07BIS1012012ReplicLect_7

07BIS1012012ReplicLect_7 - BIS101-001: Genes and Gene...

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Unformatted text preview: BIS101-001: Genes and Gene Expression Chromosome Structure, DNA Stability and Replication Lecture #7 Chapter 7 March 19, 2012April 22, 2009 January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 1 Last Lecture One Gene-One Enzyme Hypothesis Inborn Errors of Metabolism. Molecular Basis of Dominant and Recessive Traits. Complete, Incomplete and Co-Dominance ABO Blood Groups Rh Disease March 19, 2012 BIS101001, Spring 2009--Genes and Gene Expression, R.L. Rodriguez 2009 2 This Lecture: Key Concepts Homologous recombination involves several DNA modifying enzymes. Watson and Crick show the structure of DNA to be a double helix consisting of nucleotides, linked by phosphodiester bonds. The two strands of the helix are held together with hydrogen bonds. The structure of DNA suggests a mechanism for replication in which one strand is use as a template for the synthesis of a new strand. Weak chemical bonds and interactions enable DNA to be replicated Meselson and Stahl demonstrate that DNA replicates semiconservatively. March 19, 2012April 22, 2009 Many enzymes take part in the replication process. BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 3 January 31, 2012 Molecular Basis of Meiotic Recombination:Mov Meselson-Radding Model and Holliday Structure Endonuclease DNA polymerase Strand Displacement DNA ligase Branch migration Resolvase January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 4 5' to 3' Synthesis: The Movie January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 5 In this Lecture DNA as the Genetic Material: A series of experiments conducted between 1928 and 1952 demonstrated beyond a doubt that genes were composed of DNA, and not lipids, polysaccharides, or protein. Structure of DNA: Watson and Crick deduce the structure of DNA from 15 published facts and X-ray crystallography data showing that the phosphate/sugar backbone is on the outside of a double helix and the hydrophobic bases are on the inside. Weak chemical bonds and interactions: The great stability of DNA is due to five physico-chemical interactions; ionic bonds, hydrophobic interactions, -bonds, Van der Waals forces and hydrogen bonding. Replication: The weak bonds between the strands of DNA permit DNA to be replicated in a semiconservative fashion. The constraints on replication require that DNA be replicated in a semidiscontinuous fashion. BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 6 January 31, 2012 Milestones of Modern Genetics April 25, 2003 -- 50th anniversary of the discovery of DNA April 22, 2008 January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 7 DNA Composition (everything you need to know) 1. DNA consists of three components: q q q The pentose sugar, deoxyribose Phosphate Four nitrogenous bases (A, T, G and C) 1. 2. Deoxyriboses are linked by covalent by phosphodiester bonds between the 3'OH of the sugar and the phosphate group on the 5' carbon of the adjacent deoxyribose. The nitrogenous bases are of two types: q q Purines are double ring structures (A and G). G (guanine) can be distinguished from A (adenine) by the oxygen on the double ring Pyrimidines are single ring structures (T and C). C can be distinguished from T by the NH2 on the single ring structure. 1. 2. In RNA, U substitutes for T and ribose substitutes for dexoribose (ribose has an OH at both the 3' and 2' positions and U lacks the methyl group found on T). In double stranded DNA (or in a DNA/RNA hybrid), strands are antiparallel -- the two ends of the DNA molecule must have opposing 5' PO4 and 3'OH groups. BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 http://www.youtube.com/watch?v=ZGHkHMoyC5I 8 January 31, 2012 The Nitrogenous Bases January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 9 The Nitrogenous Bases DNA RNA January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 10 DNA composition is species specific Chargaff documented that base compositions varied among species. Based on his chemical analysis of many different DNAs, he proposed the Chargaff's Rules which state that in all species: [ A ] = [ T ] and [ C ] = [ G ] and [ A ] + [ G ] = [ C ] + [ T ]. January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 11 Nucleotides: The Sugars O O - P O N N O H N H 5' H 4' 3' O C H O ON P O N N H N H N N H N H 5' H O C H O 1' 2' 4' 3' O OP O O 1' 2' H O O - OH O P O Deoxyribose Ribose January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 12 Nucleotides: The Phosphates OOOOOP O H C H O O N N N H N N H H P O O O- P O O OH N N N N H H P O O O- P O O O- P O O N H N H N N H H H C H O N H C H O N OH Deoxyadenosine Monophosphate 2' position if OH, then