Using DNA Barcodes to Identify and Classify Living Things:
I. Use BLAST to Find DNA Sequences in Databases (Electronic PCR)
- Perform a BLAST search as follows:
- Do an Internet search for "ncbi blast."
- Click on the link for the result BLAST: Basic Local Alignment Search Tool. This will take you to the Internet site of the National Center for Biotechnology Information (NCBI).
- Under the heading "Basic BLAST," click on "nucleotide blast."
- Enter the primer set you used into the search window. These are the query sequences.
The following primers were used in this experiment:
- rbcLaf 5’- ATGTCACCACAAACAGAGACTAAAGC-3’ (forward primer)
- rbcLa rev 5’- GTAAAATCAAGTCCACCRCG-3’ (reverse primer)
Animal COI gene
- LepF1 5’- ATTCAACCAATCATAAAGATATTGG -3’ (forward primer)
- LepR1 5’- TAAACTTCTGGATGTCCAAAAAATCA-3’(reverse primer)
- VF1F 5’- TCTCAACCAACCACAAAGACATTGG-3’ (forward primer)
- VF1R 5’- TAGACTTCTGGGTGGCCAAAGAATCA-3’ (reverse primer)
- Omit any non-nucleotide characters from the window because they will not be recognized by the BLAST algorithm.
- Under "Choose Search Set," select "NCBI Genomes (chromosome)" from the pull-down menu.
- Under "Program Selection," optimize for "Somewhat similar sequences (blastn)."
- Click on "BLAST." This sends your query sequences to a server at the National Center for Biotechnology Information in Bethesda, Maryland. There, the BLAST algorithm will attempt to match the primer sequences to the DNA sequences stored in its database. A temporary page showing the status of your search will be displayed until your results are available. This may take only a few seconds or more than 1 minute if many other searches are queued at the server.
- The results of the BLAST search are displayed in three ways as you scroll down the page:
- First, a "Graphic Summary" illustrates how significant matches, or "hits," align with the query sequence. Why are some alignments longer than others?
- This is followed by "Descriptions of sequences producing significant alignments," a table with links to database reports.
Next is an "Alignments" section, which provides a detailed view of each primer sequence ("Query") aligned to the nucleotide sequence of the search hit (S"bjct," "subject"). Notice that hits have matches to one or both of the primers:
- The accession number is a unique identifier given to a sequence when it is submitted to a database, such as Genbank. The accession link leads to a detailed report on the sequence.
- Note the scores in the "e" column on the right. The Expectation or E value is the number of alignments with the query sequence that would be expected to occur by chance in the database. The lower the E value, the higher the probability that the hit is related to the query. For example, an E value of 1 means that a search with your sequence would be expected to turn up one match by chance.
- What is the E value of your most significant hit, and what does it mean? What does it mean if there are multiple hits with similar E values?
- What do the descriptions of significant hits have in common?
|Lep or VF
Predict the length of the product that the primer set would amplify in a PCR reaction (in vitro).
Determine the type of DNA sequence amplified by the primer set:
- In the "Alignments" section, select a hit that matches both primer sequences.
- Which nucleotide positions do the primers match in the subject sequence?
- The lowest and highest nucleotide positions in the subject sequence indicate the borders of the amplified sequence. Subtracting one from the other gives the difference between the coordinates.
- However, the PCR product includes both ends, so add 1 nucleotide to the result that you obtained in Step 3.c. to determine the exact length of the fragment amplified by the two primers.
- What value do you get if you calculate the fragment size for other species that have matches to the forward and reverse primer? Why is this so?
- Click on the accession link (beginning with "ref") to open the data sheet for the hit used in Question 3 above.
- The data sheet has three parts:
Identify the feature(s) located between the nucleotide positions identified by the primers, as determined in 3.b. above.
- a. The top section contains basic information about the sequence, including its basepair (bp) length, database accession number, source, and references to papers in which the sequence is published.
- The bottom section lists the nucleotide sequence.
- The middle section contains annotations of gene and regulatory "FEATURES," with their beginning and ending nucleotide positions ("xx..xx"). These features may include genes, coding sequences (cds), regulatory regions, ribosomal RNA (rRNA), and transfer RNA (tRNA).
