the final step in a sanger dna sequencing reaction is to run the dna fragments on a gel. what purpose does this serve?
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Sanger Sequencing Steps & Method
Learn about Sanger Sequencing steps or the chain termination method and how DNA sequencing works and how to read Sanger Sequencing results accurately for your research.
Sanger Sequencing Steps & Method
Sanger Sequencing Steps & Method
Sanger Sequencing Steps & Method What is Sanger Sequencing?
Sanger sequencing, also known as the “chain termination method”, is a method for determining the nucleotide sequence of DNA. The method was developed by two time Nobel Laureate Frederick Sanger and his colleagues in 1977, hence the name the Sanger Sequence.
To review the general structure of DNA, please see Figure 2.
How Does Sanger Sequencing Work?
Sanger sequencing can be performed manually or, more commonly, in an automated fashion via sequencing machine (Figure 1). Each method follows three basic steps, as described below.
Sanger Sequencing Steps
There are three main steps to Sanger sequencing.
1. DNA Sequence For Chain Termination PCR
The DNA sequence of interest is used as a template for a special type of PCR called chain-termination PCR. Chain-termination PCR works just like standard PCR, but with one major difference: the addition of modified nucleotides (dNTPs) called dideoxyribonucleotides (ddNTPs). In the extension step of standard PCR, DNA polymerase adds dNTPs to a growing DNA strand by catalyzing the formation of a phosphodiester bond between the free 3’-OH group of the last nucleotide and the 5’-phosphate of the next (Figure 2).
In chain-termination PCR, the user mixes a low ratio of chain-terminating ddNTPs in with the normal dNTPs in the PCR reaction. ddNTPs lack the 3'-OH group required for phosphodiester bond formation; therefore, when DNA polymerase incorporates a ddNTP at random, extension ceases. The result of chain-termination PCR is millions to billions of oligonucleotide copies of the DNA sequence of interest, terminated at a random lengths (n) by 5’-ddNTPs.
In manual Sanger sequencing, four PCR reactions are set up, each with only a single type of ddNTP (ddATP, ddTTP, ddGTP, and ddCTP) mixed in.
In automated Sanger sequencing, all ddNTPs are mixed in a single reaction, and each of the four dNTPs has a unique fluorescent label.
2. Size Separation by Gel Electrophoresis
In the second step, the chain-terminated oligonucleotides are separated by size via gel electrophoresis. In gel electrophoresis, DNA samples are loaded into one end of a gel matrix, and an electric current is applied; DNA is negatively charged, so the oligonucleotides will be pulled toward the positive electrode on the opposite side of the gel. Because all DNA fragments have the same charge per unit of mass, the speed at which the oligonucleotides move will be determined only by size. The smaller a fragment is, the less friction it will experience as it moves through the gel, and the faster it will move. In result, the oligonucleotides will be arranged from smallest to largest, reading the gel from bottom to top.
In manual Sanger sequencing, the oligonucleotides from each of the four PCR reactions are run in four separate lanes of a gel. This allows the user to know which oligonucleotides correspond to each ddNTP.
In automated Sanger sequencing, all oligonucleotides are run in a single capillary gel electrophoresis within the sequencing machine.
3. Gel Analysis & Determination of DNA Sequence
The last step simply involves reading the gel to determine the sequence of the input DNA. Because DNA polymerase only synthesizes DNA in the 5’ to 3’ direction starting at a provided primer, each terminal ddNTP will correspond to a specific nucleotide in the original sequence (e.g., the shortest fragment must terminate at the first nucleotide from the 5’ end, the second-shortest fragment must terminate at the second nucleotide from the 5’ end, etc.) Therefore, by reading the gel bands from smallest to largest, we can determine the 5’ to 3’ sequence of the original DNA strand.
In manual Sanger sequencing, the user reads all four lanes of the gel at once, moving bottom to top, using the lane to determine the identity of the terminal ddNTP for each band. For example, if the bottom band is found in the column corresponding to ddGTP, then the smallest PCR fragment terminates with ddGTP, and the first nucleotide from the 5’ end of the original sequence has a guanine (G) base.
In automated Sanger sequencing, a computer reads each band of the capillary gel, in order, using fluorescence to call the identity of each terminal ddNTP. In short, a laser excites the fluorescent tags in each band, and a computer detects the resulting light emitted. Because each of the four ddNTPs is tagged with a different fluorescent label, the light emitted can be directly tied to the identity of the terminal ddNTP. The output is called a chromatogram, which shows the fluorescent peak of each nucleotide along the length of the template DNA.
BIO 140 LAB Final (DNA Sequencing) Flashcards
Study with Quizlet and memorize flashcards terms like To carry out Sanger sequencing, a mix is needed containing, The primer used in Sanger sequencing, A laboratory might use ddNTPs (dideoxyribonucleotides) to _____. and more.
