When Your Insert Isn't There
By Dr. Zubair Khalid, DVM, MS, PhD ·
Quick Answer
- When a plasmid insert is missing, truncated, or rearranged, the cause is usually traced to errors in PCR amplification, restriction enzyme digestion, or recombination during bacterial propagation.
- The practical next step is to confirm the defect by sequencing the purified plasmid before repeating the cloning workflow with corrected conditions.
- Sequencing remains the only definitive confirmation of insert identity, since colony PCR and restriction mapping can miss small deletions or rearrangements.
Understanding the Problem of Incorrect Inserts in Plasmids
Plasmid construction is a foundational procedure in molecular biology laboratories. The goal is straightforward: to produce a circular DNA molecule that carries a specific insert sequence under the control of appropriate regulatory elements. In practice, however, the final plasmid often does not match the intended design. The insert may be entirely absent, shorter than expected, longer than expected, or present in a rearranged order. Each of these outcomes produces a construct that fails to express the intended protein or RNA product, and the failure may not become apparent until downstream experiments produce unexpected results.
The frequency of incorrect inserts varies with the assembly method, the insert length, the sequence composition, and the skill of the person performing the work. A researcher who understands the common failure points can design the workflow to detect problems early and avoid wasting time on constructs that cannot work. This article covers the mechanisms that produce incorrect inserts, the diagnostic methods used to identify them, and the practical steps that reduce the risk of assembling a plasmid that does not contain what you think it contains.
At a Glance
| Failure Type | Common Cause | Detection Method | Prevention Strategy |
|---|---|---|---|
| Missing insert | Incomplete ligation or assembly reaction | Colony PCR with insert-specific primers | Verify vector and insert concentrations before assembly |
| Truncated insert | PCR polymerase errors or partial digestion | Restriction mapping and sequencing | Use high-fidelity polymerase and confirm digestion completeness |
| Rearranged insert | Recombination between repeated sequences | Sequencing across the full insert | Avoid repeated sequences and use recombination-deficient host strains |
| Point mutations in insert | PCR replication errors | Sanger sequencing of the full insert | Minimize PCR cycle number and use proofreading polymerase |
| Vector-only colonies | Incomplete digestion of vector | Colony PCR and antibiotic selection | Increase restriction enzyme units and digestion time |
Core Principles of Plasmid Assembly
The Central Role of the Insert Sequence
The insert is the DNA fragment that carries the genetic information you intend to express or study. Its sequence must be exact, because even a single nucleotide change can alter an amino acid, disrupt a regulatory element, or shift the reading frame. The insert is generated by PCR amplification from a template, by restriction digestion from a donor plasmid, or by chemical synthesis. Each of these methods has its own error profile.
PCR amplification is the most common source of insert DNA. The polymerase enzyme copies the template, but it does so with a measurable error rate. Standard Taq polymerase has a higher error rate than proofreading polymerases, and the error rate increases with the number of amplification cycles. If the insert is long, the probability of at least one error in the final product increases. The practical consequence is that a PCR product that looks correct on a gel may contain a mutation that only appears after sequencing.
Restriction digestion of a donor plasmid produces a fragment that is already present in the donor. This method avoids PCR errors, but it requires that the desired insert be flanked by the correct restriction sites. If the donor plasmid contains multiple copies of the same restriction site, the digestion may produce a fragment of the wrong size. The insert may also be partially digested, leaving a fragment that is longer than expected.
Chemical synthesis produces the insert directly from a designed sequence. This method is accurate for most sequences, but it can fail for sequences that are difficult to synthesize, such as those with high GC content or long repeats. The synthesis company usually provides the sequence of the final product, but the researcher should verify it by sequencing before use.
