Prove that an algebra is a -algebra iff is closed under countable increasing unions.

Proof so far:

Suppose that is a -algebra, then if , then I have

Now, if , then since . This direction is simple.

On the other hand, if A is closed under countable increasing unions, that means that for:

, then I have And I need to show that is a -algebra.

Suppose that , I need to show that:

1.

2.

How should I proceed from here? Thanks.