15BIS1012012GeneRegLect15

15BIS1012012GeneRegLect15 - Regulation of Gene Expression...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Regulation of Gene Expression Regulation of Prokaryotic and Eukaryotic Gene Expression Lecture #15 Chapter 14.1-3 and 15 March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 1 This Lecture Evidence for Gene Regulation: We now know that not all genes are on (expressed) all the time, but rather a subset of genes are expressed in response to a environmental signals or stages of development. Gene regulation provides a means for single- and multicellular organisms to respond to internal and external signals (e.g, drugs, hormones and metabolites) and respond to these signals in a tissue-specific or developmental-specific fashion. Positive and Negative Gene Regulation: Gene expression can be controlled by turning genes up when they are needed (positive regulation) or down when not needed (negative regulation) Most genes use a combination of (+) and (-) control. Complementation was used to characterize the Lactose Operon: Merodiploids carrying mutations in various parts of the lactose operon have been used to dissect the molecular mechanism of gene regulation. Galactose Metabolism in Yeast: One of the best examples of eukaryotic gene regulation involves the metabolism of the monosaccharide, galactose by yeast. Both positive and negative control mechanisms are used. March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 2 Gene Regulation Gene regulation provides a means for single- and multicellular organisms to respond to internal and external changes in their environment. As the external chemical environment changes (or as the needs of a growing and differentiating organism change), gene regulation modulates the biosynthetic processes of DNA replication, transcription, and translation. Differential gene expression is essential for the development and differentiation of eukaryotic organisms. In addition to enabling the cell or organism to adapt to changes in the environment, gene regulation ensures the efficient or parsimonious use of cellular resources by selectively expressing only those genes needed at the time. BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 3 March 19, 2012 Example of Differential Gene Regulation Adult hemoglobin consists of 2 and 2 chains. In utero, the fetus uses 2 together with the 2 chains to make fetal hemoglobin which carries oxygen from the mother's blood instead of from the lungs in an adult. At birth, the gene for the chain is turned off and the chain gene is turned on. The represents a switch in gene expression from fetal to adult hemoglobin at the time of birth. March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 4 Evidence for Gene Expression* Selective Gene Expression: The proteins found in a cell at any particular time represent only a fraction of the total number of proteins encoded by the cell. For example, although the E. coli genome has the capacity to encode about 6000 genes, only about 600 to 800 proteins are present at any instant in the life cycle of the cell. Differential Gene Expression: Within the same cell, different proteins are present in different amounts. Responsive Gene Expression: By changing the environment of the cell, it is possible to selectively increase the amount of one protein over another. Selective, differential, and responsive gene expression is accomplished through specific protein/DNA and protein/protein interactions involving regulatory DNA sequences such promoters, operators and ribosome binding (SD) sites, enhancers. Protein/DNA interactions can be negative (i.e., repress transcription or translation) or positive (i.e, enhance transcription or translation). March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 5 Examples of Positive and Negative Control For most genes, a combination of positive and negative regulation is used to achieve the precise level of gene expression for the cell. Positive Control p gene Negative Control p gene p gene p gene March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 6 Lactose Operon One of the best examples of regulated gene expression is found in E. coli and involves the metabolism (or fermentation) of the milk sugar, lactose. E. coli is capable of utilizing various carbon sources for energy. When grown on the disaccharide lactose, E. coli produces the enzyme galactosidase ( -gal), which breaks lactose down into the simple monosaccharides, glucose and galactose. March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 7 Lactose Operon E. coli cells grown in the presence of lactose contain about 3000 -gal molecules/cell. However, when the cells are switched from lactose to a glucose-containing medium, the number of gal molecules drops to about 3 molecules/cell. This in shown in the graph at the left. IPTG is an analog of lactose and can inducer