The Assembly Reaction
The insert must be joined to the vector backbone to form a circular plasmid. The traditional method uses restriction enzymes to create compatible ends on both the insert and the vector, followed by ligation with DNA ligase. The ligation reaction is a competition between the desired insert-vector product and several unwanted products, including vector alone, insert alone, and insert-insert concatemers. The ratio of insert to vector in the reaction is a critical variable. Too little insert produces empty vector, and too much insert produces multiple inserts in a single plasmid.
Modern assembly methods such as Gibson Assembly and Golden Gate Assembly use different mechanisms to join the fragments. Gibson Assembly relies on overlapping sequences at the ends of the fragments, and it uses an exonuclease to chew back the ends, a polymerase to fill in the gaps, and a ligase to seal the nicks. Golden Gate Assembly uses type IIS restriction enzymes that cut outside their recognition site, producing a defined overhang that can be designed to be unique for each junction. Both methods are efficient, but they have their own failure modes. Gibson Assembly can produce errors if the overlapping sequences are not perfectly matched, and Golden Gate Assembly can fail if the type IIS enzyme cuts at an unexpected site.
Common Causes of Incorrect Inserts
PCR Amplification Errors
The polymerase chain reaction is a powerful tool, but it is not infallible. The polymerase enzyme can incorporate the wrong nucleotide, and the error rate depends on the enzyme and the reaction conditions. The error rate of Taq polymerase is approximately one error per 10,000 nucleotides, while proofreading polymerases have error rates that are 10 to 50 times lower. The error rate also increases with the number of amplification cycles, because each cycle copies the product of the previous cycle, and errors are not corrected in the later cycles.
The practical consequence is that a PCR product that is used directly for cloning may contain errors that are not visible on a gel. The errors are more likely to be present if the insert is long, if the PCR was run for many cycles, or if the template DNA was damaged. The errors can be point mutations, deletions, or insertions, and they can occur anywhere in the insert.
The solution is to use a proofreading polymerase for cloning PCR, to minimize the number of cycles, and to sequence the final construct. The PCR product should also be purified to remove the template DNA and the primers, because these can interfere with the assembly reaction.
Restriction Enzyme Star Activity
Restriction enzymes are designed to cut DNA at a specific recognition sequence, but they can also cut at other sequences under certain conditions. This phenomenon is called star activity, and it occurs when the enzyme is used under non-optimal conditions. The conditions that promote star activity include high enzyme concentration, low salt concentration, high pH, the presence of organic solvents such as glycerol, and incubation at a temperature that is too high or too low.
Star activity produces fragments that are shorter than expected, because the enzyme cuts at a site that is similar to the recognition sequence but not identical. The result is a truncated insert, or a vector that is cut at an unintended site. The truncated insert may be too short to contain the full coding sequence, and the vector may be cut in a region that is essential for replication or selection.
The solution is to use the restriction enzyme under the conditions recommended by the manufacturer, to limit the incubation time, and to use the minimum amount of enzyme needed for complete digestion. The digestion should be checked on a gel before proceeding to the assembly reaction.
Incomplete Digestion
Restriction digestion is a reaction that proceeds to completion only if the enzyme is present in sufficient quantity and the incubation time is long enough. If the digestion is incomplete, the vector or the insert will have a sticky end that is not fully cut. The result is a plasmid that is missing a portion of the insert, or a vector that is not linearized.
Incomplete digestion is more likely when the DNA is contaminated with proteins or other substances that inhibit the enzyme, or when the DNA is methylated at the restriction site. The solution is to purify the DNA before digestion, to use a sufficient amount of enzyme, and to incubate for the recommended time. The digestion should be checked by a gel to confirm that the vector is fully linearized and the insert is the correct size.
Recombination and Rearrangement
The insert can also be rearranged during the assembly process. This can happen when the insert contains repeated sequences, such as direct repeats or inverted repeats. The repeats can recombine with each other, producing a plasmid that has a deletion or an inversion. The recombination can occur in the test tube during the assembly, or it can occur in the bacterial host during the propagation of the plasmid.
