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Manipulating DNA
Manipulating DNA
• Scientists use their knowledge of the structure of DNA and its chemical properties to study and change DNA molecules
• Different techniques are used to study and change DNA molecules
• Genetic Engineering – making changes in the DNA code of a living organism
• Bacteria are the workhorses of modern biotechnology
Tools of Molecular Biology: Recombinant DNA
• Recombinant DNA techniques can help biologists produce large quantities of a desired protein
• To work with genes in the laboratory, biologists often use bacterial plasmids, small, circular DNA molecules
– Plasmids can:• can carry virtually any gene,• can act as vectors, DNA carriers that move genes from one
cell to another, and• are ideal for gene cloning, the production of multiple identical
copies of a gene-carrying piece of DNA.
Recombinant DNA
• Recombinant DNA is produced by combining two ingredients:• a bacterial plasmid and • the gene of interest.
• To combine these ingredients, a piece of DNA must be spliced into a plasmid
Creating Recombinant DNA
• In order to create Recombinant DNA, there needs to be:– DNA extraction
• Cells opened to separate DNA from other cell parts
– Cutting DNA
• DNA too large to study, so biologists “cut” them into smaller fragments using restriction enzymes. Many restriction enzymes are known and each one cuts DNA at a specific sequence of nucleotides
• Produces pieces of DNA called restriction fragments with “sticky ends” important for joining DNA from different sources.
– Splicing DNA back together
• DNA ligase connects the DNA pieces into continuous strands by forming bonds between adjacent nucleotides
Restriction EnzymesRecognition site (recognition sequence)for a restriction enzyme
Restrictionenzyme
Sticky end
Sticky end
A restriction enzyme cuts the DNA into fragments.
1
Restriction EnzymesRecognition site (recognition sequence)for a restriction enzyme
Restrictionenzyme
Sticky end
Sticky end
DNA
A DNA fragment is added from another source.
A restriction enzyme cuts the DNA into fragments.
1
2
Restriction EnzymesRecognition site (recognition sequence)for a restriction enzyme
Restrictionenzyme
Sticky end
Sticky end
DNA
A DNA fragment is added from another source.
A restriction enzyme cuts the DNA into fragments.
Fragments stick together bybase pairing.
1
2
3
Restriction EnzymesRecognition site (recognition sequence)for a restriction enzyme
Restrictionenzyme
Sticky end
Sticky end
DNA
DNAligase
Recombinant DNA molecule
A DNA fragment is added from another source.
A restriction enzyme cuts the DNA into fragments.
Fragments stick together bybase pairing.
DNA ligase joins the fragments into strands.
1
2
3
4
Restriction Enzymes
Recognition sequences
DNA sequence
Restriction enzyme EcoRI cuts the DNA into fragments. Sticky end
Restriction Enzymes
Recognition sequences
DNA sequence
Restriction enzyme EcoRI cuts the DNA into fragments. Sticky end
Finding the Gene of Interest
• How can a researcher obtain DNA that encodes a particular gene of interest? First, you have to have an idea of what the gene is you want to work with (get a genomic library). Then:
• Using a nucleic acid probe consisting of a short single strand of DNA with a complementary sequence and labeled with either a radioactive isotope or a fluorescent dye.
• Or by synthesizing it through reverse transcriptase (viral enzyme that makes DNA)
• Or by making it by scratch with machines
DNA Profiling and Forensic Science
• DNA profiling• can be used to determine if two samples of genetic
material are from a particular individual and• has rapidly revolutionized the field of forensics, the
scientific analysis of evidence from crime scenes.
– To produce a DNA profile, scientists compare sequences in the genome that vary from person to person.
DNA Profiling and Forensic Science
– DNA profiling can be used to• test the guilt of suspected criminals,• identify tissue samples of victims,• resolve paternity cases,• identify contraband animal products, and• trace the evolutionary history of organisms.
DNA Profiling Techniques: Making Copies of DNA
• Polymerase Chain Reaction (PCR) technique– Allows biologists to make many copies of a
specific piece of DNA– DNA strands separated with heat, then cooled
to allow DNA Polymerase to start making new copies of DNA
– A few dozen heat and cool cycles results in many copies of DNA
Making Copies of DNADNA polymerase adds complementary strand
DNA heated to separate strands
DNA fragment to be copied
PCRcycles 1
DNAcopies 1
2
2
3
4
4
8
5 etc.
16 etc.
DNA Profiling Techniques: STR Analysis
– Short tandem repeats (STRs) are:• short sequences of DNA that are repeated many times,
tandemly (one after another), in the genome.
– STR analysis• Proves two samples come from the same person - Everyone
has these repetitive DNA sequences, but in different lengths and a different number of them
• compares the lengths of STR sequences at specific sites in the genome
• uses gel electrophoresis, a method for sorting DNA by size
DNA Profiling Techniques: Gel Electrophoresis
• Gel electrophoresis– Used to separate DNA fragments. DNA
fragments placed in a gel and electricity is applied to the gel. DNA molecules are negatively charged and move towards the positive end of the gel. Smaller DNA fragments move faster and farther
– This technique used to compare the genomes of different organisms or even different people
Gel Electrophoresis
DNA plus restriction enzyme
Mixture of DNA fragments
Gel
Power source
Longer fragments
Shorter fragments
Gel Electrophoresis
Gel Electrophoresis
DNA from
Victim
DNA from
Suspect #1
DNA from
Suspect #2
DNA from
Suspect #3
DNA from
Suspect #4
Blood found at Crime Scene
Which suspect should have more questioning?