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Chapter 10Chapter 10
DNA Sequencing
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ObjectivesObjectives Compare and contrast the chemical (Maxam/Gilbert)
and chain termination (Sanger) sequencing methods. List the components and molecular reactions that occur
in chain termination sequencing. Discuss the advantages of dye primer and dye
terminator sequencing. Derive a text DNA sequence from raw sequencing data. Describe examples of alternative sequencing methods,
such as bisulfite sequencing and pyrosequencing.
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Sequencing MethodsSequencing Methods Maxam/Gilbert chemical sequencing Sanger chain termination sequencing Pyrosequencing Array sequencing
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Maxam-Gilbert sequencing is performed by chain breakage at specific nucleotides.
DMS
G
GG
G
FA
GA
GG
AG
AA
H
CTT
C
TC
CT
H+S
CC
CC
Maxam-Gilbert SequencingMaxam-Gilbert Sequencing
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Sequencing gels are read from bottom to top (5′ to 3′).
G G+A T+C C
3′AAGCAACGTGCAG5′
Longer fragments
Shortest fragmentsG
A
Maxam-Gilbert SequencingMaxam-Gilbert Sequencing
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Chain Termination (Sanger) Chain Termination (Sanger) SequencingSequencing A modified DNA
replication reaction. Growing chains are
terminated by dideoxynucleotides.
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Chain terminates at ddG
Chain Termination (Sanger) Chain Termination (Sanger) Sequencing Sequencing
The 3′-OH group necessary for formation of the phosphodiester bond is missing in ddNTPs.
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Template area to be sequenced
-3′ OHTCGACGGGC…
5′OP-Primer
Template
Chain Termination (Sanger) Chain Termination (Sanger) Sequencing Sequencing A sequencing reaction mix includes labeled
primer and template.
Dideoxynucleotides are added separately to each of the four tubes.
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ddATP + ddAfour dNTPs dAdGdCdTdGdCdCdCdG
ddCTP + dAdGddCfour dNTPs dAdGdCdTdGddC
dAdGdCdTdGdCddC dAdGdCdTdGdCdCddC
ddGTP + dAddGfour dNTPs dAdGdCdTddG
dAdGdCdTdGdCdCdCddG
ddTTP + dAdGdCddTfour dNTPs dAdGdCdTdGdCdCdCdG
A
C
G
T
Chain Termination (Sanger) Chain Termination (Sanger) Sequencing Sequencing
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Chain Termination (Sanger) Chain Termination (Sanger) SequencingSequencing With addition of enzyme (DNA polymerase), the
primer is extended until a ddNTP is encountered.
The chain will end with the incorporation of the ddNTP.
With the proper dNTP:ddNTP ratio, the chain will terminate throughout the length of the template.
All terminated chains will end in the ddNTP added to that reaction.
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Chain Termination (Sanger) Chain Termination (Sanger) SequencingSequencing The collection of fragments is a
sequencing ladder. The resulting terminated chains are
resolved by electrophoresis. Fragments from each of the four tubes are
placed in four separate gel lanes.
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Sequencing gels are read from bottom to top (5′ to 3′).
G A T C
3′GGTAAATCATG5′
Longer fragments
Shorter fragmentsddG
ddG
Chain Termination (Sanger) Chain Termination (Sanger) SequencingSequencing
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Cycle SequencingCycle Sequencing
Cycle sequencing is chain termination sequencing performed in a thermal cycler.
Cycle sequencing requires a heat-stable DNA polymerase.
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Fluorescent DyesFluorescent Dyes Fluorescent dyes are multicyclic
molecules that absorb and emit fluorescent light at specific wavelengths.
Examples are fluorescein and rhodamine derivatives.
For sequencing applications, these molecules can be covalently attached to nucleotides.
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Fluorescent DyesFluorescent Dyes In dye primer sequencing, the primer contains
fluorescent dye–conjugated nucleotides, labeling the sequencing ladder at the 5′ ends of the chains.
In dye terminator sequencing, the fluorescent dye molecules are covalently attached to the dideoxynucleotides, labeling the sequencing ladder at the 3′ ends of the chains.
ddA
ddA
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ACGT
The fragments are distinguished by size and “color.”
Dye Terminator SequencingDye Terminator Sequencing A distinct dye or “color” is used for each of the
four ddNTP. Since the terminating nucleotides can be
distinguished by color, all four reactions can be performed in a single tube.
A
T
G
T
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Capillary
GT
C
TG
A
Slab gel
GATC G A T C
Dye Terminator SequencingDye Terminator Sequencing
The DNA ladder is resolved in one gel lane or in a capillary.
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The DNA ladder is read on an electropherogram.
CapillarySlab gel
5′ AGTCTG
Electropherogram
Dye Terminator Sequencing Dye Terminator Sequencing
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5′ AGTCTG 5′ AG(T/A)CTG 5′ AGACTG
T/T T/A A/A
Automated SequencingAutomated Sequencing Dye primer or dye terminator sequencing on capillary
instruments. Sequence analysis software provides analyzed
sequence in text and electropherogram form. Peak patterns reflect mutations or sequence changes.
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Alternative Sequencing Methods:Alternative Sequencing Methods:PyrosequencingPyrosequencing Pyrosequencing is based on the generation of
light signal through release of pyrophosphate (PPi) on nucleotide addition. DNAn + dNTP DNAn+1 + PPI
PPi is used to generate ATP from adenosine phosphosulfate (APS). APS + PPI ATP
ATP and luciferase generate light by conversion of luciferin to oxyluciferin.
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DNA sequence: A T C A GG CC T
Nucleotide added : A T C A G C T
Alternative Sequencing Methods:Alternative Sequencing Methods:Pyrosequencing Pyrosequencing Each nucleotide is added in turn. Only one of four will generate a light signal. The remaining nucleotides are removed
enzymatically. The light signal is recorded on a pyrogram.
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Alternative Sequencing Methods:Alternative Sequencing Methods:Bisulfite SequencingBisulfite Sequencing Bisulfite sequencing is used to detect
methylation in DNA. Bisulfite deaminates cytosine, making
uracil. Methylated cytosine is not changed by
bisulfite treatment. The bisulfite-treated template is then
sequenced.
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The sequence of treated and untreated templates is compared.
GTC
Methylated sequence: GTCMeGGCMeGATCTATC MeGTGCA…
Treated sequence: MeGGCMeGATUTATC MeGTGUA…
DNA Sequence:
(Untreated) reference: ...GTCGGCGATCTATCGTGCA…
Treated sequence: ...GTCGGCGATUTATCGTGUA…
This sequence indicates that these Cs are methylated.
Alternative Sequencing Methods:Alternative Sequencing Methods:Bisulfite Sequencing Bisulfite Sequencing
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SummarySummary Genetic information is stored in the order or
sequence of nucleotides in DNA. Chain termination sequencing is the standard
method for the determination of nucleotide sequence.
Dideoxy-chain termination sequencing has been facilitated by the development of cycle sequencing and the use of fluorescent dye detection.
Alternative methods are used for special applications, such as pyrosequencing (for resequencing and polymorphism detection) or bisulfite sequencing (to analyze methylated DNA).