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Quality control

Learning outcomes

After having completed this chapter you will be able to:

  • Find information about a sequence run on the Sequence Read Archive
  • Run fastqc on sequence reads and interpret the results
  • Trim adapters and low quality bases using cutadapt

Material

Download the presentation

Exercises

Download and evaluate an E. coli dataset

Exercise: If you haven’t already done so, create a directory called workdir in your home directory and make the directory your current directory.

If working with Docker

If you have mounted your local directory to /root/workdir, this directory should already exist.

Answer
cd ~
mkdir workdir
cd workdir

Check out the dataset at SRA.

Exercise: Browse around the SRA entry and answer these questions:

A. Is the dataset paired-end or single end?

B. Which instrument was used for sequencing?

C. What is the read length?

D. How many reads do we have?

Answers

A. paired-end

B. Illumina MiSeq

C. 2 x 251 bp

D. 400596

Make a directory reads in ~/workdir and download the reads from the SRA database using prefetch and fastq-dump from SRA-Tools into the reads directory:

Running sra-tools for the first time

If you run sra-tools for the first time, you have to set a config file. We’ll be just using a minimal file for now. To do that run:

vdb-config --interactive
When the GUI pops up, press the key X. The config file will be created in ~/.ncbi/user-settings.mkfg.

mkdir reads
cd reads
prefetch SRR519926
fastq-dump --split-files SRR519926

Exercise: Check whether the download was successful by counting the number of reads in the fastq files and compare it to the SRA entry.

Tip

A read in a fastq file consists of four lines (more on that at file types). Use Google to figure out how to count the number of reads in a fastq file.

Answer

e.g. from this thread on Biostars:

## forward read
echo $(cat SRR519926_1.fastq | wc -l)/4 | bc

## reverse read
echo $(cat SRR519926_2.fastq | wc -l)/4 | bc

Run fastqc

Exercise: Run fastqc on the fastq files.

Tip

fastqc accepts multiple files as input, so you can use a wildcard to run fastqc on all the files in one line of code. Use it like this: *.fastq.

Answer
fastqc *.fastq

Exercise: Download the html files to your local computer, and view the results. How is the quality? Where are the problems?

Answer

There seems to be:

  • Low quality towards the 3’ end (per base sequence quality)
  • Full sequence reads with low quality (per sequence quality scores)
  • Adapters in the sequences (adapter content)

We can probably fix most of these issues by trimming.

Trim the reads

We will use cutadapt for trimming adapters and low quality bases from our reads. The most used adapters for Illumina are TruSeq adapters. To run cutadapt you need to specify the adapter sequences with options -a (or --adapter) and -A. A reference for the adapter sequences can be found here.

Exercise: The script below will trim the sequence reads. However, some parts are missing. We want to:

  • trim bases with a quality lower then 10 from the 3’ and 5’ end of the reads,
  • keep only reads with a read length not shorter than 25 base pairs.

Fill in the missing options and execute the script to trim the data.

Hint

Check out the helper of cutadapt with: 

cutadapt --help


#!/usr/bin/env bash

TRIMMED_DIR=~/workdir/trimmed_data
READS_DIR=~/workdir/reads

mkdir -p $TRIMMED_DIR

cutadapt \
--adapter AGATCGGAAGAGCACACGTCTGAACTCCAGTCA \
-A AGATCGGAAGAGCGTCGTGTAGGGAAAGAGTGT \
[QUALITY CUTOFF OPTION] \
[MINIMUM LENGTH OPTION] \
--output $TRIMMED_DIR/paired_trimmed_SRR519926_1.fastq \
--paired-output $TRIMMED_DIR/paired_trimmed_SRR519926_2.fastq \
$READS_DIR/SRR519926_1.fastq \
$READS_DIR/SRR519926_2.fastq
Answer

Your script should look like this:

#!/usr/bin/env bash

TRIMMED_DIR=~/workdir/trimmed_data
READS_DIR=~/workdir/reads

mkdir -p $TRIMMED_DIR

cutadapt \
--adapter AGATCGGAAGAGCACACGTCTGAACTCCAGTCA \
-A AGATCGGAAGAGCGTCGTGTAGGGAAAGAGTGT \
--quality-cutoff 10,10 \
--minimum-length 25 \
--output $TRIMMED_DIR/paired_trimmed_SRR519926_1.fastq \
--paired-output $TRIMMED_DIR/paired_trimmed_SRR519926_2.fastq \
$READS_DIR/SRR519926_1.fastq \
$READS_DIR/SRR519926_2.fastq

The use of \

In the script above you see that we’re using \ at the end of many lines. We use it to tell bash to ignore the newlines. If we would not do it, the cutadapt command would become a very long line, and the script would become very difficult to read. It is in general good practice to put every option of a long command on a newline in your script and use \ to ignore the newlines when executing.

Exercise: Run fastqc on the trimmed fastq files and answer these questions:

A. Has the quality improved?

B. How many reads do we have left?

Answers

Running fastqc: 

cd ~/workdir/trimmed_data
fastqc paired_trimmed*.fastq

A. Yes, low quality 3’ end, per sequence quality and adapter sequences have improved.

B. 315904