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Biochemistry-II - Molecular Cloning of DNA

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    Study Guide Biochemistry - II – Molecular Cloning of DNA 1. DNA and Information Modern genetics — whether called biotechnology , genetic engineering , or DNA technology — is centered on one main goal: understanding and manipulating DNA based on its information content . This means studying the exact sequence of nucleotides (A, G, C, and T) in a DNA molecule. Why DNA Is Hard to Study Chemically At first glance, all DNA molecules look chemically very similar . DNA from baker’s yeast and DNA from the organism that produces penicillin have nearly the same chemical structure, yet they encode very different biological properties . This is because: • DNA has a highly regular chemical structure • What really matters is not the overall composition , but the order (sequence) of bases As a result, genes cannot be separated using ordinary chemical techniques. 1.1 Sequence vs. Composition To understand why sequence matters more than composition, consider a simple example. Imagine a short DNA molecule made of four bases , one each of A, G, C, and T. • The base composition is the same in every case • But the sequence can vary There are 4! (4 × 3 × 2 × 1 = 24) different possible sequences, such as: • AGCT • ATGC

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    Study Guide • CGTA • TGAC …and many more All of these molecules contain exactly the same amounts of A, G, C, and T , yet they carry different information . Because their chemical properties are so similar, traditional chemistry methods are very poor at separating them . As DNA molecules get longer, this problem becomes dramatically worse. 1.2 The Problem of Scale The challenge increases even more when we consider the size of genomes . • The human genome contains about 3 billion base pairs • Any single gene makes up only a tiny fraction of that total DNA This makes isolating and studying one specific gene extremely difficult. Not All DNA Is Informational In complex, multicellular organisms: • A large portion of DNA does not directly encode proteins or RNA • This noninformational DNA increases genome size without adding obvious function As a result: • Useful DNA sequences are diluted among vast amounts of other DNA • Studying a specific gene becomes even harder Why DNA Technology Was Revolutionary Before the development of DNA cloning and sequencing techniques , it was nearly impossible to obtain:

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    Study Guide • Enough copies of a specific DNA sequence • Pure DNA corresponding to a single gene Modern DNA technology solved this problem by enabling scientists to: • Separate nucleic acids based on sequence • Analyze DNA according to its information content , not just its chemistry Key Takeaways • DNA technology focuses on sequence 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 possible 2. DNA Hybridization What Is DNA Hybridization? DNA (and RNA) hybridization is a technique used to compare and analyze nucleic acid sequences . It works by allowing single strands of DNA or RNA to come together and form Watson – Crick base pairs . • Sequences that match well will pair easily • These paired strands form a double helix , called a hybrid • The better the match, the more stable the hybrid Hybridization can occur between: • DNA – DNA • RNA – RNA • DNA – RNA

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    Study Guide The extent of base pairing tells us how closely related two sequences are in terms of their information content. 2.1 Melting Temperature (T ₘ ) When a DNA double helix is heated, the two strands eventually separate. The temperature at which half of the DNA is double - stranded and half is single - stranded is called the melting temperature (T ₘ ) . • Above T ₘ → DNA is mostly single - stranded • Below T ₘ → DNA is mostly double - stranded What Determines T ₘ ? 1. Base Composition • G – C base pairs have three hydrogen bonds • A – T base pairs have two hydrogen bonds Because G – C pairs are stronger: • DNA with high G+C content has a higher T ₘ • DNA rich in A+T has a lower T ₘ 2. Salt Concentration • High salt (e.g., NaCl) raises T ₘ • Positive Na ⁺ ions shield the negative charges on the DNA backbone • This reduces repulsion and stabilizes the double helix 3. Solvent Conditions • Some organic solvents increase repulsion between phosphates • This lowers T ₘ and destabilizes the helix

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    Study Guide What If Sequences Are Only Partly Complementary? If two nucleic acid strands are similar but not identical : • Some regions will base - pair correctly • Other regions will contain mismatches Whether these strands stay together depends on the hybridization conditions . 2.2 Stringency: How Strict the Conditions Are The term stringency describes how demanding the hybridization conditions are. Figure 1 High Stringency • High temperature • Low salt • Few or no organic solvents