BIO 140 LAB Final (DNA Sequencing)
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To carry out Sanger sequencing, a mix is needed containing
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DNA template Primer DNA polymerase
dideoxyribonucleotides (ddNTPs)
four deoxyribonucleotides (A, T, C, G)
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The primer used in Sanger sequencing
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Has a nucleotide sequence complementary to the area that flanks the region you want to sequence
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Terms in this set (8)
To carry out Sanger sequencing, a mix is needed containing
DNA template Primer DNA polymerase
dideoxyribonucleotides (ddNTPs)
four deoxyribonucleotides (A, T, C, G)
The primer used in Sanger sequencing
Has a nucleotide sequence complementary to the area that flanks the region you want to sequence
A laboratory might use ddNTPs (dideoxyribonucleotides) to _____.
sequence a DNA fragment
The dideoxyribonucleotides (ddNTPs) used in DNA sequencing work to ______
stop the growth of a DNA strand at a particular base (A, T, G, or C)
Since dideoxy (Sanger) sequencing is based on chain termination, why are normal dNTPs (deoxyribonucleotides like A, T, G, and C) also included in the reaction?
to produce a range of sizes of DNA synthesis products that terminate at variety of lengths
Dideoxy (aka Sanger) DNA sequencing makes use of monomers called ddNTPs that stop DNA synthesis in predictable ways. This allows researchers to determine the sequence of bases present in a strand of DNA.
The master mix of ingredients used in the DNA synthesis reactions of Sanger dideoxy sequencing include few ddNTPs relative to normal dNTPs (A's, T's, G's, and C's). What would be the most likely outcome of a Sanger dideoxy DNA synthesis reaction if ddNTPs were present in large numbers relative to the number of dNTPs (normal A's, T's, G's, and C's)?
(NOTE: a daughter strand is a strand of DNA that is replicated from the original DNA template that is being sequenced)
Most daughter strands would be very short.
In the last steps of Sanger sequencing, the DNAs produced in a DNA sequencing reaction are analyzed on the basis of their ______.
size or length
The final step in a Sanger DNA sequencing reaction is to run the DNA fragments on a gel. What purpose does this serve?
It separates DNA fragments generated during the sequencing reaction based on one-nucleotide differences in their size.
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Since dideoxy sequencing is based on the chain termination, why are normal deoxynucleotides also included in the reaction? A)to create DNA synthesis products long enough to allow running a gel B) to enhance the chain termination ability of the deoxynucleotides C)to produce a range of DNA synthesis products that terminate at every occurrence of a particular base D)to provide a substrate for DNA polymerase
C)to produce a range of DNA synthesis products that terminate at every occurrence of a particular base
The reason for using Taq polymerase for PCR is that _____. A)it binds more readily than other polymerases to the primers B)it is heat stable and can withstand the heating step of PCR C)it has regions that are complementary to the primers only minute amounts are needed for each cycle of PCR
A)it binds more readily than other polymerases to the primers
A laboratory might use dideoxyribonucleotides to _____.
A)produce cDNA from mRNA B.)visualize DNA expression C.)separate DNA fragments D.)sequence a DNA fragment
D) Sequence a DNA fragment
Which of the following is a representation of gene density? has ~ 20,000 genes. B) Humans have ~ 20,000 protein- encoding genes in 2900 Mb. has a genome 40 times the size of a human. D) Humans have 2900 Mb per genome.
B) Humans have ~20,000 protein-encoding genes in 2900 Mb.
In a single PCR cycle consisting of 15 seconds at 94°C, 30 seconds at 50°C, and 1 min at 72°C, what is happening in the step run at 50°C?
A) The DNA to be amplified is being denatured. B) Primers are being denatured. C) Primers are annealing to the DNA to be amplified. D) DNA polymerase is being inactivated. E) DNA polymerase is extending new DNA from the primers.
C) Primers are annealing to the DNA to be amplified.
Which of the following is in the correct order for one cycle of polymerase chain reaction (PCR)?
A) Denature DNA; anneal primers; extend primers. B)Denature DNA; add fresh enzyme; anneal primers; add dNTPs; extend primers. D) Extend primers; anneal primers; denature DNA. E)Anneal primers; denature DNA; extend primers.
A) Denature DNA; anneal primers; extend primers.
The final step in a Sanger DNA sequencing reaction is to run the DNA fragments on a gel. What purpose does this serve?
A) It separates DNA fragments based on their charge. B) It changes the length of the DNA fragments. C) It separates DNA fragments generated during the sequencing reaction based on one- nucleotide differences in their size. D) It adds ddNTP to the end of each DNA fragment.
C) It separates DNA fragments generated during the sequencing reaction based on one-nucleotide differences in their size.
Why might the cricket genome have eleven times as many base pairs as that of ?
A) Crickets must have more noncoding DNA. B) Crickets have higher gene density. C) Crickets must make many more proteins. are more complex organisms.
A)Crickets must have more noncoding DNA.
What information is critical to the success of polymerase chain reaction (PCR) itself?
A) The sequence of restriction- enzyme recognition sites in the DNA to be amplified must be known. B) The sequence of restriction- enzyme recognition sites in the DNA to be amplified and in the plasmid where the amplified DNA fragment will be cloned must be known. C) The complete DNA sequence of the DNA to be amplified must be known. D) The DNA sequence of the ends of the DNA to be amplified must be known.
D) The DNA sequence of the ends of the DNA to be amplified must be known.
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