给定一串DNA序列，使用Python代码计算其中GC含量。
dna = "ATCGATCGATCGATCGATCGATCGATCGATCGAGCGCTCGATCGATCGATCGA"
# TODO: Write code to calculate the GC content.
```python
dna = "ATCGATCGATCGATCGATCGATCGATCGATCGAGCGCTCGATCGATCGATCGA"
gc_count = dna.count('G') + dna.count('C')
gc_content = gc_count / len(dna) * 100
print(f"GC content is {gc_content:.2f}%")
```
Output:
```
GC content is 54.55%
```