II. Identify Species and Phylogenetic Relationships Using DNA Subway
The following directions explain how to use the Blue Line of DNA Subway to analyze novel DNA sequences generated by a DNA sequencing experiment. If you did not sequence your own DNA sample, you can follow these directions to use DNA sequences produced for other students. You can find supplementary instructions by clicking on the "manual" link on the DNA Subway homepage.
DNA Subway is an intuitive interface for analyzing DNA barcodes. Generally, you progress in a stepwise fashion through the button "stops" on each "branch line." An "R" indicates that analysis is available. A blinking "R" indicates an analysis is in process. A "V" means that results are ready to view.
- Create a DNA Subway Project and Upload DNA Sequences
- Log into DNA Subway at www.dnasubway.org. If you do not have an account, you will need to register first to save and share your work.
- Select "Determine Sequence Relationships" (Blue Line) to begin a project.
- Select "rbcL" or "COI" from the "Select Project Type" section. (rbcL (plant) sequences must be analyzed separately from COI (animal) sequences.)
- "Select Sequence Source" provides several ways to obtain sequences for barcode analysis:
Provide a title in the "Name Your Project" section.
Write a short description of your project in the "Description" section (optional).
Click on "Continue."
View and Build Sequences
- Upload sequence(s) in ab1 (files ending with .ab1) or FASTA format. Click "Browse" to navigate to a folder on your desktop or drive containing your sequence(s). Select a sequence by clicking on its file name. Select more than one sequence by holding down the ctrl key while clicking file names. Once you have selected the sequences you want, click "Open".
- Enter a sequence in FASTA format. Below is an example of this format. the ">" symbol demarcates the sequence name. The sequence is started on the next line.
- Import a sequence/trace from the DNALC. Click on your tracking number. Select one or more files from the list. Click to "Add" selected files.
- Select a sample sequence.
- On the "Assemble Sequences" branch line, click on "Sequence Viewer." Click on a sequence name to view an electropherogram with quality scores for each nucleotide.
Click on "Sequence Trimmer" to automatically remove Ns from the 5’ and 3’ ends of selected sequences. Click again to view the trimmed sequences. Why is it important to remove excess N’s from the ends of the sequences?
Pair Forward and Reverse Reads
- The DNA sequencing software measures the fluorescence emitted in each of four channels – A,T,C,G – and records these as a trace, or electropherogram. In a good sequencing reaction, the nucleotide at a given position will be fluorescently labeled far in excess of background (random) labeling of the other three nucleotides, producing a "peak" at that position in the trace. Thus, peaks in the electropherogram correlate to nucleotide positions in the DNA sequence.
- A software program called Phred analyzes the sequence file and "calls" a nucleotide (A, T, C, G) for each peak. If two or more nucleotides have relatively strong signals at the same position, the software calls an "N" for an undetermined nucleotide.
- Phred also examines the peaks around each call and assigns a quality score for each nucleotide. The quality scores corresponds to a logarithmic error probability that the nucleotide call is wrong, or, conversely, to the accuracy of the call.
|1 in 10
|1 in 100
|1 in 1,000
|1 in 10,000
|1 in 100,000
- The electropherogram viewer represents each Phred score as a blue bar. The horizontal line equals a Phred score of 20, which is generally the cut-off for high-quality sequence. Thus any bar at or above the line is considered a high-quality read. What is the error rate and accuracy associated with a Phred score of 20?
- Every sequence "read" begins with nucleotides (A,T,C,G) interspersed with Ns. In "clean" sequences, where experimental conditions were near optimal, the initial Ns will end within the first 25 nucleotides. The remaining sequence will have very few, if any, internal Ns. Then, at the end of the read the sequence will abruptly change over to Ns.
- Large numbers of Ns scattered throughout the sequence indicate poor quality sequence. Sequences with average Phred scores below 20 will be flagged with a "Low Quality Score Alert." You will need to be careful when drawing conclusions from analyses made with poor quality sequence. What do you notice about the electropherogram peaks and quality scores at nucleotide positions labeled "N"?
