Gene regulation in prokaryotes and eukaryotes

2. Gene regulation in prokaryotes and eukaryotes • How do organisms respond correctly to their environment? • How do they control their growth and development? • How do they control which genes are expressed?

3. Proteins interacting with DNA turn prokaryotic genes on or off in response to environmental changes • The process by which genetic information flows from genes to proteins is called gene expression Prokaryotes Figure 11.1A

4. genes for related enzymes are often controlled in groups called operons lac operon regulator gene promotor operator p i gene o z gene y gene a gene DNA binding site of RNA polymerase codes for repressor protein codes for b-galactosidase, which clips lactose molecules codes for permease enzyme that transports lactose into cells

5. lac operon RNA polymerase regulator gene operator o p i gene z gene y gene a gene DNA promotor no transcription repressor protein blocks binding of RNA polymerase repressor protein Regulatory proteins bind to control sequences in DNA and turn operons on or off

6. 2 RNA polymerase binds to promoter o p i gene z gene y gene a gene DNA transcription proceeds 3 repressor mRNA transcript lactose b-galactosidase 1 lactose the (inducer) inactivates the repressor so that it cannot bind to the operator permease galactose glucose cell membrane lactose

7. Two types of repressor-controlled operons Promoter Operator Genes DNA Activerepressor Activerepressor Tryptophan Inactiverepressor Inactiverepressor Lactose lac OPERON trp OPERON Figure 11.1C

8. Eukaryotic gene control • Transcription is a major control point • Individual genes are regulated, rather than operons • Activation is more important than repression

9. Activators bind to enhancer regions of DNA and to other transcription factors • These interactions turn the transcription of eukaryotic genes on or off Enhancers Promoter Gene DNA Activatorproteins Transcriptionfactors Otherproteins RNA polymerase Bendingof DNA Figure 11.8 Transcription

10. Control of transcription • Control access of enzymes to DNA: 1. Methylation of DNA - inhibits 2. Acetylation of histones - promotes

11. How do eukaryotes control genes of a single metabolic pathway? • genes for individual enzymes of one pathway are often located far apart in the genome • same enhancer region(s) common to all genes of a pathway.

12. Eukaryotic RNA may be spliced in more than one way • After transcription, alternative splicing may generate two or more types of mRNA from the same transcript Exons DNA RNAtranscript RNA splicing or mRNA Figure 11.9

13. Translation and later stages of gene expression are also subject to regulation • The lifetime of an mRNA molecule helps determine how much protein is made • Initiation of translation

14. The protein may need to be activated in some way • Rate of protein degradation Folding of polypeptide andformation of S–S linkages Cleavage Initial polypeptide(inactive) Folded polypeptide(inactive) Active formof insulin Figure 11.10

15. DNAdoublehelix(2-nmdiameter) multiple levels of DNA packing Histones “Beads ona string” Nucleosome(10-nm diameter) Tight helical fiber(30-nm diameter) • DNA packing tends to block gene expression Supercoil(200-nm diameter) 700nm Metaphase chromosome Figure 11.6

16. In female mammals, one X chromosome is inactive in each cell EARLY EMBRYO TWO CELL POPULATIONSIN ADULT Active X Orange fur Allele fororange fur Inactive X Cell divisionandX chromosomeinactivation X chromosomes Inactive X Black fur Active X Allele forblack fur Figure 11.7

17. Chromosome DNA unpackingOther changes to DNA GENE TRANSCRIPTION GENE Exon RNA transcript Intron Addition of cap and tail Splicing Tail Cap mRNA in nucleus NUCLEUS Flowthroughnuclear envelope mRNA in cytoplasm CYTOPLASM Breakdown of mRNA Translation Broken-down mRNA Polypeptide Cleavage/modification/activation ACTIVE PROTEIN Breakdownof protein Broken-down protein Figure 11.11

18. Review: Multiple mechanisms regulate gene expression in eukaryotes • Each step of gene expression can be turned on or off, speeded up, or slowed down • most important control is usually the start of transcription • DNA can be mobile; position will affect transcription. Transposons

20. What is a gene? • First version: one gene, one polypeptide • Now: DNA segment that is transcribed to RNA • Proteins and RNA function in cells

21. Nature vs nurture • Do environments or genes determine phenotype? • Himalayan rabbits and fur color • Twin studies - identical cp. fraternal

22. Complex human traits • Language - FoxP2 • Depression • Social bonds Do 460 bp determine our life partner?

23. Genome imprinting • Same gene is expressed differently, depending on whether it was inherited from the male or female parent • Ex. Corn kernel color • Ex. Xsome 15 deletion • Gene is “reset” during gamete formation