gene regulation chapter 13. gene regulation 2 prokaryotic regulation: bacteria do not need the...
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Gene RegulationChapter 13
Gene Gene RegulationRegulation 2Prokaryotic Regulation:
Bacteria do not need the same enzymes and other proteins all of the time. - They need only: 1. The enzymes required to break
down the nutrients available to them or
2. The enzymes required to synthesize whatever metabolites are absent under the present circumstances.
Gene Gene RegulationRegulation 3Prokaryotic Regulation:
The Operon Model (Jacob & Monod 1961)
An operon consists of three components:1. Promoter
DNA sequence where RNA polymerase first attaches
Short segment of DNA2. Operator
DNA sequence where active repressor binds
Short segment of DNA
Gene Gene RegulationRegulation 4Prokaryotic Regulation:
The Operon Model (Jacob & Monod 1961)
3. Structural Genes One to several genes coding for enzymes of a metabolic pathway Translated simultaneously as a block Long segment of DNA
A regulator gene is located outside of the operon. It codes for a repressor that controls whether the operon is active or not.
Gene Gene RegulationRegulation 5Repressible Operons:
The trp Operon - Normally turned ON
If tryptophan (an amino acid) is ABSENT:
Repressor is unable to attach to the operator (expression is normally “on”)
RNA polymerase binds to the promoter
Transcription & translation occur
Enzymes for synthesis of tryptophan are produced
Tryptophan will be produced by E. coli
6The trp Operon
Gene Gene RegulationRegulation 7Repressible Operons:
The trp Operon - Genes repressed
If tryptophan IS present enzymes are not needed and following occurs:
Tryptophan combines with repressor, causing it to change shape, thus acting as a corepressor
Repressor becomes functional
Blocks transcription & synthesis of enzymes and tryptophan is NOT produced
8The trp Operon
9Summary of repressible trp operon
Operon usually ON, must be turned OFF
Repressor Transcriptionbound ? occurs?
NO -------> YES YES -------> NO
*** Corepressors are frequently the products in the pathway. In this case, tryptophan is the corepressor.
Gene Gene RegulationRegulation 10Inducible Operons:
The lac Operon - Normally turned OFF
When E. coli is denied glucose & is given lactose instead, it immediately begins to make three enzymes needed for the metabolism of lactose.
These enzymes are encoded by three structural genes which are adjacent to one another on the chromosome. They are controlled by one regulator gene that codes for a one repressor.
Gene Gene RegulationRegulation 11Inducible Operons:
The lac Operon - Normal OFF state
If lactose (a sugar that can be used for food) is absent:Repressor attaches to the operatorRNA polymerase cannot bind to promoterTranscription of structural genes is blocked
Enzymes needed to digest lactose NOT made
12The lac Operon
Gene Gene RegulationRegulation 13Inducible Operons:
The lac Operon - Induced state
If lactose IS present:
It combines with repressor and renders it unable to bind to operator by causing shape of repressor to change
RNA polymerase binds to the promoter
Transcription of genes occurs
The three enzymes necessary for lactose catabolism are produced
Lactose will be digested by enzymes
14The lac Operon
15Summary of inducible lac operon
Operon usually OFF, must be turned ON
Repressor Transcriptionbound ? occurs?
YES -------> NO NO -------> YES
*** Inducers are frequently the reactants in the pathway. In this case, the lactose is the inducer.
Gene Gene RegulationRegulation 16
The lac Operon - Further control
E. coli preferentially break down glucose. Thus, they have a way to ensure that the lac operon is only turned on maximally when glucose is absent.
This involves use of cyclic AMP which is abundant when glucose is absent.
- Cyclic AMP binds to a molecule called catabolite activator protein (CAP).
Gene Gene RegulationRegulation 17
The lac Operon - Further control (2)
The cAMP-CAP complex then binds to a CAP binding site next to the lac operon promoter.
• When CAP binds to DNA, the DNA bends.
- This exposes the promoter to RNApolymerase which is now better able to bind to the promoter.
Gene Gene RegulationRegulation 18
The lac Operon - Further control (2)
When glucose IS present:
There is little cAMP in the cell
- CAP is not activated by cAMP
- lac operon does NOT function maximally and cell will preferentially use glucose as its food source.
19Action of CAP
Gene Gene RegulationRegulation 20
Animations for the OperonsAnimations for the Operons
http://highered.mcgraw-hill.com/olc/dl/120080/bio27.swfhttp://highered.mcgraw-hill.com/olc/dl/120080/bio27.swf
Trp Operon
http://highered.mcgraw-hill.com/olc/dl/120080/bio26.swf
lac Operon
Gene Gene RegulationRegulation 21
Eukaryotic Regulation
A variety of mechanisms to control gene expression:
Five primary levels of control:Nuclear levels
Chromatin Packing
Transcriptional Control
Posttranscriptional Control
Cytoplasmic levels Translational Control
Posttranslational Control
22Regulation of Gene Expression:
Levels of Control in Eukaryotes
Gene Gene RegulationRegulation 23
Chromatin Structure
Eukaryotic DNA associated with histone proteins Together make up chromatinAs seen in the interphase nucleus
Nucleosomes:DNA wound around balls of eight molecules of histone proteins
Looks like beads on a stringEach bead a nucleosome
The levels of chromatin packing determined by degree of nucleosome coiling
24Levels of Chromatin Structure
Gene Gene RegulationRegulation 25
Chromatin PackingEuchromatin
Loosely coiled DNAAppears lightly stained in micrographsTranscriptionally active - capable of being transcribed
HeterochromatinTightly packed DNAAppears darkly stained in micrographsTranscriptionally inactive
Gene Gene RegulationRegulation 26
Chromatin Packing
Barr BodiesFemales have two X chromosomes, but only one is active
Other is tightly packed along its entire length
Inactive X chromosome is called a Barr body
Inactive X chromosome does not produce gene products
27X-Inactivation in Mammalian Females
Gene Gene RegulationRegulation 28
Transcriptional Control
Transcription controlled by DNA-binding proteins called transcription factors
Bind to a promoter adjacent to a gene
Transcription activators bind to regions of DNA called enhancers. Might be brought near region of promoter by hairpin loops in DNA.