The bacterial host is a common source of recombination. The host cells have their own recombination systems, and they can act on the plasmid DNA. The recombination is more likely when the insert contains sequences that are homologous to the vector or to other parts of the insert. The solution is to use a host strain that is deficient in recombination, such as a recA-negative strain, and to avoid the sequences that are prone to recombination.
Diagnostic Strategies for Identifying Incorrect Inserts
Colony PCR
Colony PCR is a quick method to check whether the insert is present in the plasmid. The method uses a small amount of a bacterial colony as the template for a PCR reaction, and the primers are designed to amplify a region that spans the insert-vector junction. If the insert is present, the PCR product will be the expected size. If the insert is missing, the PCR product will be smaller, and if the insert is rearranged, the PCR product may be a different size.
Colony PCR is a useful screening tool because it is fast and it does not require the purification of the plasmid. The limitation is that it only checks the region that is amplified by the primers. It does not detect errors that are outside the amplified region, and it does not detect point mutations. The PCR product can also be a false positive if the primers bind to the vector or to the host chromosome.
The colony PCR should be used as a first screen, and the positive colonies should be confirmed by a more definitive method.
Restriction Mapping
Restriction mapping is a method that uses restriction enzymes to cut the plasmid at specific sites, and the resulting fragments are separated by gel electrophoresis. The pattern of the fragments is compared to the pattern that is expected from the insert sequence. If the insert is missing, the pattern will show a fragment that is smaller than expected. If the insert is truncated, the pattern will show a fragment that is smaller than expected, and if the insert is rearranged, the pattern will show a fragment that is not expected.
Restriction mapping is more definitive than colony PCR, because it covers the entire plasmid. The limitation is that it requires the purification of the plasmid, and it requires the knowledge of the restriction sites in the insert and the vector. The restriction mapping can also be difficult to interpret if the plasmid is large or if the insert contains many restriction sites.
Sequencing
Sequencing is the definitive method for confirming the insert sequence. The sequencing reaction reads the DNA sequence of the plasmid, and the result is compared to the expected sequence. The sequencing can detect point mutations, deletions, insertions, and rearrangements. The sequencing can be done on the entire insert, or it can be done on the regions that are most likely to contain errors.
The sequencing is the most accurate method, but it is also the most expensive and the most time-consuming. The sequencing is used to confirm the final construct, and it is not used for the initial screening. The sequencing is also used to confirm the insert after the plasmid has been propagated, because the insert can be mutated during the propagation.
Practical Workflow for Verifying the Insert
Step 1: Design the Insert and the Primers
The design of the insert and the primers is the first step in the workflow. The insert sequence is designed to include the coding sequence, the regulatory elements, and the restriction sites or the overlapping sequences for the assembly. The primers are designed to amplify the insert, and they should be designed to include the restriction sites or the overlapping sequences.
The design should also consider the sequence composition of the insert. The insert should not contain the sequences that are difficult to amplify, such as the GC-rich regions or the repeated sequences. The insert should also avoid the sequences that are recognized by the restriction enzymes that are used for the assembly.
Step 2: Amplify the Insert
The insert is amplified by PCR using a proofreading polymerase. The PCR is run for the minimum number of cycles that produces a sufficient amount of the product. The PCR product is purified to remove the template DNA and the primers.
The PCR product is checked by a gel to confirm that it is the correct size. The PCR product is also quantified to determine the amount of the insert that is available for the assembly.
Step 3: Digest the Vector and the Insert
The vector and the insert are digested with the restriction enzymes that are used for the assembly. The digestion is run for the recommended time and temperature, and the digestion is checked by a gel to confirm that the vector is linearized and the insert is the correct size.
The digested vector and the insert are purified to remove the restriction enzymes and the buffer.
Step 4: Assemble the Plasmid
The vector and the insert are assembled using the ligation or the assembly method. The ratio of the insert to the vector is optimized to produce the highest number of the correct colonies.
