Due: at 11pm on Thursday, January 17.
Submit via this
turnin page.
You will use, modify, and extend a program to compute the GC content of DNA data. The GC content of DNA is the percentage of nucleotides that are either G or C.
DNA can be thought of as a sequence of nucleotides. Each nucleotide is adenine, cytosine, guanine, or thymine. These are abbreviated as A, C, G, and T. A nucleotide is also called a nucleotide base, nitrogenous base, nucleobase, or just a base.
Biologists have multiple reasons to be interested in GC content.
If you are curious, Wikipedia has more information about GC content. That reading is optional and is not required to complete this assignment.
Your program will read files produced by a high-throughput sequencer — a machine that takes as input some DNA, and produces as output a file containing a sequence of nucleotides.
Here are the first 8 lines of output from a particular sequencer:
@SOLEXA-1GA-2_2_FC30DNN:1:2:574:1722 CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC +SOLEXA-1GA-2_2_FC30DNN:1:2:574:1722 hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh @SOLEXA-1GA-2_2_FC30DNN:1:2:478:1745 GTGGGGGTGATGTCCACGATTACGCCGACCGGCTGG +SOLEXA-1GA-2_2_FC30DNN:1:2:478:1745 hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh
The nucleotide data is in the second line, the sixth line, the tenth line, etc. Your program will not use the rest of the file, which provides information about the sequencer and the sequencing process that created the nucleotide data.
Obtain the files you need by downloading the homework2.zip file. (This is a large download — be patient.)
Unzip the homework2.zip file to create a homework2 directory/folder. You will do your work here. The homework2 directory/folder contains:
You will do your work by modifying two files — dna_analysis.py and answers.txt — and then submitting the modified versions. Add your name to the top of each of these files.
Each problem will ask you to make some changes to the program dna_analysis.py (or to write text in the answers.txt file, or both). When you do so, you will generally add to the program. Do not remove changes from earlier problems when you work on later problems; your final program should solve all the problems.
In either file, keep the amount of characters within a particular line below 80. One technique to do this in python would be to break large equations into smaller ones by storing subexpressions in variables.
By the end of the assignment, we would like dna_analysis.py to produce output of the exact form:
GC-content: ___ AT-content: ___ G count: ___ C count: ___ A count: ___ T count: ___ Sum count: ___ Total count: ___ seq length: ___ AT/GC Ratio: ___ GC Classification: ___
Where ___ is replaced by values that you will calculate. Of course, the exact values in each category will vary depending on the input data that you are using. We expect the formatting of your program output to exactly match this.
You will submit answers.txt as a text file. Plain text is the standard for communicating information among programmers, because it can be read on any computer without installing proprietary software. You can edit text files using IDLE or another text editor. If you use a word processor, then be sure to save the files as text. Windows users should never use Notepad for any purpose, because Notepad will mangle the line endings in the file; WordPad or Notepad++ are better alternatives.
It is a good idea to check the correctness of your program by comparing it to a computation done in some other way, such as by hand or by a different program. We have provided the test-small.fastq file for this purpose. First, examine the file by hand to determine the GC content. Then, run your program to verify that it provides the correct answer for this file.
Run your program by opening a shell or command prompt (not IDLE's Python interpreter), navigating to your homework2 directory, then typing the following command.
On Mac/Linux:
python dna_analysis.py data/test-small.fastq
On Windows:
python dna_analysis.py data\test-small.fastq
If you get a "can't open file 'dna_analysis.py'" error or a "No such file or directory" error, then perhaps your working directory is not homework2, or you mistyped the file name.
After you have confirmed that your program runs correctly on test-small.fastq, run your program on each of the 6 real sample_N.fastq files provided, by executing 6 commands such as
python dna_analysis.py data/sample_1.fastq
or if you are a Windows user,
python dna_analysis.py data\sample_1.fastq
You will have to change sample_1.fastq to a different file name in the subsequent commands. Be patient — you are processing a lot of data, and it might take a minute or so to run.
(If you are interested, sample_3.fastq and sample_4.fastq are from Streptococcus pneumoniae TIGR4, and sample_5.fastq is from Human herpesvirus 5.)
If you have already used the Output Comparison Tool (referenced at the bottom of the page), you might notice that some of your results are different than the example results. Don't worry about this — this issue will be resolved in Problem 6.
Cut and paste the line of output regarding GC-content from sample_1.fastq into your answers.txt file. For example, your answer might look like
GC-content: 0.42900139393
(Note that this is not the answer you should expect to get, this is just an example of the format that your answer should be in.)
In your program, comment out these lines
seq = "" linenum = 0
by prefixing them by the # character. Re-run the program, just as you did for Problem 2. In answers.txt, explain what happened, and why it happened. Now, restore the lines to their original state by removing the # that you added.
