MCB3895-004 Lecture #10 Sept 25/14

MCB3895-004 Lecture #10Sept 25/14

SRA, Illumina data QC

Underwstanding the BBC cluster• What is the cluster? Many individual computers

controlled by a "head node"• The head node is what you log onto by default

using SSH• It is bad etiquette to run things off the head

node• Can slow down the entire system

Using the cluster - method 1

• When you SSH in, use the "qlogin" command to take you to a subordinate note

• Running programs here will not disrupt the head node

• You need to stay connected to the network until all of your programs are completed

Checking a qlogin job

• Use the terminal command "top"

• Shows all processes running on your node

• kill a process by pressing "k" and then entering its PID when prompted

Using the cluster - method 2

• Use the command "qsub" combined with a shell script

• e.g., qsub script.sh• shell is a programming language commonly

used for controlling actual processes• The BBC has example scripts for you to modify:

http://bioinformatics.uconn.edu/understanding-the-bbc-cluster-and-sge/

• This method allows you to walk away once your script is running

qsub bash script#!/bin/bash

################################################################## TEMPLATE SGE SCRIPT - BLAST EXAMPLE ######################## /common/sge_templates/template_single.sh ###########################################################################

# Specify the name of the data file to be usedINPUTFILENAME="test.fasta"

# Name the directory (assumed to be a direct subdir of $HOME) from which the file is listedPROJECT_SUBDIR="test"

# Specify name to be used to identify this run#$ -N blastp_job

# Email address (change to yours)#$ -M bioinformatics@uconn.edu

# Specify mailing options: b=beginning, e=end, s=suspended, n=never, a=abortion#$ -m besa

qsub bash script# Specify that bash shell should be used to process this script#$ -S /bin/bash

# Specify working directory (created on compute node used to do the work)WORKING_DIR="/scratch/$USER/$PROJECT_SUBDIR-$JOB_ID"

# If working directory does not exist, create it# The -p means "create parent directories as needed"if [ ! -d "$WORKING_DIR" ]; thenmkdir -p $WORKING_DIRfi

# Specify destination directory (this will be subdirectory of your home directory)DESTINATION_DIR="$HOME/$PROJECT_SUBDIR/$JOB_ID-$INPUTFILENAME"

# If destination directory does not exist, create it# The -p in mkdir means "create parent directories as needed"if [ ! -d "$DESTINATION_DIR" ]; thenmkdir -p $DESTINATION_DIRfi

qsub bash script# navigate to the working directory

cd $WORKING_DIR

# Get script and input data from home directory and copy to the working directory

cp $HOME/$PROJECT_SUBDIR/$INPUTFILENAME ./test.fasta

cp $HOME/template_single.sh .

# Specify the output file

#$ -o $JOB_ID.out

# Specify the error file

#$ -e $JOB_ID.err

# Run the program

blastp -query $INPUTFILENAME -db /usr/local/blast/data/refseq_protein -num_alignments 5 -num_descriptions 5 -out my-results

# copy output files back to your home directory

cp * $DESTINATION_DIR

# clear scratch directory

rm -rf $WORKING_DIR

Checking a qsub job

• Use "qstat" to understand the status of your job

• Shows jobs waiting to be executed• Monitor a running job's status using qstat -j <job_number>

• Retrieve information about a completed job using qaact -j <job_number>

Cluster etiquette

• Never run something on the head node!• Always check that your processes will run

correctly before starting a large task! • Best strategy: run commands individually using a

reduced input dataset• If using a loop to execute multiple commands, only

go through a single iteration (e.g., use die)

The first part of Assignment #4

• Write a perl script that subsamples the first ~10000 reads of your input fasta file(s)

• Allows you to do quick troubleshooting• Can be modified later to examine the effect of

sampling depth

SRA

• "Sequence Read Archive"• http://www.ncbi.nlm.nih.gov/sra• The part of NCBI that holds raw sequencing

data• Generally, this is where you need to put your

raw data when you publish genomic research

SRA

A SRA record

SRA run browser

For kicks…

• Go to http://www.ncbi.nlm.nih.gov/sra• Search "Escherichia coli MG1655"• Note different results

• Different sequencing platforms• Note mutant strains!

