DiscoveriesaboutthenatureofDNAinthe1940smade（61）___possibleforcloningexperimentstoprogress．In1944thediscovery（62）___geneticinformationforeachcellwaskeptinthecell'sDNAgavescientistsnewwaystoattempt（63

　　Discoveries about the nature of DNA in the 1940s made （61）___ possible for cloning experiments to progress．In 1944the discovery （62）___ genetic information for each cell was kept in the cell's DNA gave scientists new ways to attempt （63）___ （clone） animals．

　　The first cloned animal was a tadpole （64）___ was cloned in 1952．Thomas J．King and Robert W．Briggs believed，based on their clones，that young cells were more viable （可生长发育的） for the cloning process than adult cells．

　　The next successful cloning experiments also resulted （65）___ cloned frogs．John Gurdon cloned South African frogs in 1962．His experiments proved that the previous theory that adult cells were （66）___ （able） to be used in the cloning process was wrong．From 1962to 1965，more frog clones were created from adult frog cells．

　　While animal cloning had been the focus of cloning experiments，the 1960s also saw （67）___ types of cloning．In 1964F．C．Steward （68）___ （undertake） an experiment in which he took an adult cell from a carrot plant and successfully cloned the plant．

　　So far more than 20different animals，（69）___ （include） dogs，cats，and the famous Dolly，have been cloned，（70）___ human cloning research has been forbidden in many countries．