Extremely Long String Help

[Destry23000]

Hi,
I'm working with DNA and am trying to run a simple program to clean up the sequences I get from online databases. I need to convert all lower case letters to uppercase ones and ignore anything that isn't an A, T, C, or G. I want this new string in a new global variable. Unfortunately the code I currently have (below), which does work, is EXTREMELY slow. Does anyone know a better/faster/more efficient means of doing this? I currently have it running over the human genome which is 6.6 billion characters long and it has been going for 7 days and counting.

def CleanText(Text):
Gen = ""
global Genome
Genome = Gen
for i in Text:
if i == "a":
Gen += "A"
elif i == "t":
Gen += "T"
elif i == "c":
Gen += "C"
elif i == "g":
Gen += "G"
Genome = Gen

---

Sorry, I forgot to format it correctly.

def CleanText(Text):
Gen = ""
global Genome
Genome = Gen
for i in Text:
if i == "a":
Gen += "A"
elif i == "t":
Gen += "T"
elif i == "c":
Gen += "C"
elif i == "g":
Gen += "G"
Genome = Gen