• Try "Escherichia coli MG1655 Pacbio"

Downloading SRA data

• Possible to do from web browser, but transferring large files is cumbersome

• Better: use NCBI's SRA Toolkit on the BBC server to perform file conversion while downloading

/opt/bioinformatics/sratoolkit.2.3.5-2-centos_linux64/bin/fastq-dump --split-files SRR826450

• Decompresses files, splits paired ends into separate files

The fastq file format

• 4 lines per sequence:• Line 1: begins with "@", followed by sequence ID• Line 2: raw sequence data• Line 3: begins with "+", may have sequence ID• Line 4: Phred quality score for each position, in ASCII

@SEQ_IDGATTTGGGGTTCAAAGCAGTATCGATCAAATAGTAAATCCATTTGTTCAACTCACAGTTT+!''*((((***+))%%%++)(%%%%).1***-+*''))**55CCF>>>>>>CCCCCCC65

ASCII from low (left) to high (right):

!"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~

http://en.wikipedia.org/wiki/FASTQ_format

Phred scores

• Developed by old program "Phred" during human genome project, adopted as standard throughout field

• Phred score = -10log(P(base call error))• e.g.,

Phred score of 10 = 90% base call accuracyPhred score of 20 = 99% base call accuracyPhred score of 30 = 99.9% base call accuracyPhred score of 40 = 99.99% base call accuracyetc.

FastQC - QC for raw reads

• FastQC is the most common software to understand the quality of raw sequencing reads

• http://www.bioinformatics.babraham.ac.uk/projects/fastqc/

• Runs using a java applet• Using the server, we have to run via command

line

FastQC screenshot

Starts into specific figures

Summary stats

What it thinks of yourrun quality - NOT HARDAND FAST RULES!!

FastQC - Per base quality

• Blue line: mean

• Red line: median

• Boxes: 25-75% range

• Whiskers:• 10-90%

range

Phre

d sc

ore

FastQC - Per read quality• Highlights

systematic problems

• e.g., a region of flowcell is problematic

FastQC - Per base sequence content

• Unbiased sequences should have the same content across all bases

• Will show biases if some sequence is hugely overrepresented

• e.g., adapter contamination

• e.g., biased fragmentation

FastQC - Per Sequence GC content

• Unbiased sequencing should have a normally distributed %GC content

• Deviations may indicated contamination

• e.g., adapter• e.g., two

species with different %GC contents

FastQC - Per base N count

• Ns indicate that the base caller could not determine a base at that position

• Global N abundance generally correlates with sequence quality

FastQC - Sequence length

• Some methods yield non-uniform read lengths

• e.g., Pacbio (shown)

• Illumina will only show one uniform value here

FastQC - Duplicate sequences

• An unbiased library should have few duplicates

• A few duplicates may indicate saturated template sequencing

• High duplication may indicate adapter contamination or enrichment bias

FastQC - Kmer content

• Tests for kmers enriched as a certain read position

• Graphs 6 worst, tabulates the rest

• Can indicate sequencing/library bias

• Can indicate contamination by one sequence, e.g., primers, adapters

FastQC - Overrepresented sequences• May indicate how read diversity is limited, e.g.,

adapter/primer contamination• May be biological, e.g., repeats

FastQC - Adapter content

• Specifically looks for known adapter/primer contamination

• Indicates reads are longer than insert size

FastQC - Per tile sequence quality

• Shows flowcell tiles that are particularly error-prone

• Illumina data only, and only if positional metadata is included with reads

Running FastQC on the server

• Very simple: $ fastqc <input_file>• Produces a .html file as output• Transfer the html to your computer and open it

using your favorite web browser

Getting rid of adapters using Trimmomatic• Trimmomatic is a standard method to remove

adapter contamination• http://www.usadellab.org/cms/?

page=trimmomatic• Bolger et al. 2014 Bioinformatics btu170

Running Trimmomatic• Admittedly, a complex syntaxjava -jar <path to trimmomatic.jar> PE [-threads <threads] [-phred33 | -phred64] [-trimlog <logFile>] <input 1> <input 2> <paired output 1> <unpaired output 1> <paired output 2> <unpaired output 2> <step 1> ...

java -jar /opt/bioinformatics/Trimmomatic-0.32/trimmomatic-0.32.jar PE SRR826450_1.fastq SRR826450_2.fastq output_forward_paired.fq output_forward_unpaired.fq output_reverse_paired.fq output_reverse_unpaired.fq ILLUMINACLIP:/opt/bioinformatics/Trimmomatic-0.32/adapters/TruSeq3-PE.fa:2:30:10

Assignment #4

• Download these two E. coli K-12 MG1655 genome sequencing reads from NCBI SRA: SRR826450, SRR956947

• What are the differences?• Write script to subsample fastq files• Analyze your input data using fastqc• If appropriate, adapter trim using Trimmomatic