How to Find the Reverse Complement of a DNA Sequence

Last Updated: May 6, 2021

The reverse complement of a DNA sequence signifies the contents of the opposite strand in a DNA molecule. DNA molecules are constructed as such because each nucleotide has a complementary nucleotide on the other strand to which a non-covalent bond exists.

Method 1

Method 1 of 2:
By Hand

1
Trace through the sequence backwards, starting from the last nucleotide in the sequence.
2
As you pass over each nucleotide, add its complementary nucleotide to the next line, beginning the complemented string from the left-hand side of the page. Remember, guanine (G) bonds to cytosine (C) and adenine (A) bonds to thymine (T).

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Method 2

Method 2 of 2:
Programmatically (Python 2)

1
Create or accept an input file. This article assumes that the input is in FASTA format, with a single sequence per file. The following steps also assume that all nucleotides are ATGC bases.
2
Read in the file. For FASTA format:
- Discard the first line of the file.
- Remove all remaining newlines and other trailing whitespace.
```
def init(sequence):
    with open(argv[1]) as input:
        sequence = "".join([line.strip() for line in input.readlines()[1:]])
    return sequence
```
3
Create a hash table that maps each nucleotide to its complement.
```
complement = {'A': 'T', 'C': 'G', 'G': 'C', 'T': 'A'}
```
4
Iterate through the sequence and use a hash table lookup to construct the complementary sequence. Reverse the resulting vector.
```
def reverse_complement(seq):
    bases = [complement[base] for base in seq]
    bases = reversed(bases)
    return bases
```
5
Print the contents of the vector.=
```
result = reverse_complement(seq)
print ''.join(result)
```
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