Tree of Life

My schedule of Torah readings is all messed up, so I decided to just start at the beginning this past week in Genesis 1. As I began the first reading (Genesis 1.1-6.8) and came to the part about the fall of Adam and Eve I wondered about something: When Adam and Eve fall into sin God cuts them off from the Tree of Life. He puts an angel in place to guard the tree so that Adam and Eve can't get eternal life. Why does he do this? Why does he guard the tree instead of just destroying it? Is the guarding of the tree--rather than destroying it--a symbol of hope: there can yet be eternal life, but this is not the path to it?
In the New Testament the tree of life shows up again in Revelation 22.1 "Then the angel showed me the river of the water of life, bright as crystal, flowing from the throne of God and of the Lamb 2 through the middle of the street of the city; also, on either side of the river, the tree of life* with its twelve kinds of fruit, yielding its fruit each month. The leaves of the tree were for the healing of the nations." The tree of life ends up not in a garden but in the city of God. Now it doesn't bring eternal life, for Jesus has done that, instead it brings healing to the nations.
One other thing which makes one think a bit in the fall story is that Eve eats of the tree of good and evil believing that it will make her wise ("the tree was to be desired to make one wise.... Gen.3.6). But the real wisdom comes from not our attempts to grab what we think is wise, but to take on God's wisdom. The book of Proverbs talks about the tree of life and says:
Prov. 3:13 Blessed is the one who finds wisdom, and the one who gets understanding,
14 for the gain from her is better than gain from silver and her profit better than gold. 15 She is more precious than jewels, and nothing you desire can compare with her. 16 Long life is in her right hand; in her left hand are riches and honor. 17 Her ways are ways of pleasantness, and all her paths are peace. 18 She is a tree of life to those who lay hold of her; those who hold her fast are called blessed.