revcomp.clj

(ns user)

(defn- reverse-base [base]
  (let [mapping { :A \T :T \A :G \C :C \G }
        key     (keyword (.toString base))]
    (key mapping)))

(defn reverse-complement [sequence]
  (let [rev-seq (reverse (.toUpperCase sequence))]
    (map reverse-base rev-seq)))

(println "Reverse complement gives" (reverse-complement "TGAC"))

Thanks for that link. Does help to clear it up at least a little bit for me. Trouble is that the core 'complement' cannot be easily accessed then (apart from prepending "clojure.core/"). Would be nice if we could say:
(println bio/complement(bio/reverse-str "TGACC"))
i.e. adding "bio" in front.

@wmacgyver: This is the code that we're trying to get going:

  (ns bio)
  (defn reverse-str
    "Reverse a string"
    [s]
    (apply str (reverse s))
  )
  
  (defn complement
    "Complement a base"
    [base]
    (let [mapping { :A \T :T \A :G \C :C \G }
      key (keyword (.toString base))]
      (key mapping)))
  )
  
  (println bio/complement (bio/reverse-str "TGACC"))

Output should be: "GGTCA"