advanced techniques in molecular biology
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Advanced Techniques in Molecular Biology. Section 6.3. Outline. Techniques Polymerase chain reaction Restriction fragment length polymorphism + Southern blotting DNA sequencing Applications Gene therapy Other applications. ADVANCED TECHNIQUES. Polymerase chain reaction - PowerPoint PPT PresentationTRANSCRIPT
Section 6.3
Advanced Techniques in Molecular Biology
OutlineTechniques
1. Polymerase chain reaction2. Restriction fragment length polymorphism + Southern
blotting3. DNA sequencing
Applications1. Gene therapy2. Other applications
ADVANCED TECHNIQUES•Polymerase chain reaction•Restriction length polymorphism & Southern blotting•DNA sequencing
Polymerase Chain Reaction (PCR)Technique for making many copies of DNA from a
small sampleThe sample DNA is said to be “amplified”
Contrast with cloning in a plasmid: direct method of isolating and replicating a desired DNA sequence
Technique is modeled after DNA replication
Uses some of the same machinery
Steps1. De-naturation of DNA (separation of strands)2. Priming of DNA3. Elongation of new DNA strand
Separation of strandsIn DNA replication, which enzyme performs this
function?
In PCR, heat (94°C - 96°C) is used to denature the strandsBreaks the H-bonds
Priming of DNA templateDNA primers are engineered in the lab
designed to be complementary to the template
Primers are annealed to the 3’ ends of the template strandsTwo different primers – the “forward” and “reverse”
primers
Temperature is reduced to allow annealing (50-65°C)
ElongationDNA Polymerase adds deoxyribonucleotides to
the 3’ end of the primer
Recall enzymes are denatured by heatDNA Pol III is denatured at 37°C
Heat-resistant polymerase is used: Taq polymeraseThermus aquaticus, hot springs bacterium
Cycle repeats over and over to produce many copies:
Video:http://highered.mcgraw-hill.com/olc/dl/120078/micro15.swf
CyclingTarget strand is not completely isolated after one roundOne cycle produces variable-length strands
After the second cycle, the target strand is isolated.Constant-length strands are produced.
Third cycle onwards: The number of target strands increases exponentially
Restriction Fragment Length Polymorphism (RFLP)
Polymorphism: Any difference in DNA sequence that can be detected between individuals. Can be in either a coding, or a non-coding region.
Individuals within a species are polymorphic.Coding polymorphisms are allelesNon-coding polymorphisms:
includes variable number tandem repeats (microsatellites), restriction fragment length
RFLP analysis: The principle
Restriction fragment: A region that is flanked by restriction sites. The sequence in between the sites is the “target” sequence.
The length of the target sequence is polymorphicIt will be different between individuals
The differences in restriction fragment length can be used to individualize DNA samples
Steps1. DNA is digested with restriction enzymes & denatured2. The digested sample is separated by gel electrophoresis3. Radioactive probes specific to the target sequence are
hybridized to the sequences4. Distinctive banding pattern will be detected, depending
on the location of restriction sites
Video:http://highered.mcgraw-hill.com/olc/dl/120078/bio20.swf
Why are probes needed?Genomic DNA is an extremely large source of DNAThe sample will appear to the eye as a continuous
smear of bandsNeed to “highlight” the target sequence
Southern blottingThe separated DNA needs to be transferred out of the gel
in order to hybridize with the probe
Procedure:Nylon membrane is placed on the gelElectric current is applied:
(+) behind nylon; (-) behind gelNegatively-charged DNA will transfer to the nylon
Video:http://highered.mcgraw-hill.com/olc/dl/120078/bio_g.swf
Detecting the target sequenceNylon membrane is immersed in a
solution with the radioactive probes
Allow probes to hybridize by complementary base pairing
Nylon is placed on X-ray filmExposure of film will occur where the
radioactive probes are locatedAn autoradiogram is the pattern of bands
on the X-ray film
DNA SequencingSanger dideoxy method
Based on the process of DNA replicationUtilizes DNA synthesis reactions to determine sequence
of bases in synthesized strand
Sequencing reactionsRequires four separate synthesis reactions
In each of the four reaction tubes, place the following components:
DNA to be sequenced (denatured first)a short, radioactively-labelled primer , complementary to
end of templateDNA polymerasefree nucleotides
“regular” deoxyribonucleotides (dNTPs), as well as a deoxyribonucleotide analogue (ddNTPs)
Deoxyribonucleotide analogueCalled a dideoxy analogueLike a deoxyribonucleotide, except does not have a
–OH group on the 3’ carbon
Free nucleotides cannot be added onto the 3’ end of a dideoxy analogue
Sequencing setup:Each reaction tube will locate a different nucleotide
(base) where it is incorporated into the new strand
One tube each for G, A, T, and C"G" tube: all four dNTP's, ddGTP and DNA polymerase"A" tube: all four dNTP's, ddATP and DNA polymerase"T" tube: all four dNTP's, ddTTP and DNA polymerase"C" tube: all four dNTP's, ddCTP and DNA polymerase
ProcessIn each reaction tube, allow synthesis to occurDNA polymerase will add on free nucleotides to the
end of the primerChain elongation will occurWhenever a ddNTP is incorporated into the chain,
synthesis will STOP
e.g., sequence to be elucidated:
5’-TTACGTACGTAA-3’ If a ddATP is incorporated instead of dATP, termination of
synthesis will occurHowever, sometimes a regular dATP will be incorporated,
allowing several possible fragments:
5’-TTA-3’5’-TTACGTA-3’5’-TTACGTACGTA-3’5’-TTACGTACGTAA-3’
This same process occurs in each of the four reaction tubes, but for different bases
To ensure that productive chain elongation occurs, dNTP’s will greatly outnumber ddNTP’s.
