bio-read-alignment-bowtie2-alignment
$
npx mdskill add GPTomics/bioSkills/bio-read-alignment-bowtie2-alignmentAlign short DNA reads to a reference genome using Bowtie2 with flexible alignment modes
- Solves alignment of ChIP-seq, ATAC-seq, or other short-read sequencing data
- Uses Bowtie2 CLI tool and samtools for sorting output
- Chooses between end-to-end or local alignment based on input read characteristics
- Produces sorted BAM files directly from FASTQ input using pipeline integration
SKILL.md
.github/skills/bio-read-alignment-bowtie2-alignmentView on GitHub ↗
---
name: bio-read-alignment-bowtie2-alignment
description: Align short reads using Bowtie2 with local or end-to-end modes. Supports gapped alignment. Use when aligning ChIP-seq, ATAC-seq, or when flexible alignment modes are needed.
tool_type: cli
primary_tool: bowtie2
---
## Version Compatibility
Reference examples tested with: samtools 1.19+
Before using code patterns, verify installed versions match. If versions differ:
- CLI: `<tool> --version` then `<tool> --help` to confirm flags
If code throws ImportError, AttributeError, or TypeError, introspect the installed
package and adapt the example to match the actual API rather than retrying.
# Bowtie2 Alignment
**"Align DNA reads with Bowtie2"** → Map short reads to a reference genome using Bowtie2's end-to-end or local alignment modes.
- CLI: `bowtie2 -x index -1 R1.fq -2 R2.fq | samtools sort -o aligned.bam`
## Build Index
```bash
# Build index from reference FASTA
bowtie2-build reference.fa reference_index
# With threads (faster)
bowtie2-build --threads 8 reference.fa reference_index
# Creates: reference_index.1.bt2, .2.bt2, .3.bt2, .4.bt2, .rev.1.bt2, .rev.2.bt2
```
## Basic Alignment
```bash
# Paired-end reads
bowtie2 -p 8 -x reference_index -1 reads_1.fq.gz -2 reads_2.fq.gz -S aligned.sam
# Single-end reads
bowtie2 -p 8 -x reference_index -U reads.fq.gz -S aligned.sam
# Direct to sorted BAM
bowtie2 -p 8 -x reference_index -1 r1.fq.gz -2 r2.fq.gz | \
samtools sort -@ 4 -o aligned.sorted.bam -
```
## Alignment Modes
```bash
# End-to-end mode (default) - align entire read
bowtie2 --end-to-end -x index -1 r1.fq -2 r2.fq -S aligned.sam
# Local mode - soft-clip ends for better alignment
bowtie2 --local -x index -1 r1.fq -2 r2.fq -S aligned.sam
```
## Sensitivity Presets
```bash
# Very fast (less sensitive)
bowtie2 --very-fast -x index -1 r1.fq -2 r2.fq -S aligned.sam
# Fast
bowtie2 --fast -x index -1 r1.fq -2 r2.fq -S aligned.sam
# Sensitive (default)
bowtie2 --sensitive -x index -1 r1.fq -2 r2.fq -S aligned.sam
# Very sensitive (slower but more accurate)
bowtie2 --very-sensitive -x index -1 r1.fq -2 r2.fq -S aligned.sam
# Local mode equivalents
bowtie2 --very-sensitive-local -x index -1 r1.fq -2 r2.fq -S aligned.sam
```
## ChIP-seq Alignment
```bash
# Typical ChIP-seq settings
bowtie2 -p 8 \
--very-sensitive \
--no-mixed \
--no-discordant \
-x index -1 chip_1.fq.gz -2 chip_2.fq.gz | \
samtools view -bS -q 30 -F 4 - | \
samtools sort -o chip.sorted.bam -
```
## ATAC-seq Alignment
```bash
# ATAC-seq with size selection
bowtie2 -p 8 \
--very-sensitive \
-X 2000 \ # Max fragment length
--no-mixed \
--no-discordant \
-x index -1 atac_1.fq.gz -2 atac_2.fq.gz | \
samtools view -bS -q 30 - | \
samtools sort -o atac.sorted.bam -
```
## Fragment Size Options
```bash
# Set expected insert size range
bowtie2 -p 8 \
-I 100 \ # Minimum fragment length
-X 500 \ # Maximum fragment length
-x index -1 r1.fq -2 r2.fq -S aligned.sam
```
## Read Group and Output Options
```bash
# Add read group
bowtie2 -p 8 \
--rg-id sample1 \
--rg SM:sample1 \
--rg PL:ILLUMINA \
--rg LB:lib1 \
-x index -1 r1.fq -2 r2.fq -S aligned.sam
```
## Multi-mapping Reads
```bash
# Report up to k alignments per read
bowtie2 -k 5 -x index -1 r1.fq -2 r2.fq -S aligned.sam
# Report all alignments
bowtie2 -a -x index -1 r1.fq -2 r2.fq -S aligned.sam
```
## Output Unmapped Reads
```bash
# Write unmapped reads to separate files
bowtie2 -p 8 \
--un-conc-gz unmapped_%.fq.gz \
-x index -1 r1.fq.gz -2 r2.fq.gz -S aligned.sam
```
## Key Parameters
| Parameter | Default | Description |
|-----------|---------|-------------|
| -p | 1 | Number of threads |
| -x | - | Index basename |
| -1/-2 | - | Paired-end reads |
| -U | - | Single-end reads |
| -I | 0 | Min fragment length |
| -X | 500 | Max fragment length |
| -k | 1 | Report up to k alignments |
| --no-mixed | off | Suppress unpaired alignments |
| --no-discordant | off | Suppress discordant alignments |
## Alignment Statistics
```bash
# Bowtie2 prints alignment summary to stderr
bowtie2 -p 8 -x index -1 r1.fq -2 r2.fq -S aligned.sam 2> alignment_stats.txt
```
Example output:
```
1000000 reads; of these:
1000000 (100.00%) were paired; of these:
50000 (5.00%) aligned concordantly 0 times
900000 (90.00%) aligned concordantly exactly 1 time
50000 (5.00%) aligned concordantly >1 times
95.00% overall alignment rate
```
## Related Skills
- read-qc/fastp-workflow - Preprocess reads before alignment
- alignment-files/alignment-sorting - Post-alignment processing
- chip-seq/peak-calling - ChIP-seq analysis
- atac-seq/atac-peak-calling - ATAC-seq analysis
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