In genetic the reverse complement of a sequence is formed by **reversing** the sequence and then taking the complement of each symbol.
The four nucleotides in DNA is Adenine (A), Cytosine (C), Guanine (G) and Thymine (Thymine). 
- A is the complement of T 
- C is the complement of G.
This is a bi-directional relation so:
- T is the complement of A
- G is the complement of C.
For this kata you need to complete the reverse complement function that take a DNA string and return the reverse complement string.
**Note**: You need to take care of lower and upper case. And if a sequence conatains some invalid characters you need to return "Invalid sequence".
This kata is based on the following [one](http://www.codewars.com/kata/complementary-dna/ruby) but with a little step in addition.
def reverse_complement(dna):
    # your code here
    reverse = {'A': 'T', 'T':'A', 'C':'G', 'G':'C'}
    r_c = ''
    for nucl in reversed(dna):
        if nucl not in reverse:
            return "Invalid sequence"
        else: r_c += reverse[nucl]
    return r_c
def reverse_complement(dna):
    # your code here
    reverse = {'A': 'T', 'T':'A', 'C':'G', 'G':'C'}
    r_c = ''
    for nucl in reversed(dna):
        if nucl not in reverse:
            return "Invalid sequence"
        else: r_c += reverse[nucl]
    return r_c