Adenosine Monophosphate OH H OH H Deoxyadenosine Diphosphate 2' position - if OH, then Adenosine Diphosphate Deoxyadenosine Triphosphate 2' position - if OH, then Adenosine Triphosphate January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 13 DNA Structure 3' 5' 3 - 5' 14 O O P O H H O O C C O P O DNA is a plectonemic coil consisting of two polynucleotide chains of DNA. Because the 1' carbon of the O sugar bonded to the nitrogenous base limits the O rotation of the base, the -O P O H H O strands of DNA must be N CH H arranged in an antiparallel N N O N fashion. N O H C H N N DNA is also a polyanion with A T a negative charge on every phosphodiester bond. O H O Electrostatic repulsion H -O P O N N N causes DNA to writhe or twist H O N violently in solution. Without N N N H stabilizing forces, N H C H O H phosphodiester bonds will C G break and the DNA will degrade. 3' O BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 January 31, 2012 -O P O H H O - What are the factors contributing to DNA stability? Ionic bonds Hydrophobic interactions -bonds Van der Waals interaction Hydrogen bonds January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 15 Factors contributing to DNA stability 1. Ionic bonds involve an attraction between unlike charges (e.g., between the (-) carboxyl group of aspartic acid and the (+) amino group of lysine). The ability of cations like Na+ to neutralize the effects of electrostatic repulsion is a good example of the stabilizing effects of ionic bonds. Na+ acts as a counter ion that neutralize the (-) charge on the phosphodiester bonds. Writhing subsides and the backbone of the helix becomes more rigid or stiff. This promotes the stacking of the bases, one over the other. All of this leads to greater stability of the helix. This phenomenon underscores the reasons sodium is typically added to DNA solutions in the laboratory. + Na Cl + H2O = Na+ OCl- January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 16 DNA stability: Ionic bonds The ability of cations like Na+ to neutralize the effects of electrostatic repulsion is a good example of the stabilizing effects of ionic bonds. Na + acts as a counter ion that binds to the (-) charge on the phosphate group. Writhing subsides and the backbone of the helix becomes more rigid or stiff. This promotes the stacking of the bases, one over the other. All of this leads to greater stability of the helix. This phenomenon underscores the reasons NaCl is typically added to DNA solutions in the laboratory. - - - -- - - - - - - - - - - - - - - - - Na+ - January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 17 DNA stability: Hydrophobicity Hydrophobicity stabilizes DNA by forming ordered shells of water around each base. This ordering of water molecules into shells decreases entropy (or the tendency toward molecular randomness) and therefore is thermodynamically unfavorable. For this reason, shells tend to fuse together, thus reducing order or increasing entropy. Since fewer water molecules are ordered in fused shells as compared to shells around individual bases, a smaller number of water molecules remain in an ordered state. This increased entropy is thermodynamically favorable. = H 20 January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 18 Base Stacking Two views of base stacking in DNA. Figure 1 shows a van der Waal space filling model of single stranded DNA. The DNA backbone is shown as a yellow ribbon. On the right (Fig. 2), hydrogen bonding between between complementary base pairs and between stacked bases are shown (black arrow). BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 19 January 31, 2012 DNA stability:van der Waals interactions van der Waals forces also contribute to DNA structure. Because of fluctuating charge distributions in atoms and molecules (dipole moments), they will attract each until they reach a point where the attractive force and the repulsive force balance out. This occurs when outer shell electrons in each atom begin to overlap. Half the distance between their respective nuclei is called Van der Waals radii. These forces are about as strong as the weakest H bonds and therefore promote DNA stability. These forces are particularly important in DNA/protein interactions where a close fit between molecules is required. March 19, 2012April 22, 2009 s orbitals of Cl2 January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 20 DNA stability: Hydrogen bonds Hydrogen bonds are weak; only -0.3 to -3.0 kcal/mol (Gibbs energy) above thermal noise. Hydrogen bonds are only second to the phosphodiester bonds in terms of contribution to DNA stability. The weakest hydrogen bond in DNA is -0.9 kcal/mol (AA), whereas thermal noise is about +0.6 kcal/mol. Watson/Crick base pairing between antiparallel strands of DNA: q q (a) reduces rotation around the phosphodiester bond and thus reducing writhing and helix instability (b) protects reactive groups on the bases from chemical modification that could lead to mutation. The weak nature of these bonds allows strands of DNA to separate or denature during replication or transcription. It should also be noted that because of intramolecular complementarity, single stranded DNA molecules (and RNA molecules) can form double helices with themselves. The transfer RNA (tRNA) is a good example of this. Gibbs energy (Gibbs free energy) is the thermodynamic potential that measures the "useful" or "processinitiating" work obtainable in a closed thermodynamic system. As a process changes states (e.g., from double strand DNA to single strand DNA) Gibbs energy changes from - G to + G. January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 21 Intramolecular complementarity Transfer RNA January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 22 Alternate DNA Forms: January 9, 1981 March 19, 2012April 22, 2009 January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 23 Alternate DNA Forms DNA structure can vary in nature. Although the double-stranded B helix is the predominant form, other helical and some completely unexpected forms can be found. B-DNA A-DNA Z-DNA March 19, 2012April 22, 2009 January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 24 Alternate DNA Structures: A form DNA The A form helix is not the predominant form of doublestranded RNA A-form DNA or RNA has 11 bases per turn rather instead of the 10 in B-DNA A-form DNA is wider and shorter than the B helix, and the distinction between the major and minor grooves is reduced This helix is favored under conditions of dehydration A-DNA Minor groove Major groove January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 25 Alternate DNA Structures: Z form DNA The Z helix (so-named because the backbone has a zigzag shape) is a more radical departure from the B helix theme. Although double stranded, the Z helix is a left-handed helix and has 12 base pairs per turn instead of the 10 in B-DNA and 11 in A-DNA. Z DNA appears longer and slimmerlooking than the B helix. Unlike B-DNA and A-DNA, Z-DNA is dependent on the nucleotide sequence. Alternating G and C promotes the formation of Z-DNA. Therefore, stretches of Z-DNA can be found interspersed in BDNA chromosomes. The presence of regions of Z DNA near genes on the same molecule can influence their expression by promoting protein binding. January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 26 Copying Mechanism 5' 3' DNA Replication A G T C T C G A A G A A C G G C C G T T T 3' A C G G T G C C 5' A T T C G T A A T T C T A A G A T C A G C January 31, 2012 G A T C G 27 3'OH 5' 3' 3' OH G C BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 A T 3 Models for DNA Replications March 19, 2012April 22, 2009 January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 28 Semiconservative Copying Mechanism January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 29 Alternative Copy Mechanisms Conservative Dispersive March 19, 2012April 22, 2009 January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 30 Density (isopicnic) gradient ultracentrifugation In 1957, Mat Meselson and Franklin Stahl used the heavy isotope of nitrogen (N15) and ultracentrifugation to demonstrate that bacterial DNA replicates in a semiconservative fashion and not in a dispersive or conservative fashion. N14 N15 January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 31 Meselson-Stahl Experiment According to the conservative model, "hybrid" density DNA is never produced. While the dispersive model would produce hybrid DNA at G1, it produces hybrid DNA at G2 and G3 as well. Only the semiconservative model produces hybrid at G1 and light at G2. This is only consistent with the Watson and Crick structure for DNA. BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 32 January 31, 2012 Chromosome Replication January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 33 Replication of DNA With the exception of some viruses and bacteriophage, most organisms, from bacteria to man, replicate their DNA in a semiconservative and bidirectional fashion. The Meselson and Stahl experiment confirmed the validity of the semiconservative model, while dispelling the conservative and dispersive models. Bidirectional DNA replication was first visualized in the bacteria E. coli and later in demonstrated in higher eukaryotes, including man. Autoradiography (in combination with conditional lethal DNA initiation mutants of E. coli) was used to radio-label the E. coli chromosome at the origin and terminus of replication. Chromosomes replicating in only one direction would give one type of autoradiographic image (A) while a