The assembly reaction is transformed into the bacterial host, and the cells are plated on the selective medium.
Step 5: Screen the Colonies
The colonies are screened by colony PCR to identify the colonies that contain the insert. The positive colonies are then confirmed by restriction mapping and sequencing.
The confirmed colonies are used to prepare the plasmid DNA for the downstream applications.
Records and Measurements
The records and the measurements are the key to the troubleshooting of the incorrect insert. The records should include the design of the insert, the PCR conditions, the digestion conditions, the assembly conditions, and the screening results. The measurements should include the size of the PCR product, the size of the digested fragments, and the sequence of the insert.
The records are used to identify the cause of the incorrect insert. For example, if the PCR product is the correct size but the insert is truncated, the cause is likely to be the restriction digestion. If the PCR product is the correct size and the digestion is correct, but the insert is rearranged, the cause is likely to be the recombination.
The records are also used to compare the results of the different experiments. The records are used to identify the conditions that produce the correct insert, and the conditions that produce the incorrect insert.
Common Failure Patterns
The Insert Is Missing
The insert is missing when the plasmid contains only the vector backbone. The cause is usually the incomplete ligation or the assembly reaction. The insert is not joined to the vector, and the vector is recircularized without the insert.
The failure is detected by the colony PCR, which shows no amplification of the insert. The failure is also detected by the restriction mapping, which shows a fragment that is the size of the vector alone.
The solution is to increase the ratio of the insert to the vector, and to check the digestion of the vector and the insert. The vector should be fully linearized, and the insert should be present in the correct amount.
The Insert is Truncated
The Insert is truncated when the plasmid contains a portion of the insert, but not the full insert. The cause is likely the PCR error, the restriction enzyme star activity, or the incomplete digestion.
The failure is detected by the colony PCR, which shows a product that is smaller than expected. The failure is also detected by the restriction mapping, which shows a fragment that is smaller than expected.
The solution is to use a proofreading polymerase, to minimize the number of PCR cycles, and to check the digestion conditions. The insert should be sequenced to confirm the full sequence.
The Insert is Rearranged
The Insert is rearranged when the plasmid contains the insert in the wrong order. The cause is likely the recombination of the repeated sequences.
The failure is detected by the restriction mapping, which shows a fragment that is different from the expected pattern. The failure is also detected by the sequencing, which shows the insert in the wrong order.
The solution is to avoid the repeated sequences in the insert, and to use a recombination-deficient host strain. The insert should also be sequenced to confirm the correct order.
Biosafety and Regulatory Context
The work with the recombinant DNA is subject to the biosafety and the regulatory oversight. The NIH Guidelines for Research Involving Recombinant or Synthetic Nucleic Acid Molecules provide the official framework for the oversight of the recombinant DNA work. The guidelines classify the experiments into the risk groups, and they require the institutional oversight for the experiments that involve the risk.
The biosafety principles are described in the Biosafety in Microbiological and Biomedical Laboratories, which is published by the CDC and the NIH. The principles include the risk assessment, the containment, and the institutional oversight. The risk assessment is used to determine the level of the containment that is required for the experiment. The containment is the physical and the biological barriers that prevent the release of the recombinant DNA. The institutional oversight is the review of the experiment by the institutional biosafety committee.
The researcher should be aware of the biosafety requirements for the experiment, and they should follow the guidelines. The researcher should also be aware of the biosafety requirements for the disposal of the recombinant DNA and the bacterial cultures.
Quality Control and Reproducibility
The quality of the insert is the key to the reproducibility of the experiment. The quality is determined by the accuracy of the insert sequence, and the quality is confirmed by the sequencing. The quality is also determined by the purity of the plasmid DNA, and the purity is confirmed by the spectrophotometry and the gel.