Reduces the probability of a ddNTP being incorporated whenever the complementary base is encountered.
End result:Each reaction tube will contain fragments of different
lengthsFragment length depends on where a ddNTP was
added to the chain
Through random incorporation of nucleotides, theoretically a fragment should exist that corresponds to every location of that base in the sequence
Analysis
Gel electrophoresisto separate fragments in each sample
Lanes:Sequencing reaction for GSequencing reaction for ASequencing reaction for CSequencing reaction for T
Allow the gel to runSouthern blotDetect fragments (expose X-ray film)
Primers were radioactive
Recall shorter fragments will migrate fartherFragments will differ by only one base pair
Reading the sequenceRead backwards from the positive electrode to
determine the sequence
APPLICATIONS•Gene therapy•PCR applications•RFLP applications
Gene therapyRefers to any method for treating genetic diseases
that involves altering the DNA sequenceInserting genesDeleting genesAltering expression of genes
Can act on either the germ line cells (results will be heritable), or the somatic cells
InsertionInserting genes can be accomplished by introducing
vectors into the host cellViral transfectionDirect injection of DNA
Insertion can occur at a random location: risk of altering existing host gene
Altering expressionUse an antisense oligonucleotide
“oligonucleotide” – A short nucleic acid (RNA) strand “antisense” – Complementary to a functional mRNA
Introduce short antisense RNA strandsComplementary base-pairing with mRNA will occur
prevents translationUse to de-activate specific mRNA’s associated with disease
Effectiveness of antisense gene therapy has so far been limited
Clinical trials:HIV/AIDSCancerHigh cholesterolEbola hemorrhagic feverPain management in cancer patients
Read section 6.4 to find out more about this
Applications of PCRUseful when only a small
amount of DNA is availableArchaeological samples
“degraded DNA"Forensic investigations
DNA evidence may be limited
Medical diagnosise.g., HIV virus. Amplification allows detection before
immune system symptoms are widespread
Applications of RFLPGenetic screeningSome genetic diseases are associated with particular RFLP
banding patternse.g., Sickle cell anemia – base pair substitution occurs within
restriction site for DdeI
Similar techniques can be used to screen for known genetic mutationsDigest DNA and hybridize probes that are complementary to
mutationsRequires blood sample or another biological samplePrenatal screening: use amniotic fluid
DNA Fingerprinting
Forensic investigations and Paternity testingLocation of restriction sites is unique to individualsDigest genomic DNA with several RE’s
Banding pattern should be particular to each individual
Compare suspect banding patterns with those from crime scene samples or from childForensics: Looking for 100% concordancePaternity: Looking for 50% concordance
Side note: DNA profiles today...RFLP is time-consuming and requires large amounts of DNAPCR-based techniques are actually used today for
generating DNA profiles
Why do you think RFLP-based DNA fingerprinting is an unattractive alternative for forensic investigations?
VNTR’s (microsatellites) are the markers of choice The copy number will vary between individuals
PCR is used to selectively amplify certain VNTR loci so the number of repeats can be determined
Separation occurs by electrophoresis, but within a narrow glass capillary tube instead of a slab of gel
Who da babydaddy??? Assign “names” to RFLP variants Determine genotypes of sources
Compare: Child should share one RFLP variant with father, one with mother
As a rule, Child/AF mix should not have more than three bands
Source Genotype
Mother B/E
Child B/D
Alleged father (A.F.)
A/C
Child/A.F. mix A/B/C/D
A
BC
D
EIS THE ALLEGED FATHER THE BABYDADDY?? NO! Follow link for more detail
A
BC
D
Source Genotype
Mother B/D
Child C/D
Alleged father (A.F.)
A/C
Child/A.F. mix A/C/D
IS THE ALLEGED FATHER THE BABYDADDY?? YES!
To catch a killer...Two suspectsTwo samples recovered from sceneVictim shares no bands with either
suspect
Crime Scene 2 sample:Victim is the source
Crime Scene 1 sample:Whodunnit?
Homework