of -gal without being metabolized. IPTG is a gratuitous inducer. -gal enzyme -gal mRNA 0 IPTG 10 IPTG 15 20 min 8 March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 Genetics of the Lactose Operon Most of what we now know about gene regulation stem from studies of the lactose operon conducted by Jacob and Monod at the Pasteur Institute in Paris in the 1950s. Their approach to understanding this inductive form of gene regulation was to destroy, by mutation, the genetic elements responsible for these effects. By treating E. coli cells with a chemical mutagen, they isolated a number of mutants that fell into two general groups. The first group was unable to use lactose as a carbon source (called lactose deficient or lac-). The second group included those that expressed -galactosidase even in the absence of lactose. These were called lac constitutive mutants and represent the loss of regulation. Constitutive mutants could subdivided into two categories: repressor deficient and operator mutants. BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 9 March 19, 2012 Genetics of the Lactose Operon Using these mutants, complementation analysis and interrupted mating experiments revealed that the ability of E. coli to ferment lactose is encoded by three genes (lacZ, lacY and lacI) and an intergenic region containing a promoter (P) and operator (O). A collection of linked genes, all involved in the same metabolic process, is called an "operon." Although linked to the Z and Y genes, the "A" (transacetylase) is not required for lactose fermentation. Lactose Operon P I PO Z Y (A) repressor -galactosidase permease (transacetylase) 10 March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 Lactose Operon: A polycistron Because the Z, Y and A genes are transcribed as one mRNA molecule from one promoter, this region is called a polycistron and the mRNA is referred to as a polycistronic message. Ribosome binding sites (AGGA) precede the AUG codons for the Z, Y and A genes, so that three separate protein products can be translated from this polycistronic mRNA. The Y gene encodes a permease that permits lactose to enter the cell. The repressor is an allosteric protein whose interactions with the operator are controlled by an effector molecule called the inducer. The inducer is allolactose, a metabolite of lactose, which binds to the repressor and alters the repressor's affinity for the operator. BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 11 March 19, 2012 Operon Model Introduction to Lac Operon March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 12 Merodiploid (partial diploid) F-factors are like minichromosomes (plasmids) in E. coli. Using the plasmid, F-lac, to pick up some of the different lac mutations, Jacob and Monod were able to construct merodiploids (partial diploids) to study the nature of each mutation. These studies revealed that constitutive mutants mapped to two different locations in the operon. Some were located in the lacI gene while others were located in the intergenic region designated "O". Bacterial F-factors-Conjugation-Merodiploids BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 13 March 19, 2012 Bacterial Genetics Bacterial Genetics: Sexual exchange of genetic information in bacteria can be used to identify and map genes. Plasmids: Bacteria contain extrachromosomal elements called plasmids or F factors. Plasmids play a important role in the transfer of genetic material from male (F+ or Hfr) bacteria to female bacteria. Bacterial chromosomes: The bacterial chromosome is circular, not linear. Conjugation: Chromosomal material can be passed through a "sex pilus" from donor (or male) bacterium to recipient (female) cell. By opening the circular chromosome at any point on its circumference it can be transferred into the female cell as a "linearly permutation" of the circle. Complementation: Complementation can occur when the wild type gene is placed in the presence of a mutant gene, without recombination occurring (e.g. on a plasmid). In this instance, the wild type phenotype is restored and it is said that the wild type gene complements the mutant phenotype. Complementation analysis can be used to determine how many genes are involved in a particular phenotype. March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 14 Lac Regulation Movie Allolactose-Repressor Binding-Allosteric Confirmational Change March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 15 Merodiploids and Complementation In this merodiploid, the I- gene makes no functional repressor. However, because the repressor protein is a diffusible gene product, it can act at both operators (O+) to control expression of the Z+ gene on both chromosomes. The repressor protein is said to act in "trans" or I+ is "trans-dominant" to I-. II March 19, 2012 + P+ P + O+ O + Z+ Z + = inducible control of -gal 16 