The reproducibility is the ability to repeat the experiment and to obtain the same result. The reproducibility is improved by the use of the standard protocols, the use of the quality controls, and the use of the records. The reproducibility is also improved by the use of the measurement science, which is the science of the measurement and the calibration.
The NIST Laboratory Programs provide the standards for the measurement science, and they are used to ensure the quality of the measurements. The standards are used to calibrate the instruments, and they are used to validate the methods.
Professional Escalation Criteria
The researcher should escalate the problem to the supervisor or the laboratory manager when the problem cannot be resolved by the troubleshooting. The escalation is appropriate when the insert is still incorrect after the troubleshooting, or when the insert is incorrect in a way that is not expected.
The escalation is also appropriate when the problem is the result of the equipment failure, or when the problem is the result of the contamination. The escalation is also appropriate when the problem is the result of the design, and the design needs to be changed.
The escalation should include the records of the experiment, the measurements, and the troubleshooting. The escalation should also include the proposed solution, and the request for the guidance.
Decision Framework for Triage and Root-Cause Assignment
When a sequencing result shows a missing, truncated, or rearranged insert, the immediate reaction is often to repeat the cloning with the same conditions and hope for a different outcome. This approach wastes time and reagents. A structured triage framework that assigns the failure to a specific step in the workflow reduces the number of repeated attempts and produces a record that is useful for future cloning projects.
Step 1: Classify the Observed Defect
The first decision point is to classify the defect precisely from the sequencing trace or the restriction digest pattern. The classification determines which root causes are possible and which are not. Use the following categories:
| Defect Category | Sequencing Evidence | Restriction Digest Evidence | Likely Cause Group |
|---|---|---|---|
| Complete absence | No insert sequence, vector backbone only | Fragment pattern matches empty vector | Ligation failure, vector recircularization, insert loss during purification |
| Internal deletion | Insert present but a segment is missing | One or more fragments shorter than expected | PCR mispriming, recombination between repeats, exonuclease activity |
| Terminal truncation | Insert ends before the expected boundary | Junction fragment shorter than expected | Partial restriction digestion, incomplete assembly overlap |
| Point mutation | Single or multiple nucleotide changes | Digest pattern unchanged | PCR polymerase error, template damage |
| Rearrangement | Insert sequence present but in wrong order | Unexpected fragment sizes or extra fragments | Recombination, concatemer formation, misassembly |
| Insertion of extra sequence | Additional nucleotides between insert and vector | Junction fragment longer than expected | PCR primer misannealing, vector contamination |
The classification should be written into the laboratory notebook with the sequencing file name, the date, and the construct name. This record is the starting point for the root-cause analysis.
Step 2: Trace the Defect to the Workflow Stage
Once the defect is classified, the next decision is to identify which stage of the workflow could have produced it. The workflow stages are template preparation, PCR amplification, restriction digestion, assembly, transformation, and bacterial propagation. Each defect class maps to a limited set of stages.
For a complete absence of the insert, the failure is most likely in the assembly or ligation stage. The vector was recircularized without the insert, or the insert was not present in the reaction at a sufficient concentration. The decision is to check the insert concentration and the vector linearization before repeating the assembly.
For an internal deletion, the failure is likely in the PCR amplification or in the recombination during the bacterial propagation. The decision is to examine the PCR product on a high-resolution gel before the assembly, and to consider the host strain.
For a terminal truncation, the failure is likely in the restriction digestion. The enzyme cut at a site that is not the intended site, or the digestion was incomplete. The decision is to check the digestion conditions and the enzyme specificity.
For a point mutation, the failure is likely in the PCR amplification. The decision is to use a proofreading polymerase and to reduce the cycle number.
For a rearrangement, the failure is likely in the recombination or in the assembly method. The decision is to check the sequence composition of the insert and the host strain.
Step 3: Apply the Single-Variable Test
The most efficient way to identify the root cause is to change one variable at a time and observe the effect on the outcome. This approach is called the single-variable test, and it is the standard method for troubleshooting in molecular biology.
