From where the execution starts in Drupal Generally in any HTML file execution starts from index.html and PHP file starts from index.php.

But in Drupal from which file the execution is started, i know that it is started from bootstrap file but what calls the bootstrap file ?

What is the execution sequence when i write http://example.com which will load/install drupal ?
index.php inside Drupal root folder is the starting point of execution. It calls the bootstrap functions. Please see the code inside drupal 7 index.php file below.

define('DRUPAL_ROOT', getcwd());

require_once DRUPAL_ROOT . '/includes/bootstrap.inc';
drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);
menu_execute_active_handler();