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 Lac Repressor Mutation Movie March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 17 Lactose Repressor The lac repressor acts as a homotetramer (four identical subuntis. Functional repressor has two (2) principal domains: the DNA domain at the N-terminus that binds the operator; and the C-terminal domain that binds the inducer lactose. operator binding site inducer binding site March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 18 Lactose Repressor When the inducer, (lactose, allolactose or IPTG) is bound in the inducer binding site, it induces an allosteric conformational change in the DNA binding site that prevents binding of the repressor to the operator. operator binding closed bound inducer March 19, 2012 Lactose BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 19 Merodiploids and Complementation In this merodiploid, the I+ gene makes functional repressor. However, because the operator has mutated (Oc), the repressor can only bind to O+ not Oc. In this case, the operator mutant (Oc) is cis-dominant to (O+) and the I+ gene. The phenotype of this merodiploid is constitutive. I+ I+ March 19, 2012 P+ P+ O+ Oc Z+ Z+ = constitutive expression of -gal 20 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 Lac Operator Mutation Movie March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 21 Merodiploids and Complementation In the merodiploid shown below, the Is gene makes a repressor with a binding site for the operator DNA sequence but no binding site for the inducer. In this case, the Is binds to the operator and cannot be induced off with lactose or IPTG. This mutant repressor is call the super repressor and it is trans-dominant to I+. Is I March 19, 2012 + P+ P + O+ O + Z+ Z + = -gal unducible 22 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 Superrepressor Mutation Movie March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 23 Operator Nucleotide Sequence -10 March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 24 Lac Repressor Binding Operator DNA -35 2 operator binding sites separated by a 93 bp loop CAP binding at -60 -10 Repressor tetramer March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 25 Operon Model: Negative Control What about positive control? March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 26 Operon Model: CAP and Positive Regulation CAP-cyclic AMP Binding to CAP site March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 27 Lactose Operon: Positive Control Catabolite Repression and Positive Regulation: Unlike the controlled environment of the laboratory, bacteria in nature frequently find themselves in mixed chemical environments containing a mixture of sugars to use as a carbon source (e.g., glucose, lactose, maltose, arabinose etc). Consistent with the Law of Parsimony, cells will utilize the simplest carbon source (e.g., glucose) before they use more complex sugars. To utilize the more complex sugars before all glucose is expended would require the wasteful synthesis of the enzymes to break down the more complex sugars. This would be a needless use of the cell's energy and resources (e.g., transcriptional and translational machinery). How does the cell know to use glucose first and ignore the more complex sugars? March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 28 Catabolite Repression and Positive Control The wild type lac promoter is weak because of a nonconsensus -10 region (TATGTTG) instead of TATPuATG. (The mutant lac UV5 promoter is TATAATG). Therefore, even when the repressor is bound by allolactose and prevented from binding to the lac operator, RNAP holoenzyme binds the promoter but is slow to form the open complex (also called isomerization). Isomerization needs help from a positive activator protein called CAP (catabolite activator protein) to stimulate open complex formation and subsequent transcription initiation. CAP must be first complex with cyclic AMP (cAMP) before it can bind , (as a dimer) to the palindromic sequence upstream of the -35 region of the lac promoter. CAP/cAMP binds these sequences and bends the DNA 90. This promotes isomerization at -10 by RNAP holoenzyme and transcriptional activity. The concentration of cAMP in the cell is inversely proportional to glucose concentration (i.e., the higher the glucose concentration, the lower the concentration of cAMP). A breakdown product of glucose (an unknown catabolite) either increases the level of a cAMP phosphodiesterase (cleaves the cyclic 5'-3' phosphodiester bond) or inhibits adenyl cyclase (responsible for producing this bond from ATP). BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 29 March 19, 2012 CAP Binding to the Lac Promoter In the presence of glucose and in the absence of lactose, repressor binds the operator. In presence of glucose and lactose, a small amount of lac mRNA is