What would happen if you commented out this line?
gc_count = 0
Explain (in answers.txt).
Augment your program so that, in addition to computing and printing the GC ratio, it also computes and prints the AT content. The AT content is the percentage of nucleotides that are A or T.
Two ways to compute the AT content are:
You may use whichever approach you prefer.
Check your work by manually computing the AT content for file test-small.fastq, then comparing it to the output of running your program on test-small.fastq.
Run your program on sample_1.fastq. Cut-and-paste the relevant line of output into answers.txt.
Augment your program so that it also computes and prints the number of A nucleotides, the number of T nucleotides, the number of G nucleotides, and the number of C nucleotides.
When doing this, add at most one extra loop to your program. You can solve this part without adding any new loops at all, by reusing an existing loop.
Check your work by manually computing the results for file test-small.fastq, then comparing them to the output of running your program on test-small.fastq.
Run your program on sample_1.fastq. Cut-and-paste the relevant lines of output into answers.txt (the lines that indicate the G count, C count, A count, and T count).
For each of the 11 .fastq files, compare the following three quantities:
In other words, compute the three numbers for test-small.fastq and determine whether they are equal or different. Then do the same for test-high-gc-1.fastq, etc.
For at least one file, at least two of these metrics will differ. In your answers.txt file, state which file(s) and which metrics. (If all the metrics are equal for each file, then your code contains a mistake.) In your answers.txt file, write a short paragraph that explains why.
Explaining why (or debugging your code if all the metrics were the same) might require you to do some detective work. For instance, to understand the issue, you may need to load a file into a text editor and examine it. We strongly suggest that you start with the smallest file for which the numbers are not all the same. Perusal of the file may help you. Failing that, you can manually compute each of the counts, and then compare your manual results to what your program computes to determine where the error lies. A final approach would be to modify your program, or create a new program, to compute the three metrics for each line of a file separately: if the metrics differ for an entire file, then they must differ for some specific line, and then examining that line will help you understand the problem.
If all of the three quantities that you measured in problem 6 are the same, then it would not matter which one you used in the denominator when computing the GC content. In fact, you saw that the numbers are not the same. In file answers.txt, state which of these quantities can be used in the denominator and which cannot, and why.
If your program incorrectly computed the GC content (which should be equal to (G+C)/(A+C+G+T)), then state that fact in your answers.txt file. Then, go back and correct it, and also update any incorrect answers elsewhere in your answers.txt file.
Sometimes biologists use the AT/GC ratio, defined as (A+T)/(G+C), rather than the GC-content, which is defined as (G+C)/(A+C+G+T).
Modify your program so that it also computes the AT/GC ratio.
Check your work by manually computing the results for file test-small.fastq. Compare them to the output of running your program on test-small.fastq.
Run your program on sample_1.fastq. Cut-and-paste the relevant lines of output into answers.txt (the line that indicates the AT/GC ratio).
The GC content can be used to categorize microorganisms.
Modify your program to print out a classification of the organism in the file.
If the GC content is above 60%, the organism is considered “high GC content”.
If the GC content is below 40%, the organism is considered “low GC content”.
Otherwise, the organism is considered “moderate GC content”.
Biologists can use GC content for classifying species, for determining the
melting temperature of the DNA (useful for both ecology and
experimentation, for example
PCR is
more difficult on organisms with high
GC content), and for other purposes. Here are some examples:
The GC content of Streptomyces coelicolor A3(2) is 72%.
The GC content of Yeast (Saccharomyces cerevisiae) is 38%.
The GC content of Thale Cress (Arabidopsis thaliana) is 36%.
The GC content of Plasmodium falciparum is 20%.
Again, test your work. The test-small.fastq file has low GC content. We have provided four other test files, whose names explain their GC content: test-moderate-gc-1.fastq, test-moderate-gc-2.fastq, test-high-gc-1.fastq, test-high-gc-2.fastq.
After your program works for all the test files, run it on sample_1.fastq. Cut-and-paste just the relevant line of output from your program into answers.txt.
You are almost done!
At the bottom of your answers.txt file, in the “Collaboration” part, state which students or other people (besides the course staff) helped you with the assignment, or that no one did.
At the bottom of your answers.txt file, in the “Reflection” part, reflect on this assignment. What did you learn from this assignment? What do you wish you had known before you started? What would you do differently? What advice would you offer to future students?
Submit the following files via this turnin page.
Please validate your dna_analysis.py program's output using the Diff Checker before submitting your assignment. You can compare your output to the files given in the expected_output directory of the homework2 files.
Answer a survey about how much time you spent on this assignment.