The variables that can be changed are:
- The polymerase enzyme (standard versus proofreading)
- The number of PCR cycles
- The restriction enzyme concentration and the incubation time
- The insert to vector ratio in the assembly
- The assembly method (ligation, Gibson, Golden Gate)
- The bacterial host strain
- The purification method for the PCR product and the digested vector
The single-variable test should be designed so that the outcome is easy to measure. The outcome can be the number of colonies, the size of the PCR product, the restriction digest pattern, or the sequencing result. The test should be run with a positive control, which is a construct that is known to be correct, and a negative control, which is a reaction without the insert.
The result of the single-variable test is recorded in a table that shows the variable that was changed, the condition that was used, and the outcome. The table is used to identify the variable that produces the correct insert.
Step 4: Use the Decision Tree for the Assembly Method
The assembly method is a common source of the incorrect insert, and the decision tree can help to identify the cause.
- If the insert is missing, check the insert concentration in the assembly. The insert should be present in a 2:1 to 3:1 molar ratio to the vector. If the insert concentration is too low, the vector will recircularize without the insert.
- If the insert is truncated, check the restriction digestion of the insert. The insert should be fully digested to produce the correct fragment. If the digestion is incomplete, the insert will be longer than expected, and if the digestion is star activity, the insert will be shorter.
- If the insert is rearranged, check the sequence of the insert for the repeated regions. The repeated regions can recombine during the assembly or during the bacterial growth. The insert should be redesigned to avoid the repeats, or the host strain should be changed to a recombination-deficient strain.
The decision tree is a practical tool because it gives a clear path from the observed defect to the likely cause and the next step.
Record System for Cloning Attempts
A structured record system is essential for the troubleshooting of the incorrect insert. The record system should capture the information that is needed to identify the cause of the failure and to avoid repeating the same mistake.
The Cloning Attempt Log
The cloning attempt log is a table that records the details of each cloning attempt. The log should include the following fields:
- The date of the attempt
- The name of the plasmid and the insert
- The assembly method
- The PCR polymerase and the cycle number
- The restriction enzymes and the digestion conditions
- The insert to vector ratio
- The bacterial host strain
- The number of colonies obtained
- The screening method and the result
- The sequencing result
The log is used to compare the attempts and to identify the conditions that produce the correct insert. The log is also used to identify the conditions that produce the incorrect insert, and to avoid them in the future.
The Gel Image Archive
The gel images are the primary evidence for the size of the PCR product and the digested fragments. The gel images should be archived with the date, the sample names, and the ladder. The gel images should be annotated to show the expected size of the fragments and the observed size.
The gel image archive is used to compare the results of the different attempts. The archive is also used to identify the patterns that are associated with the specific defects, such as the truncated insert or the rearranged insert.
The Sequencing Trace Archive
The sequencing traces are the definitive evidence for the insert sequence. The sequencing traces should be archived with the file name, the plasmid name, and the date. The traces should be compared to the expected sequence, and the differences should be noted.
The sequencing trace archive is used to identify the exact location of the mutation or the deletion. The archive is also used to determine whether the mutation is a point mutation, a deletion, or a rearrangement.
The Decision Record
The decision record is a table that records the decisions that were made during the troubleshooting. The record should include the observed defect, the likely cause, the variable that was changed, and the outcome.
The decision record is used to track the progress of the troubleshooting and to avoid repeating the same test. The record is also used to identify the patterns in the failures, which can be used to improve the cloning protocol.
Common Failure Patterns and Their Specific Causes
The failure patterns are the recurring outcomes that are observed in the cloning experiments. The patterns are useful because they provide a quick way to identify the cause of the failure.
Pattern 1: The Empty Vector
The empty vector is the most common failure pattern. The plasmid contains only the vector backbone, and the insert is absent. The cause is usually the incomplete ligation or the assembly reaction. The vector was recircularized without the insert.
