JSFiddle - React, Tailwind, and code Playground

HTML

<span>Source:</span><textarea id=source>TCACCGGCTTCGGATTACGATCCCATGCGGTCTCGATCGATTCCCCTTGGGGAAATCTATCTTCAGAATTCG</textarea>
<span>Result:</span><textarea id=result></textarea>

CSS

*[id]
{
    display:block;
    color: red;
    margin-left:10px;
    width:90%
}

JavaScript

function DNA(strand) {
        return strand
            .concat("||TACG")
            .replace(/[aA](?=.*?(T))|[tT](?=.*?(A))|[cC](?=.*?(G))|[gG](?=.*?(C))|\|\|....$/g, "$1$2$3$4");
}
updateText =  function() {
    
    var source = $("#source").val();
    $("#result").text(DNA(source));
    

}
updateText();

$("#source").bind('input propertychange',updateText);