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Molecular Genetics 3
Transcript of Molecular Genetics 3
Prokaryotes regulate gene expression (and therefore their metabolism) almost entirely by regulating transcription.
The lack of a nucleus makes this very efficient. Eukaryotes regulate gene expression (and therefore their metabolism) at every step of protein synthesis from pre-transcription to post-translation Refer to organized clusters of genes that all contribute to a particular metabolic task.
2 Major types: Inducible and Repressible Operons: Inducible: Repressible: "Up Regulation": Precursor Enzyme 1 Enzyme 2 Enzyme 3 - - Gene 1 Gene 2 Gene 3 Gene 4 Gene 5 Product Inhibition of
enzyme activity Inhibition of
transcription Francois Jacob & Jacques Monod
Nobel Prize: 1965 For metabolic pathways that are usually "off".
Ex: The Lac Operon (digests lactose) For metabolic pathways that are usually "on".
Ex: The Trp Operon (synthesizes tryptophan) When Lactose is absent:
The repressor protein (made by the LacI gene) is able to attach to the operator (a region of the promoter)
RNA polymerase cannot transcribe the structural genes (LacZ, LacY and LacA) that the cell needs to be able to digest lactose, since it can not attach to the promoter.
This is how things remain in the cell as long as there is no lactose present. When Lactose is present:
The inducer molecule (allolactose, a form of lactose) binds to the repressor protein.
This changes the shape of the repressor protein so that it can not attach to the promoter.
RNA polymerase is then able to transcribe the structural genes that the cell needs to be able to digest lactose.
This is how things remain in the cell until lactose is digested What kind of feedback? When tryptophan is absent:
The repressor protein (made by the TrpR gene) is unable to attach to the operator
RNA polymerase can transcribe the structural genes (TrpE, TrpD, TrpC, TrpB, & TrpA) that the cell needs to be able to synthesize tryptophan.
This is how things remain in the cell as long as there is no tryptophan present. When Tryptohan is present:
The corepressor molecule (tryptophan) binds to the repressor protein.
This changes the shape of the repressor protein so that it can attach to the promoter.
RNA polymerase can not transcribe the structural genes that the cell uses to synthesize tryptophan.
This is how things remain in the cell until tryptophan is no longer present. What kind of feedback? A way to increase the rate of transcription of an operon
Ex: The CAP/cAMP System When Lactose is present and glucose is low:
Low glucose means the amount of cyclic AMP (cAMP) is high
cAMP binds to the Catabolite Activator Protein (CAP), activating it
The Activated cAMP/CAP complex increases the rate of transcription of the lac operon (and ~100 other catabolic operons), boosting the rate of transcription several fold. When Glucose and Lactose are both present:
Normal glucose level means the amount of cyclic AMP (cAMP) is low
cAMP is not bound to CAP
CAP is inactive.
Very little lac structural gene transcription occurs. What kind of feedback? What kind of logic? What kind of logic? What kind of logic? Levels of Control 1.DNA Access 2.Pre-Transcription 3.Post-Transcription Recent research has uncovered many forms of "non-coding" RNA (ncRNA) molecules in cells.
We'll look at one example, active in regulation of gene expression post-transcription and pre-translation 4.Pre-Translation 5.Post-Translation The
points Only keep necessary genes accessible. In eukaryotes, DNA is "wound" around histone proteins.
The addition of acetyl groups (-CH2CH3) to histones causes them to become less tightly packed, allowing for access to the DNA.
Heterochromatin: more tightly packed DNA, unavailable for transcription.
Euchromatin: less tightly packed DNA, available for transcription.
Cool fact: Histone acetylation patterns seems to be heritable. Epigenetics strikes again! Recent research shows that even in interphase, the "loose" chromosomes occupy distinct nuclear regions.
"Transcription factories": Areas of the nucleus where active regions of different chromosomes interface. May be associated with common functions
Thousands of transcription factories in any nucleus All the actors need to be present for the play to begin. Eukaryotic genes interact with many "upstream" regulatory elements. These are DNA sequences that precede a transcription unit that need to have specific proteins present for RNA polymerase to begin transcription. The proteins that mediate RNA Polymerase are known as "Transcription Factors": Control of transcription factor availability is one of the major ways that cells of a multicellular organism accomplish "differential gene expression", which in turn allows cells to "differentiate" to serve different functions in the organism.
This is crucial!!! So many options lead to so many outcomes. Following transcription, 5' capping and 3' poly-adenylation are necessary for eukaryotic mRNA to remain functional and be transported to the cytoplasm for translation. "Alternative splicing" of exons allows for multiple functional (or non-functional) gene products to be made from a single primary transcript.
Anywhere from 75 - 100 percent of human genes with multiple exons probably undergo alternative splicing.
Shown here: the Troponin T gene produces 2 different mRNA sequences to produce 2 different gene products.
Not shown: A Drosophila gene that has enough exons to produce 19,000 different transcripts. Lots still left to learn RNA Interference (RNAi): Mediated by a group of tiny RNA molecules ("Micro RNA" or miRNA).
The miRNAs are produced after the transcript for them is cleaved into multiple fragments by a "dicer" protein.
The miRNAs complex with proteins.
Any mRNA with a sequence complementary to an miRNA is "tagged" with the miRNA/protein complex.
Tagged miRNA molecules are not translated.
Nobel Prize: 2006 Don't let unnecessary proteins hang around. Ubiquitin (a protein so plentiful in all eukaryotic cells it is "ubiquitous"), will tag unnecessary proteins for transport to a proteasome. Inside the proteasome, the protein is broken down.
Nobel Prize: 2004 Proteasomes are abundant in eukaryotic cells (why?) Big Question Make sure you can: Polyribosomes demonstrating the coupling of transcription and translation in prokaryotes: Prokaryotic control of metabolism through control of transcription: Wikipedias "list of RNAs"
(Oct. 2011): Coding RNAs How is gene expression controlled? Explain the structure and function of all expression control systems described in this presentation.
Compare and contrast the expression control systems utilized in prokaryotic and eukaryotic cells.
Relate prokaryotic expression control systems to prokaryotic cellular organization and feedback mechanisms.
Relate eukaryotic expression control systems to eukaryotic cellular organization and the decoupling of transcription and translation. RNAi