells in your body have the same DNA • Different patterns of gene expression cause your various cell types to have different sets of proteins, making each cell type uniquely specialized to do its job • Gene expression patterns are determined by information inside and outside the cell • Inside may include inherited proteins inherited, DNA damage, and amount of ATP • Outside may include chemical signals from other cells, mechanical signals from the extracellular matrix, and nutrient levels

3. Growth Factors are a Type of Outside Signal

4. Prokaryotic versus Eukaryotic Gene Regulation • Prokaryotic organisms primarily control what type of protein and how much of each protein is expressed by regulation of DNA transcription • Eukaryotic organisms regulate gene expression at all stages of transcription and translation • Epigenetic: DNA uncoiling and binding of transcription factors • Transcriptional: when RNA is transcribed • Post-transcriptional: RNA processing and movement to the cytoplasm • Translational: when RNA is translated • Post-translational: after a protein is made

5. Prokaryotic Gene Regulation • In bacteria and archaea, structural proteins with related functions are usually encoded together within the genome in a block called an operon and are transcribed together under the control of a single promoter

6. lac Operon • First operon identified in E. coli • Operons include regulatory region that includes promoter and surrounding areas where the transcription factors can bind • Transcription factors influence the binding of RNA polymerase • A repressor is a transcription factor that suppresses transcription of a gene in response to an external stimulus by binding to a DNA sequence within the regulatory region called the operator • An activator is a transcription factor that increases the transcription of a gene in response to an external stimulus by facilitating RNA polymerase binding to the promoter • An inducer is a small molecule that either activates or represses transcription by interacting with a repressor or an activator

7. Epigenetic Regulation • In eukaryotes, DNA is wound, folded, and compacted to fit inside the nucleus • The first level of organization, or packing, is the winding of DNA strands around histone proteins • If DNA encoding a specific gene is to be transcribed into RNA, the nucleosomes surrounding that region of DNA can slide down the DNA to open that specific chromosomal region and allow for the transcriptional machinery (RNA polymerase) to initiate transcription  • This type of gene regulation is called epigenetic regulation, or ‘around genetics’ • Changes that occur to the histone proteins and DNA do not alter the nucleotide sequence and are not permanent

8. Transcription Factors as Regulators • Transcription factors are proteins that bind to the promoter sequence and other regulatory sequences to control the transcription of the target gene in eukaryotes • Transcription factors must bind to the promoter region first and recruit RNA polymerase • Genes are organized to make transcriptional control easier • The promoter region is immediately upstream of the coding sequence • In some eukaryotic genes, there are enhancers that help increase or enhance transcription • Activators can bind to enhancers with the transcription factors bound to the promoter region and the RNA polymerase • Transcriptional repressors can bind to promoter or enhancer regions and block transcription

9. Enhancers and Transcriptional Machinery

10. RNA Splicing • RNA is transcribed, but must be processed into a mature form before translation can begin • This processing after an RNA molecule has been transcribed, but before it is translated into a protein, is called post-transcriptional modification • In eukaryotic cells, the RNA transcript often contains regions, called introns, that are removed prior to translation; the regions of RNA that code for protein are called exons • Prior to departure from the nucleus, introns are removed by splicing

11. Alternative Splicing • Alternative RNA splicing is a mechanism that allows different protein products to be produced from one gene when different combinations of introns, and sometimes exons, are removed from the transcript

12. mRNA Splicing

13. RNA Stability Influences Gene Expression • If the RNA is stable, it will be detected for longer periods of time in the cytoplasm • Each RNA molecule has a defined lifespan and decays at a specific rate, known as RNA stability • RNA stability can be influenced by protecting the RNA molecule and preventing degradation • A 5′ cap is placed on the 5′ end of the mRNA • A poly-A tail is attached to the 3′ end • RNA degradation can be increased by binding of microRNAs (or miRNAs) • Short single-stranded RNA molecules that are only 21–24 nucleotides in length that recognize and bind a specific sequence to degrade the target

14. Quick Review • Why do cells express different genes? • List the key differences between prokaryotic and eukaryotic gene regulation. • Describe the basic steps in prokaryotic gene regulation. • What is epigenetic regulation? • How do transcription factors impact gene regulation? • What is RNA splicing? • Explain how RNA stability plays a role in gene regulation.