Admin side jquery is not loaded

I want to use jquery in my admin side programming but it seems there’s no jquery loaded in the page at all and it load only at main website! Has my wordpress any problem which can’t load jquery or I should do something extra to do it. However I put this code in function.php, but it not works:

if(is_admin()){        
    function load_admin_script(){
        wp_enqueue_script('jquery_script', "/wp-includes/js/jquery/jquery.js");
    }
    add_action('admin_enqueue_scripts','load_admin_script');

 }