Revelation 7: 1-8, Tribes of Israel Set Aside

The first six seals of the Scroll have been removed.  Only one remains.

Revelation 7: 1-3, Preparations to open the scroll

After this I saw four angels standing at the four corners of the earth, holding back the four winds of the earth to prevent any wind from blowing on the land or on the sea or on any tree. 
2 Then I saw another angel coming up from the east, having the seal of the living God. He called out in a loud voice to the four angels who had been given power to harm the land and the sea: 
3 “Do not harm the land or the sea or the trees until we put a seal on the foreheads of the servants of our God.” 

A horrible natural disaster is about to unfold. The angels hold back the "four winds" from the "four corners" so that nothing stirs.  For a desert environment, this is ominous. Then an angel "from the east" comes, halting the disaster until people ("servants of God") are protected in some way.

What do the seals on foreheads represent?  It indicates some type of identification and protection. (The beast also seals people, see chapter 13.)

Revelation 7: 4-8, Those of Israel sealed by God

Then I heard the number of those who were sealed: 144,000 from all the tribes of Israel.
5 From the tribe of Judah 12,000 were sealed,
from the tribe of Reuben 12,000,
from the tribe of Gad 12,000,
6 from the tribe of Asher 12,000,
from the tribe of Naphtali 12,000,
from the tribe of Manasseh 12,000,
7 from the tribe of Simeon 12,000,
from the tribe of Levi 12,000,
from the tribe of Issachar 12,000,
8 from the tribe of Zebulun 12,000,
from the tribe of Joseph 12,000,
from the tribe of Benjamin 12,000.

Each of the twelve tribes of Israel has twelve thousand who are "sealed". Is this intended to be literal? Or is there something deeper, some symbolic meaning intended by the 12,000 from each tribe?

The tribe of Dan is replaced by Manasseh, a son of Joseph.  Why?