bidirectional replication would give a different image (B). BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 34 January 31, 2012 Bidirectional DNA Replication Using a temperature sensitive DNA initiation (dnaA) mutant of E. coli, the origin and terminus of replication were labeled with a specific activity of 3H-thymine 4 times higher than the rest of the chromosome. This labeling scheme produced the autoradiographic image shown at the right. This image is consistent with the bidirectional model for DNA replication shown below (B). Origin C Unidirectional Bidirectional March 19, 2012April 22, 2009 Terminus 35 January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 Initiation of DNA Replication at OriC The "ring" protein, DnaA is part of a superfamily of proteins, called AAA+, that are molecular "initiators" of DNA replication in the bacteria, Escherichia coli (E. coli), the archaebacteria, and in a eukaryote, Drosophila melanogaster, the fruit fly. This suggests that the AAA+ proteins is at the heart of DNA replication initiation. This indicates that DNA replication is an ancient process that evolved millions of years ago, prior to when Archae, Bacteria and Eukarya split into separate domains of life. January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 DNA Replication http://www.youtube.com/watch?v=4jtmOZaIvS0 January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 Microtopology of DNA replication DNA replication is a highly complex process. In bacteria like Escherichia coli (E. coli), DNA replication requires approximately 23 proteins, including 7 different enzymes, to accomplish DNA replication. This complex of proteins and enzymes permits both strands of DNA to be synthesized efficiently and accurately. Much of our current understanding of DNA replication comes from the biochemical analysis and genetic studies employing conditional lethal mutants. Conditional lethal mutations are the preferred way to genetically study cellular functions that are absolutely critical for cell viability. BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 38 January 31, 2012 Conditional Lethal Mutants Conditional lethal mutations affect critical cellular functions such as replication, transcription and translation. Conditional lethal mutants exhibit their phenotypes only under "restrictive" or "non-permissive" conditions. For example, under permissive conditions (e.g., low temperature) cells grow and replicate DNA normally. However, under restrictive or non-permissive conditions (e.g., high temperature) cells stop replicating DNA and die. A common type of conditional lethal is the temperature sensitive (ts) mutant. For DNA replication ts mutants fall into two categories: q q Mutants that complete ongoing rounds of replication and then stop and fail to initiate new rounds of replication ( slow stop). These mutants are also called initiation mutants. The second class of mutant is the "instant stop" type in which DNA replication stops immediately upon shifting to the restrictive temperature. Instant stop mutants typically involve those components of the replication fork required for DNA synthesis. BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 39 January 31, 2012 Genes Involved in E. Coli DNA Replication Gene dnaJ dnaK polB polC/dnaE dnaL ter dnaI dnaI lig rpoD dnaG rpoA dnaA ori rep dnaP polA rpoB rpo C dna B ssb dnaC Gene Product and/or Function DNA Replication DNA Replication DNA polymerase II Alpha subunit of DNA polymerase III DNA replication Terminus of chromosomal replication Initiation of Chromosomal replication DNA ligase gap sealing, Okazaki fragment joining RNA polymerase sigma subunit Primase-makes primer for extension of DNA polymerase DNA polymerase, alpha subunit Initiation of DNA replication Origin of chromosomal replication Helicase, unwinding activity to generate single-stranded DNA Initiation of chromosomal replication DNA polymerase I RNA polymerase beta subunit RNA polymerase beta' subunit beta' DNA replication Single-stranded binding proteins Initiation of chromosomal replication, helicase Map Location 0.5 min. 0.5 min. 2 min. 4 min. 28 min. 27-43 min. 39 min. 51 min. 66 min. 66 min. 72 min. 82 min. 83 min. 83 min. 84 min. 85 min. 89 min. 89 min. 91 min. 91 min. 99 min. January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 40 Requirements & Components for DNA Replication Rules: 1. DNA (and RNA) can only be polymerized in the 5' to 3' direction. 2. The ends of a double stranded DNA molecule must be anti-parallel. 3. DNA polymerase can not initiate the polymerization process de novo (a primer is required). 4. Primers must be removed before DNA replication is complete. Components: 1. Deoxyribonucleotide triphosphate precursors (dATP, dTTP, dCTP, dGTP). 2. 3'-OH template primer. 