Several RNA strategies to correct genetic information at transcript level are in development today. This work focuses on the exchange of small RNA fragments with twin ribozymes in order to repair or functionalise RNA. The hairpin ribozyme catalyses both cleavage and ligation of an RNA substrate depending on the stability of the ribozyme-substrate-complex. Twin ribozymes were engineered through rational design by coupling two hairpin ribozyme units to form a tandem ribozyme structure. The cleavage of the RNA substrate in both catalytic units produces three different RNA fragments: the two fragments located on the outer sides stay tightly bound to the twin ribozyme whereas the binding of the one in the middle is destabilised by a four-nucleotide loop in the ribozyme strand. This loop promotes the dissociation of the middle substrate fragment and hinders its back ligation. The subsequent addition of a repair oligonucleotide that strongly binds to this part of the twin ribozyme favours its association and ligation to the outer fragments and results in a four-nucleotide longer repair product. The whole reaction therefore mimics the repair of a four-nucleotide deletion at RNA level. The first major part of this work was to kinetically and thermodynamically characterise the repair reaction ...