To Add and Remove a Role Def: 

To Add******SPRoleDefinition newRole = SharepointHelper.FindRoleDefinition(web.RoleDefinitions, "AdminPermissions");if (newRole == null) {  newRole = new SPRoleDefinition();  newRole.Name = "AdminPermissions";  newRole.Description = "Can edit and approve pages, list items, and documents.";  newRole.BasePermissions = SPBasePermissions.CancelCheckout | SPBasePermissions.AddListItems ;  web.RoleDefinitions.Add(newRole);}To Remove**********SPRoleDefinition roleDef = SharepointHelper.FindRoleDefinition(web.RoleDefinitions, "AdminPermissions");if(roleDef!=null){ web.RoleDefinitions.Remove(roleDef);}

 

Utility Method

 

public SPRoleDefinition FindRoleDefinition(SPRoleDefinitionCollection roledefColls, string roleName)    {      foreach (SPRoleDefinition definition in roledefColls)      {        if (definition.Name.ToUpper(CultureInfo.InvariantCulture) == roleName.ToUpper(CultureInfo.InvariantCulture))          return definition;      }      return null;    }

Using SPBasePermissions enumerations you can add your base permissions which creating a Role Def.