made because there is no cAMP/CAP complex. In the absence of glucose and presence of lactose, lac mRNA transcription is high due the help of cAMP/CAP. BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 30 March 19, 2012 CAP Binding Site in Lac Promoter -75 -65 -55 -45 -35 March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 31 Relationship between glucose and cyclicAMP The net result of catabolite repression is the linkage of a positive regulatory mechanism (CAP stimulating transcription) with a negative regulatory mechanism (repressor inhibition of transcription). The modulation of transcription from the lac promoter in response to varying concentrations of glucose is shown below (assume lactose is present in the cell). [glucose] [catabolite?] [cAMP] [cAMP/CAP] [cAMP/CAP/RNAP] March 19, 2012 [-gal mRNA] 32 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 Relationship between glucose and cyclicAMP The net result of catabolite repression is the linkage of a positive regulatory mechanism (CAP stimulating transcription) with a negative regulatory mechanism (repressor inhibition of transcription). The modulation of transcription from the lac promoter in response to varying concentrations of glucose is shown below (assume lactose is present in the cell). [cAMP] [cAMP/CAP] [glucose] [catabolite?] [cAMP/CAP/RNAP] [-gal mRNA] March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 33 Transcription review Eukaryotic Gene Regulation: Galactose (Gal) Metabolism in Yeast March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 34 Example of Gal mRNA Induction by Galactose March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 35 Introduction The GAL gene system in yeast is an excellent example of eukaryotic gene regulation. In addition to common cis-acting elements like the transcription initiation or cap site, the TATA box and CAAT box, the GAL genes also employ transcriptional enhancers called "upstream activator sequences" or UASs. Regulation of the GAL genes involves induction by galactose and repression by glucose with the help of both positive and negative-acting protein transcription factors. Six genes (GAL1, 2, 5, 7, 10 and MEL1) are required for yeast to convert the disaccharide melibiose to Glu-6-P which is then used in glycolysis. The product of the MEL1 gene ( -galactosidase) is secreted outside the cell where it converts melibiose to galactose. Galactose is transported into the cell by the product of GAL2 (permease) where it is converted to an inducer (I) by the product of GAL3. The Gal3 and Gal5 genes are not regulated by galactose but GAL3 acts as an inhibitor of Gal80. BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 36 March 19, 2012 Pathway of Galactose Utilization: 6 Key Genes GAL2 permease galactose external GAL1 kinase GAL7 transferase GAL5 mutase Glu-6-P galactose internal Gal-1-P Glu-1-P -galactosidase MEL1 -Gal-Glumelibiose March 19, 2012 UDP-Glu UDP-Gal epimerase GAL10 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 glycolysis 37 Model for Gal Gene Regulations The model for GAL gene regulation in yeast is based on the following observations: (1) GAL genes are expressed in abundance when yeast cultures are shifted from galactose-free to galactose-containing media. (2) Unlinked mutations (GAL4 and GAL80) affect the expression of these genes. GAL4 is expressed constitutively and activates transcription of the GAL1, 2, 7, 10 and MEL1 genes by binding the UAS enhancers. In this position the carboxy-terminal end of GAL4 interacts with a TATA box binding factor (TBF). (3) The TBF interacts with RNAPII to stimulate transcription of the adjacent gene. GAL80 interacts with GAL4 to prevent activation of the TBF/RNAPII complex. (4) When yeast cells are grown in the presence of glucose (or a combination of glucose and galactose), the catabolite repressor protein (CRP) interacts with the DNA binding domain of GAL4 to prevent its binding to the UAS. BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 38 March 19, 2012 Mutation Analysis Mutational analysis has shown that the GAL genes (Gal1, 2, 7, 10 and Mel1) are induced by galactose and not expressed constitutively. Regulation is accomplished through the action of two additional regulatory proteins, GAL4 and GAL80. Mutations in GAL4 (gal4-) produce an uninducible phenotype. A gal80mutation produces a constitutive phenotype. As shown in the following figure, the GAL4 protein has a stimulatory affect (shaded arrows) on GAL gene transcription. GAL80 blocks (T-bar) the affect of GAL4. GAL80 is an antagonist of GAL4. The action of GAL80 is blocked when bound by the galactose sensor protein,Gal3. BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 39 March 19, 2012 Components of the Gal Regulatory Circuit GAL7 GAL10 GAL1 XII GAL2 II XIII MEL1 GAL4 XVI GAL4 GAL80 XIII Blocks GAL4 Galactose + inducer March 19, 2012 GAL3 GAL80 