1. Biology 10.2 Gene Regulation and Structure Gene Regulation and Structure

2. Protein Synthesis in Prokaryotes  A lthough prokaryotes, such as bacteria seem simple because of their small size, prokaryotes cells typically have about 2000 genes.  The human genome; the largest sequenced to date, has about 30,000 genes.  Not all genes are transcribed and translated all the time. Cells are able to regulate which genes are expressed and which are not, depending on the needs of the cell.

3. Protein Synthesis in Prokaryotes  A n example of gene regulation can be found in the bacterium Escherichia Coli. (E. Coli)  When you eat or drink a dairy product, the lactose (milk sugar) reaches the intestinal track and becomes available to the E. coli living there. The bacteria can absorb the lactose and break it down for energy or for making other compounds.  In E. Coli; recognizing, consuming, and breaking down lactose into it’s parts requires three different types of enzymes, each of which is coded for by a different gene.

4. Protein Synthesis in Prokaryotes  T he three lactose- metabolizing genes are located next to each other and are controlled by the same promoter site.  There is an on-off switch that “turns on” (transcribes and than translates) the three genes when lactose is available and “turns-off” the genes when lactose is not available.

5. Protein Synthesis in Prokaryotes  T he piece of DNA that overlaps the promoter site and serves as the on-off switch is called an operator.  In bacteria, a group of genes that code for enzymes involved in the same function, their promoter site, and the operator that controls them all function together as an operon.  In prokaryotes; gene expression is controlled by these operons.  The operon that controls the metabolism of lactose in our example is called the lac operon.

6. Protein Synthesis in Prokaryotes  W hat determines if the lac operon is in the on or off mode?  When there is no lactose in the bacterial cell, a repressor turns the operon off.  A repressor is a protein that binds to an operator and physically blocks RNA polymerase from binding to a promoter site. This blocking of the RNA polymerase STOPS the transcription of the genes in the operon.

7. Protein Synthesis in Prokaryotes  W hen lactose is present, the lactose binds to the repressor and changes the shape of the repressor.  The change in shape causes the repressor to fall off the operator.  Now the bacterial cell can begin transcribing the genes that code for the lactose-metabolizing enzymes.  By producing the enzymes only when the nutrient is available, the bacterium saves energy.

8. Protein Synthesis in Prokaryotes  In Summary:  In prokaryotes, gene expression is regulated by operons.  Gene expression is switched OFF when repressor proteins block RNA polymerase from transcribing a gene.

9. Protein Synthesis in Eukaryotes:  E ukaryote cells contain much more DNA than prokaryote cells do. Like prokaryotes cells, eukaryote cells must continually turn certain genes on/off in response to signals from their environment.  Operons have NOT been found often in eukaryote cells. Instead, genes with related functions are often scattered on different chromosomes.  Because a nuclear envelope physically separates transcription from translation in a eukaryote cell, more opportunities exist for regulating gene expression.

10. Protein Synthesis in Eukaryotes:  C ontrolling the Onset of Transcription:  Predominantly, gene regulation in eukaryotes controls the onset of transcription.  Like prokaryotes, eukaryotes cells use regulatory proteins (proteins to start, stop and regulate the process)  These regulatory proteins in eukaryotes are called transcription factors

11. Protein Synthesis in Eukaryotes:  T ranscription factors help arrange RNA polymerases in the correct position on the promoter. A gene can be influenced by many different transcription factors.  An enhancer is a sequence of DNA that can be bound by a transcription factor.  Enhancers are typically located thousands of nucleotides bases away from the promoter.

12. Protein Synthesis in Eukaryotes:  A loop in the DNA may bring the enhancer and it’s attached transcription factor (called an activator) into contact with the transcription factors and RNA polymerase at the promoter.  Transcription factors bound to enhancers can activate transcription factors bound to promoters.

13. Intervening DNA in Eukaryote cells:  W hile it is tempting to think of a gene as an unbroken stretch of nucleotides that code for a protein, this simple arrangement is usually found only in prokaryotes.  I n eukaryotes, many genes are interrupted by introns, long series of nucleotides that have NO coding information. (blank)  E xons are the portions of the genes that are translated (copied or expressed) into proteins.

14. Intervening DNA in Eukaryote cells:  A fter a eukaryotic gene is transcribed, the introns in the resulting mRNA are cut out by complex assemblies of RNA and protein called spliceosomes.  The exons that remain are “stitched” back together by the spliceosome to form a smaller RNA molecule that is than translated.

15. Intervening DNA in Eukaryote cells:  M any biologists think this organization of genes adds evolutionary flexibility. Each exon encodes a different part of a protein.  By having introns and exons, cells can occasionally shuffle exons between genes and make new genes.

16. Intervening DNA in Eukaryote cells:  T he thousands of proteins that occur in human cells appear to have arisen as combinations of only a few thousand exons.  Some genes in your cells exist in multiple copies, in clusters of as few as three or as many as several hundred.  For example, your cells may each contain 12 different hemoglobin genes, all of which came from one original hemoglobin gene.

17. Mutations:  Although changes in an organisms hereditary information are rare, they can occur.  A change in the DNA of a gene is called a mutation.  M utations in gametes can be passed on to offspring of he affected individual, but mutations in body cells affect only the individual in which they occur.

18. Mutations:  M utations that move an entire gene to a new location are called gene rearrangements.  Changes in a gene’s position can often disrupt the gene’s function because the gene is exposed to new regulatory controls in it’s new place (like moving to a place where no one spoke your language and you couldn’t communicate with anyone near you).

19. Mutations:  M utations that change a gene are called gene alterations.  Gene alterations usually result in the placement of the wrong amino acid during protein assembly. This error will usually disrupt a proteins function.  I a point mutation, a single nucleotide changes.

20. Mutations:  I n an insertion mutation, a sizable length of DNA is inserted into the gene.  Insertions often result when mobile segments of DNA, called transposons, move randomly from one position to another on chromosomes.  Transposons make up 45 percent of the human genome.  In a deletion mutation, segments of a gene are lost, often during meiosis.

21. Computer lab: U se the internet to research the following topic: give me ten facts about “mutations” that you can find from the news on the internet when you search.