- If you have good quality forward and reverse reads for any sample, click on "Pair Builder" to associate a forward read with its corresponding reverse read.
- Check the boxes for two sequences you wish to pair, and confirm your selection in the pop-up.
- Click on the "F" to the right of the reverse sequence. The entry will change to R, indicating that the sequence has been transformed into its reverse complement.
- Click on "Save" to save your pair assignments.
- Click on "Consensus Builder" to align the paired forward and reverse reads. Then, click on a forward-reverse pair to view its consensus sequence. How does the consensus sequence optimize the amount of sequence information available for analysis? Why does this occur?
- Positions highlighted in yellow mark differences in nucleotide calls between the forward and reverse reads. Do differences tend to occur in certain areas of the sequence? Why?
- Large numbers of yellow mismatches – especially in long blocks – may indicate that you have incorrectly paired sequences from two different sources (organisms), or that you failed to reverse complement the reverse strand.
A large number of mismatches in properly paired and reverse complemented sequences indicate that one or both sequences is of poor quality. Often, one of the sequencing reactions produces a high quality read that can be used on its own. To determine this:
- Return to Pair Builder to check your pairs and reverse complements.
- Click on the red "x" to redo a pairing, and toggle "F" and "R" settings, as needed.
Few or no internal mismatches indicate good quality sequence from forward and reverse reads. If you like, you can check the consensus sequence at yellow mismatches and override the judgment made by the software:
- Examine the distribution of Ns to see if they are mainly confined to one of the two sequences.
- Examine the electropherograms to see if one of the two sequences is of good quality.
- If one of the sequences seems of good quality, return to Pair Builder, and click the red x to undo the pairing.
- Continue on to Step 4.
BLAST Your Sequence
A BLAST search can quickly identify any close matches to your sequence in sequence databases. In this way, you can often quickly identify an unknown sample to the genus or species level. It also provides a means to add samples for a phylogenetic analysis.
- Click on a highlighted mismatch to see the electropherograms and Pfred scores for each read.
- Click on the desired nucleotide in the black rectangle to change the consensus sequence at that position. You should only change the consensus if you have a strong reason to believe the consensus is wrong.
- Click the button to Save Change(s).
- On the Add Sequences branch, click on "BLASTN". Then, click on the "BLAST" button next to the sequence you want to query against DNA databases.
- The returned list has information about the 20 most significant alignments (hits):
Add BLAST sequence data to your phylogenetic analysis by checking the box(es) above any accession number(s), then clicking on "Add BLAST hits to project" at the bottom of the BLAST results window.
Add Sequences to Your Analysis
- Accession number, a unique identifier given to each sequence submitted to a database. Prefixes indicate the database name – including gb (GenBank), emb (European Molecular Biology Laboratory), and dbj (DNA Databank of Japan).
- Organism and sequence description or gene name of the hit. Click on the genus and species name for a link to an image of the organism, with additional links to detailed descriptions at Wikipedia and Encyclopedia of Life (EOL).
- Several statistics allow comparison of hits across different searches. The number of mismatches over the length of the alignment gives a rough idea of how closely two sequences match. The bit score formula takes into account gaps in the sequence; the higher the score the better the alignment. The Expectation or E-value is the number of alignments with the query sequence that would be expected to occur by chance in the database. The lower the E-value, the higher the probability that the hit is related to the query. For example, an E-value of 0 means that a search with your sequence would be expected to turn up no matches by chance. Why do the most significant hits typically have E-values of 0? (This is not the case with BLAST searches with primers.) What does it mean when there are multiple BLAST hits with similar E-values?
Analyze Sequences: Select and Align
Many unknown species can be rapidly identified by a BLAST search. In this case, a phylogentic analysis adds depth to your understanding by showing how your sequence fits into a broader taxonomy of living things. If your BLAST search fails to identify your sequence, phylogenetic analysis can usually identify it to at least the family level.
- i. Click on "Upload Data" to include additional data. Either upload data in ab1 or FASTA format or import data from other sources.
- Click on "Reference Data" to select data that will let you compare your barcode sequence in an appropriate phylogenetic context.
