Biochemistry-II - Molecular Cloning of DNA - Quick Guides

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Study GuideBiochemistry-II–Molecular Cloning of DNA1.DNA and InformationModern genetics—whether calledbiotechnology,genetic engineering, orDNA technology—iscentered on one main goal:understanding and manipulating DNA based on its information content.This means studying theexact sequence of nucleotides(A, G, C, and T) in a DNA molecule.Why DNA Is Hard to Study ChemicallyAt first glance, all DNA molecules lookchemically very similar.DNA from baker’s yeast and DNA from the organism that produces penicillin have nearly the samechemical structure, yet they encodevery different biological properties.This is because:•DNA has ahighly regular chemical structure•What really matters isnot the overall composition, but theorder (sequence) of basesAs a result, genescannot be separatedusing ordinary chemical techniques.1.1Sequence vs. CompositionTo understand why sequence matters more than composition, consider a simple example.Imagine a short DNA molecule made offour bases, one each of A, G, C, and T.•The base composition is the same in every case•But thesequence can varyThere are4! (4 × 3 × 2 × 1 = 24)different possible sequences, such as:•AGCT•ATGC

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Study Guide•CGTA•TGAC…and many moreAll of these molecules containexactly the same amounts of A, G, C, and T, yet they carrydifferentinformation.Because their chemical properties are so similar, traditional chemistry methods arevery poor atseparating them.As DNA molecules get longer, this problem becomes dramatically worse.1.2The Problem of ScaleThe challenge increases even more when we consider thesize of genomes.•Thehuman genomecontains about3 billion base pairs•Any single gene makes up only atiny fractionof that total DNAThis makes isolating and studying one specific gene extremely difficult.Not All DNA Is InformationalIn complex, multicellular organisms:•A large portion of DNA doesnot directly encode proteins or RNA•Thisnoninformational DNAincreases genome size without adding obvious functionAs a result:•Useful DNA sequences aredilutedamong vast amounts of other DNA•Studying a specific gene becomes even harderWhy DNA Technology Was RevolutionaryBefore the development ofDNA cloning and sequencing techniques, it was nearly impossible toobtain:

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Study Guide•Enough copies of a specific DNA sequence•Pure DNA corresponding to a single geneModern DNA technology solved this problem by enabling scientists to:•Separate nucleic acids based on sequence•Analyze DNA according to itsinformation content, not just its chemistryKey Takeaways•DNA technology focuses onsequence information•DNA molecules are chemically similar, making separation difficult•Sequence, not base composition, determines genetic information•Genome size and noncoding DNA add to the challenge•DNA cloning and sequencing made modern genetics possible2.DNA HybridizationWhat Is DNA Hybridization?DNA (and RNA) hybridizationis a technique used tocompare and analyze nucleic acidsequences.It works by allowing single strands of DNA or RNA to come together and formWatson–Crick basepairs.•Sequences that match well will pair easily•These paired strands form adouble helix, called ahybrid•The better the match, the more stable the hybridHybridization can occur between:•DNA–DNA•RNA–RNA•DNA–RNA

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Study GuideThe extent of base pairing tells ushow closely related two sequences arein terms of theirinformation content.2.1Melting Temperature (Tₘ)When a DNA double helix is heated, the two strands eventually separate.The temperature at whichhalf of the DNA is double-stranded and half is single-strandedis calledthemelting temperature (Tₘ).•Above Tₘ→ DNA is mostly single-stranded•Below Tₘ→ DNA is mostly double-strandedWhat Determines Tₘ?1. Base Composition•G–C base pairshavethree hydrogen bonds•A–T base pairshavetwo hydrogen bondsBecause G–C pairs are stronger:•DNA withhigh G+C contenthas ahigher Tₘ•DNA rich in A+T has alower Tₘ2. Salt Concentration•High salt (e.g., NaCl)raises Tₘ•Positive Na⁺ions shield the negative charges on the DNA backbone•This reduces repulsion and stabilizes the double helix3. Solvent Conditions•Some organic solvents increase repulsion between phosphates•Thislowers Tₘand destabilizes the helix

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Study GuideWhat If Sequences Are Only Partly Complementary?If two nucleic acid strands aresimilar but not identical:•Some regions will base-pair correctly•Other regions will containmismatchesWhether these strands stay together depends on thehybridization conditions.2.2Stringency: How Strict the Conditions AreThe termstringencydescribes how demanding the hybridization conditions are.Figure 1High Stringency•High temperature•Low salt•Few or no organic solvents