The decision is to check the insert concentration and the vector concentration. The insert should be present in a 2:1 to 3:1 molar ratio to the vector. The vector should be fully linearized, and the insert should be present in the correct amount.
The empty vector is also caused by the incomplete digestion of the vector. If the vector is not fully linearized, it will recircularize without the insert. The digestion should be checked on a gel to confirm that the vector is fully linearized.
Pattern 2: The Short Insert
The short insert is the insert that is shorter than the expected sequence. The cause is the PCR amplification that produced a product that is shorter than the expected sequence, or the restriction digestion that cut the insert at an unintended site.
The decision is to check the PCR product on a gel before the assembly. The PCR product should be the expected size. If the PCR product is shorter, the PCR should be repeated with a proofreading polymerase and a reduced cycle number.
The short insert is also caused by the restriction enzyme star activity. The star activity is the cutting of the DNA at a site that is similar to the recognition sequence but is not the exact sequence. The star activity is caused by the non-optimal conditions, such as the high enzyme concentration, the low salt, or the high glycerol. The digestion should be run under the conditions recommended by the manufacturer.
Pattern 3: The Rearranged Insert
The rearranged insert is the insert that is present in the wrong order. The cause is the recombination of the repeated sequences in the insert or the vector. The recombination can occur in the test tube during the assembly, or it can occur in the bacterial host during the propagation.
The decision is to check the sequence of the insert for the repeated regions. The insert should be redesigned to avoid the repeated sequences, or the host strain should be changed to a recombination-deficient strain.
The rearranged insert is also caused by the misassembly of the fragments. The Gibson Assembly can produce the rearranged insert if the overlapping sequences are not perfectly matched. The Golden Gate Assembly can produce the rearranged insert if the type IIS enzyme cuts at an unexpected site.
Pattern 4: The Point Mutation
The point mutation is the single nucleotide change in the insert. The cause is the PCR amplification error. The error rate is higher with the non-proofreading polymerase and with the high cycle number.
The decision is to use a proofreading polymerase and to reduce the cycle number. The PCR product should be sequenced to confirm the sequence before the assembly.
Practical Implementation Steps
The following steps are the practical implementation of the decision framework and the record system.
Step 1: Set Up the Cloning Attempt Log
Create a table in the lab notebook or the electronic lab notebook that records the details of each cloning attempt. The table should include the fields listed above. The table should be filled in for each attempt, and the results should be recorded.
Step 2: Classify the Defect
When the sequencing result is obtained, classify the defect using the categories in the table. Write the classification in the log.
Step 3: Determine the Likely Cause
Use the classification to determine the likely cause. The cause is determined by the defect type and the workflow stage.
Step 4: Run the Single-Variable Test
Change one variable at a time and observe the outcome. The outcome should be measured by the number of the colonies, the size of the PCR product, or the sequencing result.
Step 5: Record the Result
Record the result of the single-variable test in the log. The result should include the variable that was changed, the condition that was used, and the outcome.
Step 6: Confirm the Correct Insert
When the correct insert is obtained, confirm the sequence by sequencing. The sequencing should be done on the purified plasmid DNA, and the result should be compared to the expected sequence.
Professional Escalation Criteria
The escalation to the supervisor or the laboratory manager is appropriate when the problem cannot be resolved by the troubleshooting. The escalation is also appropriate when the problem is the result of the equipment failure, or when the problem is the result of the contamination.
The escalation should include the records of the experiment, the measurements, and the troubleshooting. The escalation should also include the proposed solution, and the request for the guidance.
The escalation is also appropriate when the problem is the result of the design, and the design needs to be changed. The design change should be reviewed by the supervisor or the laboratory manager before the new design is used.
Frequently Asked Questions
Why is my insert missing from the plasmid?