Blocks GAL80 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 40 UAR Between GAL 1 and GAL10 Between GAL1 and GAL10, there is a 356 base pair region that contains 8 UAS sequences. GAL10 TATA TBP TBP TATA RNAPII RNAPII GAL1 GAL10 TATA TATA GAL1 GAL10 March 19, 2012 CRP TATA CRP TATA GAL1 41 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 Gal Induction by Histone Acetylation Part of the induction of the Gal genes is the recruitment of coactivators and chromatin remodeling proteins to help form the pre-initiation complex (PIC) for Gal gene transcription. After acetylation by SAGA, the SWI/SNF complex excises nucleosomes as part of PIC formation. GAL gene induction Nature Genetics Reviews 2010 March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 42 Summary of Gal Gene Regulation Components: Genes: GAL7, 10, 1, 4, 80, 2, MEL1 (GAL3 and 5 are involved but not regulated by galactose). Proteins: GAL4, GAL80, GAL3 RNAPII, TATA-box binding factor (TBF), catabolite repression protein (CRP) induced by glucose. DNA Binding Sites: UASs and TATA box. Effector Molecules: Galactose and Glucose. Interactions: 1. RNAPII/TATA = no mRNA 2. GAL4/UAS/TBF/RNAPII/TATA = mRNA 3. GAL4/UAS/GAL80 = no mRNA 4. Inducer+GAL3/GAL80 (blocks GAL4/GAL80 interaction) = mRNA 5. CRP/GAL4 = no mRNA Mutations: 1. gal4- (uninducible, UN) 2. gal80- (constitutive, C) 3. gal80s (UN, insensitive to inducer) 4. gal4(81) (Constitutive, can not bind GAL80) 5. gal4(pro26) (UN, can not bind UAS, corrected by Zn ion) March 19, 2012 6. gal3- (uniducible, UN) BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 43 Sequence of Events in Gal Regulation Galactose No Galactose [I] [80/3] [80/4] [4/TBF] = [mRNA] [I] [80/3] [80/4] [4/TBF] = no mRNA Gal + Glucose [I] [80/3] [80/4] [CRP/4] [4/TBF] = no mRNA Conclusions: 1. GAL4 is an activator protein 2. GAL80 is an antagonist of GAL4 3. I (inducer) + GAL3 are antagonists of GAL80 4. CRP is a glucose-induced antagonist of GAL4 What is the nature of the GAL4-UAS interaction? March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 44 Nature Gal4/UAS binding GAL4 regulates transcription by binding to UASs. The UASs are found in the promoter regions of all the GAL genes regulated by GAL4. The UASs act similarly, but not identically, to mammalian enhancer sequences (i.e., modular, orientation independent, act at a distance to activates transcription). Some kind of DNA binding mechanism must be invoked to explain the action of UASs and enhancers. UAS: Sequence analysis of the UASs show that they are highly conserved. UASgal are 17 base pair DNA sequences with dyad symmetry 5'-C G G G C = may be C G A C G T G C A C C A T G 5' G T T C C A C G T G C A C G G G C- March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 45 The Protein GAL4 Structure GAL4 is a 99kDal (881 amino acid) metalloprotein with two Zn ions bound in a zincfinger. GAL4 acts as a dimer and the zinc finger of each monomer binds to the major groove in the UASs. GAL4 has two separate binding domains, one for GAL80 and one for the UAS. The GAL80 binding site on GAL4 is also the binding site for TBF. Although GAL80 blocks GAL gene transcription, it does not bind DNA like the lac repressor. Gal 80 binding site UAS binding sites March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 46 Proposed GAL4 -- Cys6 Zinc Finger Lys+ Ser Cys Lys+ Leu Lys+ Lys+ Leu Arg+ Cys Ile Asp NH2 March 19, 2012 Glu Lys+ Pro Lys+ Cys Ala Lys+ Cys Leu Asn Asn Zn Zn Trp Glu Cys Arg+ COOH 47 Ala Cys BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 Comparing Lac and Gal Gene Regulation Lactose Operon Multiple genes regulated Polycistronic mRNA Regulation responsive to changes in external chemical environment Negative control by repressor Repressor acts as a homotetramer Repressor binds operator DNA sequence Operator sequence is palindromic Inducer molecule (allolactose) inactivates repressor Positive control by catabolite activation protein (CAP) Repression of CAP by a glucose catabolite that reduces cAMP levels Galactose Gene Regulation Multiple genes regulated Monocistronic mRNAs Regulation responsive to changes in external chemical environment Negative control by GAL80 GAL80 acts as a dimer GAL80 binds GAL4, not UAS DNA sequences UAS sequences are palindromic Inducer molecule (galactose) blocks GAL80 through GAL3 binding Positive control by GAL4 dimer Repression of GAL4 by a glucose-induced, catabolite repressor protein (CRP) 48 March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 Example of Gene Regulation in Humans Fetal hemoglobin turns off after birth and is replaced by the adult hemoglobin, March 19, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 49 ...
View Full Document

This note was uploaded on 03/18/2012 for the course BIS 101 taught by Professor Simonchan during the Winter '08 term at UC Davis.

Ask a homework question - tutors are online