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Study GuideUnder these conditions:•Onlynearly perfect matchesremain paired•Duplexes with mismatches fall apartLow Stringency•Low temperature•High salt•No organic solventsUnder these conditions:•Evenimperfectly matched sequencescan remain paired•More mismatches are toleratedWhy Stringency MattersBy adjusting stringency, scientists can control:•Whetheronly exact matcheshybridize•Or whetherrelated but different sequencescan form hybridsThis makes hybridization a powerful analytical tool.2.3Applications of DNA HybridizationComparing OrganismsHybridization can be used to studyevolutionary relationships.Examples:•Human and chimpanzee DNA(≈98% identical)oHybridizes underhigh-stringencyconditions•Human and bird DNAoHybridizes only underlow-stringencyconditionsThe ability (or inability) of DNA strands to hybridize reflects how closely related the organisms are.

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Study GuideKey Takeaways•DNA hybridization measuressequence similarity•Complementary strands form stable double helices•Tₘmarks strand separation•G+C content strongly affects DNA stability•Stringencycontrols how many mismatches are allowed•Hybridization is used in genetics, evolution, and biotechnology3.Restriction Enzyme Mapping3.1Restriction Enzymes: A Bacterial Defense SystemRestriction enzymes act as aprimitive immune systemin bacteria.They protect bacterial cells bycutting foreign DNA, especially DNA from infecting viruses calledbacteriophages.•When viral DNA enters a bacterium, restriction enzymescleave the DNA•The fragmented DNA cannot be replicated•Although one bacterial cell may die, thebacterial population survivesBacteria avoid destroying their own DNA bychemically modifying it, usually throughmethylation,which prevents recognition by the restriction enzymes.3.2Restriction Endonucleases in Molecular BiologyScientists userestriction endonucleasesas precise tools to cut DNA.•Endonucleasemeans the enzyme cuts DNAwithin the molecule, not from the ends•These enzymes:oCutonly DNAoCut atspecific short nucleotide sequences

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Study GuideThis predictability allows DNA to bemapped, cloned, and analyzed.Type II Restriction EnzymesThe most useful restriction enzymes in molecular biology areType II restriction enzymes.Their key features:•Recognizeshort, specific DNA sequences•These sequences are usuallypalindromic•Cut DNA at or very near the recognition sitePalindrome:A DNA sequence that reads the same5′ → 3′on both strandsExample:5′ GAATTC 3′3′ CTTAAG 5′Example: EcoRIFigure 1EcoRIis a Type II restriction enzyme fromEscherichia coli.•Recognition sequence:5′ GAATTC 3′•EcoRI cuts:oBetweenG and Aon both strandsoLeaves a5′ phosphateon the A

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Study GuideResult of EcoRI CleavageAfter cutting:•Each DNA fragment has asingle-stranded overhang•The overhang sequence is5′ AATTThese overhangs are calledcohesive ends(orsticky ends) because:•They canbase-pair with complementary sequences•This property allows different DNA fragments cut with the same enzyme to bejoinedtogetherBlunt Ends vs Cohesive EndsNot all restriction enzymes produce sticky ends.Blunt EndsSome enzymes cutstraight through the DNAwith no overhangs.Example:•SmaI•Recognition site:5′ CCCGGG 3′•Cuts in theexact centerof the palindrome•Producesblunt endsBlunt ends:•Can be ligated together•But areless efficientthan cohesive ends3.3Protection of Bacterial DNA: MethylationBacteria protect their own DNA from restriction enzymes bymethylation.Example with EcoRI:•EcoRI methylaseadds a methyl group to:

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Study GuideoThesecond adeninein the GAATTC sequence•Methylation oneither strandis sufficient•Methylated DNA isnot cleavedby EcoRIThis system ensures:•Foreign DNA is destroyed•Host DNA remains intactWhy Restriction Enzymes Are So ImportantRestriction enzymes allow scientists to:•Cut DNA at known locations•Createrestriction maps•Clone genes into plasmids•Compare DNA sequences•Perform genetic engineeringThey are the foundation ofrecombinant DNA technology.Key Takeaways•Restriction enzymes evolved asbacterial defense mechanisms•Type II enzymes cut DNA atspecific palindromic sequences•EcoRI producescohesive (sticky) ends•Some enzymes produceblunt ends•Bacteria protect their DNA bymethylation•These enzymes are essential tools inbiotechnology

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