The insert is missing because the ligation or the assembly reaction did not join the insert to the vector. The vector was recircularized without the insert. The cause is often an insufficient amount of the insert in the reaction, or the vector was not fully linearized. Increase the insert to vector ratio and confirm the digestion of the vector by gel analysis.
What causes a truncated insert in a plasmid?
A truncated insert is caused by the PCR amplification that produced a product that is shorter than the expected sequence, or by the restriction digestion that cut the insert at an unintended site. The PCR error is more likely with a non-proofreading polymerase or with too many cycles. The unintended cut is more likely with the star activity of the restriction enzyme.
How can I tell if my insert is rearranged?
A rearranged insert is detected by the restriction mapping, which shows a fragment pattern that is different from the expected pattern. The sequencing of the insert is the definitive method, and it shows the insert in the wrong order. The rearrangement is caused by the recombination of the repeated sequences in the insert.
What is the best way to confirm the insert sequence?
The best way to confirm the insert sequence is to sequence the entire insert. The sequencing is the only method that can detect the point mutations, the deletions, the insertions, and the rearrangements. The sequencing should be done on the purified plasmid DNA, and the result should be compared to the expected sequence.
How do I prevent the PCR errors in my insert?
The PCR errors are prevented by the use of a proofreading polymerase, the minimization of the number of the PCR cycles, and the use of a high-quality template. The PCR product should also be sequenced to confirm the sequence before the assembly.
What is the role of the restriction enzyme star activity in the incorrect insert?
The restriction enzyme star activity is the cutting of the DNA at a site that is similar to the recognition sequence but is not the exact sequence. The star activity is caused by the non-optimal conditions, such as the high enzyme concentration, the low salt, or the high glycerol. The star activity produces a truncated insert or a vector that is cut at an unintended site.
How do I choose the right host strain for the cloning?
The host strain should be chosen based on the recombination and the methylation. The recombination-deficient strains are used to prevent the rearrangement of the insert. The methylation-deficient strains are used to prevent the digestion of the insert by the methylation-sensitive enzymes.
When should I escalate the problem to my supervisor?
The problem should be escalated when the insert is still incorrect after the troubleshooting, or when the insert is incorrect in a way that is not expected. The escalation should include the records, the measurements, and the proposed solution. The supervisor can provide the guidance and the resources to resolve the problem.
Related Molecular Biology Guides
- Plasmid Replication: How Plasmids Replicate Independently
- Contract Recombinant Protein Expression: A Comprehensive Guide
- Custom Recombinant Protein Expression: A Practical Guide
- How to Express Recombinant Protein: A Step-by-Step Guide
- Recombinant MICB Protein Expression: A Practical Guide
References and Further Reading
- Molecular Biology and Laboratory Methods. National Library of Medicine.
- Biosafety in Microbiological and Biomedical Laboratories. CDC and NIH.
- NIH Guidelines for Recombinant or Synthetic Nucleic Acid Molecules. National Institutes of Health.
- Addgene Protocols. Addgene.
- MIQE Guidelines. Clinical Chemistry.
- NIST Laboratory Programs. National Institute of Standards and Technology.
- CRISPR/Cas9-Mediated Knockout of Physcomitrella patens Phytochromes.. Methods in molecular biology (Clifton, N.J.), 2019.
- Pathways of human cell post-replication repair.. Carcinogenesis, 1989.
- Non-functional expression of Escherichia coli signal peptidase I in Bacillus subtilis.. Journal of general microbiology, 1991.
- [[Functional arrangement of genomic DNA and structure of nuclear matrix].](https://pubmed.ncbi.nlm.nih.gov/10860485). Yakugaku zasshi : Journal of the Pharmaceutical Society of Japan, 2000.
- A switch between DNA polymerases δ and λ promotes error-free bypass of 8-oxo-G lesions.. Proceedings of the National Academy of Sciences of the United States of America, 2012.
This article is educational and does not replace validated laboratory procedures, institutional biosafety review, manufacturer instructions, or professional interpretation.