Free Random DNA Sequence Generator for Synthetic A/C/G/T Strings, GC Percentage, and FASTA Output
Create synthetic A/C/G/T sequences for teaching, coding exercises, parser tests, examples, and mock bioinformatics data. Set sequence length and approximate GC target, then inspect composition and reverse complement.
Generate a synthetic DNA sequence
How to use this tool
1. Set the rules
Choose the range, filters, list, or generation settings that match your real task.
2. Generate and inspect
Run the tool once, review the result and its summary, then adjust settings only if needed.
3. Save useful output
Copy, download, print, favorite, or keep a local history entry when you need a record.
Practical examples
Parser testing
Generate a 1,000-base FASTA string to test import and line wrapping.
GC exercise
Compare actual GC% across several short sequences with the same target.
Reverse-complement lesson
Generate a short sequence and verify base-pair rules using the displayed reverse complement.
12 useful features
10–5000 base length
Built into this page for a faster, clearer workflow.
0–100% target GC
Built into this page for a faster, clearer workflow.
A/C/G/T-only generation
Built into this page for a faster, clearer workflow.
Weighted GC sampling
Built into this page for a faster, clearer workflow.
Base-count summary
Built into this page for a faster, clearer workflow.
Actual GC calculation
Built into this page for a faster, clearer workflow.
Plain-sequence output
Built into this page for a faster, clearer workflow.
FASTA formatting
Built into this page for a faster, clearer workflow.
60-character line wrapping
Built into this page for a faster, clearer workflow.
Reverse complement
Built into this page for a faster, clearer workflow.
Copy/download/print
Built into this page for a faster, clearer workflow.
Local history
Built into this page for a faster, clearer workflow.
Reference table
| Setting | What it changes | Useful check |
|---|---|---|
| Sequence length | Sets total A/C/G/T characters | Longer sequences usually track the target GC% more closely. |
| Target GC% | Sets the probability of choosing G or C | It is probabilistic, not an exact composition constraint. |
| FASTA mode | Adds a >header and wraps lines | This is formatting only; no biological annotation is added. |
Quality checklist
0 / 5 complete
Quick knowledge check
What is the best first check before using generated output?
Microlearning guide
Randomness can cluster
A fair random process can still create streaks, repeated patterns, or uneven-looking small samples.
Uniqueness changes the pool
Sampling without replacement prevents repeats but also reduces the remaining eligible choices after every pick.
Use the right scope
These are browser utilities for planning, learning, creative work, and test data—not audited draws or high-stakes systems.
Related tools
Frequently asked questions
Are these real biological sequences?
No. The output is synthetic random A/C/G/T text generated locally for teaching, examples, parser tests, and mock data.
Does target GC guarantee an exact GC percentage?
No. Each base is sampled randomly using the target probability, so the actual percentage varies, especially for short sequences.
What is a reverse complement?
For a DNA string, the tool reverses the sequence and swaps A with T and C with G.
Can I export FASTA-style text?
Yes. Choose FASTA and set a simple header; the sequence is wrapped at 60 characters per line.
Is this suitable for laboratory or clinical work?
No. It is an educational and software-testing utility, not a laboratory design, diagnostic, or biological validation tool.