- i. Click on "Select Data." Then check boxes to select any or all of the sequences you have uploaded from your own sequencing projects, from BLAST searches, and from reference data sets. Click on Save.
- Click on "MUSCLE" to align your sequences. When the program is finished, click again to view the alignment in Jalview.
Trim unaligned ends of the sequences
- Scroll through your alignments to see similarities between sequences. Nucleotides are color coded, and each row of nucleotides is the sequence of a single organism or sequencing reaction. Columns are matches (or mismatches) at a single nucleotide position across all sequences. Dashes (-) are gaps in sequence, where nucleotides in one sequence are not represented in other sequences.
- Note that the 5’ (leftmost) and 3’ (rightmost) ends of the sequences are usually misaligned, due to gaps (-) or undetermined nucleotides (Ns). What causes these problems?
- Note any sequence that introduces large, internal gaps (-----) in the alignment. This is either poor quality or unrelated sequence that should be excluded from the analysis. To remove it, return to Select Data, uncheck that sequence, and save your change. Then click on "MUSCLE" to recalculate.
7Analyze Sequences: Create a Phylogenetic Tree
- Identify the leftmost point at which all or most sequences show corresponding nucleotide color bars. (There should be few or no gaps in the vertical column of nucleotides at this point.)
- Click in the nucleotide coordinate bar directly above this nucleotide in the first sequence. This will activate a red cursor and a pop-up menu.
- Click on "Remove left" to trim the leftmost sequences to this nucleotide position.
- Repeat Steps a-b, and click Remove right to trim the rightmost sequences.
- You can return to Select Data (Step 6.b. above) to remove any sequence that has large sequence gaps. Why is it important to remove sequence gaps and unaligned ends?
- Click Submit trimmed alignment
- Click on "PHYLIP ML" to generate a phylogenetic tree using the maximum likelihood method. A tree will open in a new window; and the MUSCLE alignment used to produce it will open in another window.
- A phylogenetic tree is a graphical representation of relationships between taxonomic groups. In this experiment, a gene tree is determined by analyzing the similarities and differences in DNA sequence.
- Look at your tree.
Find and evaluate your sequence’s position in the tree.
- The branch tips are the DNA sequences of individual species or samples you analyzed. Any two branches are connected to each other by a node (), which represents the common ancestor of the two sequences.
- The length of each branch is a measure of the evolutionary distance from the ancestral sequence at the node. Species or sequences with short branches from a node are closely related, those with longer branches are more distantly related.
- A group formed by a common ancestor and its descendants is called a clade. Related clades, in turn, are connected by nodes to make larger, less-closely related clades.
- Click on a node () to highlight sequences in that clade. Click the node again to deselect the clade. What assumptions are made when one infers evolutionary relationships from sequence differences?
- Generally, the clades will follow established phylogenetic relationships ascending from genus > family > order > class > phylum. However, gene and phylogenetic trees do disagree on some placements, and much research is focused on "reconciling" these differences. Why do gene and phylogenetic trees sometimes disagree?
Return to the menu, and click on "PHYLIP NJ" to generate a phylogenetic tree using the neighbor joining method. How does it compare to the maximum likelihood tree? What does this tell you?
If neither tree places your sequence within an identifiable clade – or if that clade is only at order level – you will need to add more sequences that may increase the resolution of your analysis. Return to Step 5, and add more reference sequences or obtain sequences within the order or family clade that contained your sequence. Then repeat Steps 6-7 to select, align, and generate trees from your refined data set.
- If your sequence is closely related to any of the reference or uploaded sequences, it will share a single node with those species.
- If your sequence is identical to another sequence, the two will diverge directly from the node without branches.
- If your sequence is distantly related to all of the species in your tree, your sequence will sit on a branch by itself – with the other sequences grouping together as a clade.
- To identify the smallest clade that includes your sequence, click on the node that is directly connected to your sequence. The sequences that are highlighted are the closest relatives of your sequence in the tree.
- Look at the scientific names of sequences within the most closely associated clade. If all members share the same genus name, you have identified your sequence as belonging to that genus. If different genus names are represented, check and see if they belong to the same family or order.