How to make question answer content type with answer as a separate node?
If you don't want to reinvent the wheel, you should have a look at the Answers module. Here are some details about it (from its project page):


  Answers enables the implementation of a Question &amp; Answer system in a Drupal site. Users can post questions and other users can answer them.


Head over to its project page for more details.

At its core, this module exists of 2 typical content types:


Question.
Answers.


The "Best Answer" submodule can be used so that the question author can "accept" one of the answers as the best one. And you could make the Rules module to either trigger all sorts of subsequent things to happen "when an answer gets marked as best answer", or to add all sorts of validations when posting an answer (or question).

The Answers module comes with integrations with other modules such as the Rules module and the Userpoints module and supports voting.

Demo

Have a look at the (new) demo site of the Answers module. This link shows various menu options within the block with title Answers Menu, i.e.:


Questions (= a view):


All: of all existing questions (= default tab).
Answered: all questions which have a "best" (= accepted) answer (= sub-tab).
Unanswered: all questions which do not yet have a "best" (= accepted) answer (= sub-tab).

Answers (= a view): lists all existing answers for any of the existing questions and which does not (yet) have any submenu options (implemented as tabs).
Rankings (= a view): contains a list of users with their actual ranking (= reputation points implemented via user points), and with each user a link to either the questions or answers from that user.
Ask Question: to post a new question (the equivalent of node/add/question), which is only shown for authenticated users though.


Disclosure: I'm a (co-)maintainer of this module.