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7. Short read quality and trimming

6. Visualizing BLAST score distributions in RStudio¶

Learning objectives:

• learn the basics of the RStudio interface.
• explore plotting in RStudio.
• explore some characteristics of the data resulting from your BLAST search.

6.1. Getting started¶

sudo passwd $USER  figuring out your username: echo My username is$USER


echo http://$(hostname):8787/  Now go to that Web address in your Web browser, and log in with the username and password from above. 6.2. Where are we?¶ We’ve moved from using the command line to using R. Much like the command line allows us to do text and database like operations on large files, R is a statistical programming language that allows us to do Excel-like operations on any size dataset. R can do anything from act like a simple interactive calculator, all the way up to automatically analyzing thousands of files and outputting the analysis as website of interactive charts and graphs. For instance, this Near Earth Object tracker is an R script that pulls data from NASA, and will let you do exploratory analysis by just pointing and clicking on a website! R is it’s own coding language, so it will take a little time before you can build your own NEO tracker, but we’re going to keep coming back to R and learning new features over the course of the workshop. So, it’s okay if you don’t feel like you ‘get’ it today. 6.3. Enter some R commands¶ (Enter the below commands into RStudio, not the command line.) Load the data you created with BLAST: blast_out <- read.table('blast/mm-second.x.zebrafish.tsv', sep='\t')  If you run View(blast_out), you’ll see the same information as in the previous section, but loaded into R for plotting and manipulation. The only problem is that the column names are kind of opaque - what does V1 mean!? To fix this, we can reset the column names like so, using the information from the BLAST outfmt documentation: colnames(blast_out) <- c("qseqid", "sseqid", "pident", "length", "mismatch", "gapopen", "qstart", "qend", "sstart", "send", "evalue", "bitscore") View(blast_out)  blast_out is called a dataframe, which is a sort of R-ish version of a spreadsheet with named columns. View can be used to present it nicely, and head(blast_out) can be used to look at just the first few rows. Another useful command is dim which will tell you the DIMENSIONS of this data frame: dim(blast_out)  That’s a big data frame! 14,524 rows (and 12 columns!) Let’s do some data visualization to get a handle on what our blast output looked like. First, let’s look at the evalue: hist(blast_out$evalue)


This is telling us that MOST of the values in the evalue column are quite low. What does this mean? How do we figure out what this is?

(You can also try plotting the distribution of -log(blast_out$evalue) - why is this more informative?) So these are a lot of low e-values. Is that good or bad? Should we be happy or concerned? We can take a look at some more stats – let’s look at the bitscore column: hist(blast_out$bitscore)


what are we looking for here? (And how would we know?)

(Hint: longer bitscores are better, but even bitscores of ~200 mean a nucleotide alignment of 200 bp - which is pretty good, no? Here we really want to rescale the x axis to look at the distribution of bitscores in the 100-300 range.)

Another question - if ‘bitscore’ is a score of the match, and ‘pident’ is the percent identity - is there a relationship between bitscore and pident?

Well, we can ask this directly with plot:

plot(blast_out$pident, blast_out$bitscore)


why does this plot look the way it does? (This may take a minute to show up, note!)

The answer is that bitscores are only somewhat related to pident; they take into account not only the percent identity but the length. You can get a napkin sketch estimate of this by doing the following:

plot(blast_out$pident * (blast_out$qend - blast_out$qstart), blast_out$bitscore)


which constructs a new variable, the percent identity times the length of the match, and then plots it against bitscore; this correlation looks much better.

6.3.1. Summary points¶

This is an example of initial exploratory data analysis, in which we poke around with data to see roughly what it looks like. This is opposed to other approaches where we might be trying to do statistical analysis to confirm a hypothesis.

Typically with small replicate sizes (n < 5) it is hard to do confirmatory data analysis or hypothesis testing, so a lot of NGS work is done for hypothesis generation and then confirmed via additional experimental work.

6.4. Some questions for discussion/points to make:¶

• Why are we using R for this instead of the UNIX command line, or Excel?

One important thing to note here is that we’re looking at a pretty large data set - with ease. It would be much slower to do this in Excel.

• What other things could we look at?