3. DNA polymerases I and III. 4. DNA polymerase 5' to 3' exonuclease activity for primer removal. 5. DNA polymerase 3' to 5' exonuclease activity for correcting errors. 6. RNA polymerase (primase) for RNA primer synthesis. (This enzyme can initiate de novo) 7. DNA ligase for sealing up nicks in DNA. 8. DNA unwinding enzymes like dnaA, helicas and rep A protein. 9. Helix destabilizing proteins (ssb). 10. DNA gyrases like topoisomerase II January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 Comparison of DNA polymerase I and III Characteristics Molecular weight De novo initiation 5'-3' polymerization Polymerization rateb 3'-5' exonuclease 5'-3' exonuclease a pol Ia 103kDal -- + ~600bp/min + + pol III 380kDal -- + ~6000bp/min + (--) A 67kDal proteolytic cleavage product of pol I (Klenow fragment) possesses the 5'-3' polyermization activity, but not the 5'-3' exonuclease activity. b The E. coli chromosome replicates at a rate of 25K to 60K bp/min/fork or ~1000bp/sec/fork. January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 42 DNA Polymerase III Monomer 140 kD 25kD 32kD 52kD 25kD 140 kD 10kD 83kD 32kD 52kD 10kD 83kD 37kD 37kD March 19, 2012April 22, 2009 Dimer January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 43 Anti parallel sugar-phosphate chains March 19, 2012April 22, 2009 January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 44 The 5' to 3" Dilemma The 5' to 3' dilemma. Because the newly synthesized strands of DNA must be antiparallel to the template strands, and the replication fork moves forward, it would appear that one strand must be synthesized in the 3' to 5' direction. However, no DNA or RNA polymerase with this activity has ever been found in nature. The solution to this dilemma is to synthesize of the one strand backwards, in the 5' to 3' direction. This is referred to as discontinuous synthesis of the lagging strand. The idea for discontinuous synthesis was first put forth by Dr. Reiji Okazaki who used 3H-thymidine and pulse-chase experiments to show that newly synthesized DNA was made in short pieces (1000 to 2000bp) on the lagging strand. January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 45 DNA Replication Fork March 19, 2012April 22, 2009 January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 46 The Priming Problem DNA polymerase cannot initiate DNA synthesis de novo. A 3'-OH template primer is required. 5'-P P P P P P HO-3' 3'-OH P P P P P P P P P P P P -5' Therefore, once the two strands of DNA have separated, a primer must be added by a polymerase which can initiate de novo. This enzyme is a special type of RNA polymerase called primase, the product of the dnaG gene of E. coli. Primase works in conjunction with collection of proteins called the primosome to synthesize small (30 to 50 bases) RNA primers at the replication fork. The 3'OH of the primer "primes" DNA synthesis. That this primer is RNA, and bears a 5' triphosphate on one end, creates another problem. DNA ligase can not use such termini as substrates. Therefore, the primer must be first erased by the 5' to 3' exonuclease activity of DNA polymerase. BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 47 January 31, 2012 DNA Replication: Lagging Strand January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 48 DNA Ligase: Mechanism of Action O DNA Strand 3' OH O P O LIGASE O 5' DNA Strand + ATP PPi + LIGASE AMP P AMP 5' P 5' DNA Strand DNA Strand + LIGASE DNA Strand DNA Strand 3' OH 3' + O O 5' DNA Strand O P O LIGASE January 31, 2012 + AMP 49 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 5' to 3' Synthesis: The Movie January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 50 3' to 5' Exonuclease Activity A C/A mismatch is detected by DNA polymerase and removed by 3'-5' exo activity January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 51 Microtopology at the Replication Fork http://www.youtube.com/watch?v=-mtLXpgjHL0&feature=endscreen&NR=1 January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 52 Concurrent Semidiscontinuous Model The semidiscontinuous synthesis model for DNA replication has been restated as the concurrent semidiscontinuous model of DNA replication. In this model the replication fork to move forward smoothly while taking into account that DNA polymerase III only functions as a dimer and is not free to migrate away from the replication fork. BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 January 31, 2012 January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 53 Concurrent Semidiscontinuous Synthesis January 31, 2012 BIS101001, Spring 2012--Genes and Gene Expression R.L. Rodriguez 2012 54 ...
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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.

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