Skip to content
MasterMath

DNA Complementary Strand Calculator

The complementary strand of a DNA sequence, its reverse complement and the messenger RNA it gives, with GC content and a base count. Paste the sequence with spaces and position numbers if you like: they are stripped out.

Reverse complement (5'→3')

—

Reverse complement (5'→3')—
Complement in the same order—
RNA if this strand is the coding one—
RNA if this strand is the template—
GC content—
Length—
Base count—

How this was worked out

    The formula

    A ↔ T · G ↔ C · in RNA, T becomes U

    What it means

    The two strands of DNA pair base by base: adenine with thymine, guanine with cytosine. The catch is direction. The strands run antiparallel, so the one that actually pairs with yours written 5' to 3' is the complement read backwards, not the complement written in the same order. That is why both appear: the first answers "what is the other strand", and the second is what an exercise that only says "complement this" usually wants.

    How to work it out by hand

    1. Swap every A for a T and every G for a C, and the other way round
    2. For the reverse complement, read that result from the end to the start
    3. For messenger RNA, replace every T with a U
    4. Count the Gs and Cs and divide by the total for GC content

    What is worth knowing

    GC content matters because a G-C pair is held by three hydrogen bonds and an A-T pair by only two: the more GC, the more heat it takes to pull the strands apart. That is why it is checked when designing primers. One warning about wording: transcription does not always start from the strand in front of you, which is why two RNAs are listed. If the question says "template strand" or "antisense strand", the second one is the answer.

    Frequently asked questions

    What is the difference between the complement and the reverse complement?

    The order. The complement swaps each base for its partner; the reverse complement does the same and then reads it backwards, which is how the real strand sits, because the two run antiparallel.

    Can I paste the sequence with spaces or numbers?

    Yes. Spaces, line breaks, dashes and position numbers are stripped. Letters that are not bases are reported separately.

    What if the sequence contains an N?

    It is dropped and reported. N is an undetermined base from a sequencing machine and has no single partner.

    Why are there two RNAs?

    Because it depends which strand you have. If it is the coding strand, the RNA is the same with U instead of T; if it is the template, the RNA is the complement.

    What is a restriction palindrome?

    A sequence whose reverse complement is itself, such as GAATTC. Restriction enzymes cut there, and this calculator shows it at a glance.