Differential Gene Expression If all cells have the same genome, how do cells become differentiated in a multicellular organism? only ~20% genes expressed in a typical cell
addition of methyl groups to bases of DNA can condense chromatin and lead to reduced transcription histone acetylation vs DNA methylation acetyl groups are attached to histone tails loosens chromatin structure, promoting transcription Figure 15.7 Nucleosome Unacetylated histones Acetylated histones Histone tails
Epigenetics inheritance of traits transmitted by mechanisms not directly involving the nucleotide sequence is called epigenetic inheritance Epigenetic modifications can be reversed, unlike mutations in DNA sequence Can be passed to future generations
Regulation of Transcription provide initial control of gene expression by making a region of DNA either more or less able to be transcribed Figure 15.6a Signal NUCLEUS Chromatin Chromatin modification: DNA unpacking involving histone acetylation and DNA demethylation DNA Gene RNA Exon Gene available for transcription Transcription Primary transcript Intron RNA processing Tail mRNA in nucleus Transport to cytoplasm Cap CYTOPLASM
Organization of a Eukaryotic Gene control elements, segments of noncoding DNA that serve as binding sites for transcription factors that help regulate transcription Figure 15.8 DNA Upstream Enhancer (distal control elements) Proximal control elements Transcription start site ExonIntron Exon Promoter IntronExon Poly-A signal sequence Transcription termination region Down- stream Transcription Exon Intron Exon Poly-A signal Primary RNA transcript (pre-mRNA) 5 Cleaved 3 end of primary transcript Intron RNA mRNA RNA processing Coding segment 3 55 3 CapUTR Start codon Stop codon UTR Poly-A tail G PP P AAA AAA Proximal control elements are located close to the promoter Distal control elements, or enhancers, may be far away from a gene
Transcription Factors Figure 15.10-3 DNA Enhancer Distal control element Activators Promoter Gene TATA box DNA- bending protein Group of mediator proteins General transcription factors RNA polymerase II RNA synthesis Transcription initiation complex To initiate transcription, eukaryotic RNA polymerase requires the assistance of proteins called transcription factors control elements must interact with specific transcription factors
Transcription Factors Figure 15.10-3 DNA Enhancer Distal control element Activators Promoter Gene TATA box DNA- bending protein Group of mediator proteins General transcription factors RNA polymerase II RNA synthesis Transcription initiation complex activator is a protein that binds to an enhancer and stimulates transcription Bound activators are brought into contact with a group of mediator proteins through DNA bending The mediator proteins in turn interact with proteins at the promoter forming transcription initiation complex
Combinatorial Control of Gene Activation A particular combination of control elements can activate transcription only when the appropriate activator proteins are present Figure 15.11 Albumin gene Crystallin gene Promoter (b) LENS CELL NUCLEUS Available activators Albumin gene not expressed Crystallin gene expressed Crystallin gene not expressed Albumin gene expressed Available activators (a) LIVER CELL NUCLEUS Control elements Enhancer
Coordinately Controlled Genes in Eukaryotes Unlike the genes of a prokaryotic operon, each of the co-expressed eukaryotic genes has a promoter and control elements These genes can be scattered over different chromosomes, but each has the same combination of control elements Activators recognize specific control elements and promote simultaneous transcription of the genes
Post-Transcriptional Regulation alternative RNA splicing, different mRNA molecules are produced from the same primary transcript Figure 15.12 DNA Primary RNA transcript mRNA or Exons RNA splicing 1 2 3 4 5 1 2 3 5 1 2 4 5 1 2 3 4 5
RNA interference (RNAi) and MicroRNAs MicroRNAs (miRNAs) are small single- stranded RNA molecules that can bind to complementary mRNA sequences These can degrade the mRNA or block its translation Figure 15.13 miRNA miRNA- protein complex Translation blockedmRNA degraded The miRNA binds to a target mRNA. 1 If bases are completely complementary, mRNA is degraded. If match is less than complete, translation is blocked. 2
bli-1 worm: Notice the large clear area on the side of the worm (a blister in the cuticle) dpy-10 adult: Dumpy worms are shorter and wider than wild-type rol-6 adult: Roller worms have twisted bodies and roll in circles. unc-22 worms: unc-22 worms tend to lie still, are often outstretched (not S- shaped), and twitch. Wild-type adult:. Wild-type worms are very active and move sinusoidally Summary of mutant phenotypes
http://www.pbs.org/wgbh/nova/body/rnai.ht ml RNAi Nova Video
How do you think this technology can help with the treatment of Huntington's disease? Hint: Huntington's disease is caused by a single autosomal dominant mutant gene. The gene produces a protein that causes brain abnormalities, which interfere with coordination, speech, and metal abilities.
The discovery of RNAi has made it possible for researchers to switch genes on and off at will, simply by inserting double-stranded RNA into cells. It also holds the promise of allowing medical scientists to turn off the expression of genes from viruses and cancer cells, and it may provide new ways to treat and perhaps even cure diseases.
Regulatin’ Genes video http://www.youtube.com/watch?v=9k_oKK4Teco http://www.youtube.com/watch?v=9k_oKK4Teco