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    Study Guide Under these conditions: • Only nearly perfect matches remain paired • Duplexes with mismatches fall apart Low Stringency • Low temperature • High salt • No organic solvents Under these conditions: • Even imperfectly matched sequences can remain paired • More mismatches are tolerated Why Stringency Matters By adjusting stringency, scientists can control: • Whether only exact matches hybridize • Or whether related but different sequences can form hybrids This makes hybridization a powerful analytical tool. 2.3 Applications of DNA Hybridization Comparing Organisms Hybridization can be used to study evolutionary relationships . Examples: • Human and chimpanzee DNA (≈98% identical) o Hybridizes under high - stringency conditions • Human and bird DNA o Hybridizes only under low - stringency conditions The ability (or inability) of DNA strands to hybridize reflects how closely related the organisms are.

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    Study Guide Key Takeaways • DNA hybridization measures sequence similarity • Complementary strands form stable double helices • T ₘ marks strand separation • G+C content strongly affects DNA stability • Stringency controls how many mismatches are allowed • Hybridization is used in genetics, evolution, and biotechnology 3. Restriction Enzyme Mapping 3.1 Restriction Enzymes: A Bacterial Defense System Restriction enzymes act as a primitive immune system in bacteria. They protect bacterial cells by cutting foreign DNA , especially DNA from infecting viruses called bacteriophages . • When viral DNA enters a bacterium, restriction enzymes cleave the DNA • The fragmented DNA cannot be replicated • Although one bacterial cell may die, the bacterial population survives Bacteria avoid destroying their own DNA by chemically modifying it , usually through methylation , which prevents recognition by the restriction enzymes. 3.2 Restriction Endonucleases in Molecular Biology Scientists use restriction endonucleases as precise tools to cut DNA. • Endonuclease means the enzyme cuts DNA within the molecule , not from the ends • These enzymes: o Cut only DNA o Cut at specific short nucleotide sequences

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    Study Guide This predictability allows DNA to be mapped, cloned, and analyzed . Type II Restriction Enzymes The most useful restriction enzymes in molecular biology are Type II restriction enzymes . Their key features: • Recognize short, specific DNA sequences • These sequences are usually palindromic • Cut DNA at or very near the recognition site Palindrome : A DNA sequence that reads the same 5′ → 3′ on both strands Example: 5′ GAATTC 3′ 3′ CTTAAG 5′ Example: EcoRI Figure 1 EcoRI is a Type II restriction enzyme from Escherichia coli . • Recognition sequence: 5′ GAATTC 3′ • EcoRI cuts: o Between G and A on both strands o Leaves a 5′ phosphate on the A

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    Study Guide Result of EcoRI Cleavage After cutting: • Each DNA fragment has a single - stranded overhang • The overhang sequence is 5′ AATT These overhangs are called cohesive ends (or sticky ends ) because: • They can base - pair with complementary sequences • This property allows different DNA fragments cut with the same enzyme to be joined together Blunt Ends vs Cohesive Ends Not all restriction enzymes produce sticky ends. Blunt Ends Some enzymes cut straight through the DNA with no overhangs. Example: • SmaI • Recognition site: 5′ CCCGGG 3′ • Cuts in the exact center of the palindrome • Produces blunt ends Blunt ends: • Can be ligated together • But are less efficient than cohesive ends 3.3 Protection of Bacterial DNA: Methylation Bacteria protect their own DNA from restriction enzymes by methylation . Example with EcoRI: • EcoRI methylase adds a methyl group to:

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    Study Guide o The second adenine in the GAATTC sequence • Methylation on either strand is sufficient • Methylated DNA is not cleaved by EcoRI This system ensures: • Foreign DNA is destroyed • Host DNA remains intact Why Restriction Enzymes Are So Important Restriction enzymes allow scientists to: • Cut DNA at known locations • Create restriction maps • Clone genes into plasmids • Compare DNA sequences • Perform genetic engineering They are the foundation of recombinant DNA technology . Key Takeaways • Restriction enzymes evolved as bacterial defense mechanisms • Type II enzymes cut DNA at specific palindromic sequences • EcoRI produces cohesive (sticky) ends • Some enzymes produce blunt ends • Bacteria protect their DNA by methylation • These enzymes are essential tools in biotechnology

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