Always present in cell, but most likely have to be activated before they will bind to DNA
29Lampbrush Chromosomes
30Initiation of Transcription
Gene Gene RegulationRegulation 31
Transcriptional Control (2)
Transposons are specific DNA sequences that have the ability to move within and between chromosomes.
Their movement to a new location sometimes alters neighboring genes by decreasing their expression
- Thus, they can act like regulator genes
- They also can be a source of mutations.
Gene Gene RegulationRegulation 32
Posttranscriptional Control
Posttranscriptional control operates within the nucleus on the primary mRNA transcript
Given a specific primary transcript:Excision of introns can varySplicing of exons can varyThus, differing versions of the mRNA transcript might leave the nucleus
Gene Gene RegulationRegulation 33
Posttranscriptional Control
Posttranscriptional control may also control speed of mRNA transport from nucleus to cytoplasmWill affect the number of transcripts arriving at rough ER
And therefore the amount of gene product realized per unit time
34Processing of mRNA Transcripts
Gene Gene RegulationRegulation 35
Translational Control
Translational control determines degree to which mRNA is translated into a protein product
Presence of 5′ cap and the length of poly-A tail on 3′ end can determine whether translation takes place and how long the mRNA is active
- Example: Long life of mRNA in RBCs that code for hemoglobin attributed to presence of 5’ cap and 3’ poly-A tail
Gene Gene RegulationRegulation 36
Posttranslational Control
Some proteins are not immediately active after synthesis.
Some need to be activated
- Folding and breaking into chains must occur in bovine insulin before it is active
Some are degraded quickly
- Cyclin proteins that control cell cycle
Gene Gene RegulationRegulation 37
Animations for Eukaryotic ControlAnimations for Eukaryotic Control
http://highered.mcgraw-hill.com/olc/dl/120080/bio28.swfhttp://highered.mcgraw-hill.com/olc/dl/120080/bio28.swf
Control of gene expression in eukaryotes
http://highered.mcgraw-hill.com/olc/dl/120080/bio31.swf
Transcription Complex and Enhancers
Gene Gene RegulationRegulation 38Effect of Mutations on
Protein Activity
A mutation is a permanent change in the sequence of bases in DNA.
Effects on proteins can range from no effect to complete inactivityGerm-line mutations
Occur in sex cells; can be passed on to future generations
Somatic mutations Occur in body cells; can’t be passed on to future generations
Can lead to development of cancer
Gene Gene RegulationRegulation 39Effect of Mutations on
Protein Activity
Point Mutations Involve change in a single DNA nucleotide
Changes one codon to a different codonCould change one amino acid for anotherEffects on protein vary:
Drastic - completely nonfunctional Reduced functionality Unaffected
Gene Gene RegulationRegulation 40Effect of Mutations on
Protein Activity
Frameshift MutationsOne or two nucleotides are either inserted or deleted from DNA
Can lead to completely new codon orderProtein can rendered nonfunctional
Normal : THE CAT ATE THE RAT After deletion: THE ATA TET HER AT
After insertion: THE CCA TAT ETH ERA T
41Point Mutation
Gene Gene RegulationRegulation 42
Nonfunctional Proteins
Examples of nonfunctional proteins:Hemophilia due to the transposon AluPhenylketonuria (PKU) due to faulty code for one enzyme
Cystic fibrosis due to inheritance of faulty code for a chloride ion channel
Androgen insensitivity due to a faulty receptor for androgens (male sex hormones)
Gene Gene RegulationRegulation 43
Carcinogenesis
Development of cancer involves a series of mutations:
•Proto-oncogenes – Stimulate cell cycle but are usually turned off. Can mutate and become oncogenes which are turned on all the time.
•Tumor suppressor genes – inhibit cell cycle
Mutation in oncogene and tumor suppressor gene:
Stimulates cell cycle uncontrollably
Leads to tumor formation
44Carcinogenesis
Gene Gene RegulationRegulation 45
Causes of Mutations
Spontaneous Errors:Happen for no apparent reasonExample of spontaneous germ-line mutation is achondroplasia, a type of dwarfism
Replication Errors: DNA polymerase proofreads new strands Generally corrects errors-1 in 1,000,000,000 replications error occurs
Gene Gene RegulationRegulation 46
Causes of Mutations
Environmental MutagensA mutagen is an environmental agent that increases the chances of a mutation
Carcinogens - Mutagens that increase the chances of cancer
-Many agricultural & industry chemicals -Many drugs Tobacco smoke chemicals Radiation (X-rays, gamma rays, UV)
47Achondroplasia andXeroderma Pigmentosum
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