'''
In this exercise you will implement a basic version of CRISPR-Cas9.

Cas9 protein contains a small piece of RNA called guide RNA (gRNA).
This gRNA is used to scan a DNA sequence. If DNA sequence complementary to 
gRNA is found AND if there is a small protospace adjacent motif (PAM) 
immediately following the matched sequence, Cas9 will cleave the DNA 
4 nucleotides before the PAM.

Replicate this behavior in Python code.

We will assume that the PAM sequence is NGG, where N stands for any 
nucleotide (A, C, T, or G). The last two nucleotides are G (guanine).

Inputs:
 - The DNA sequence to examine
 - The gRNA sequence to match against the DNA
 
Both sequences will be given in the 5' to 3' direction, the RNA is the 
complement to the DNA.

Output:
  - A list containing the sequence after it has been examined by Cas9

E.g., if a sequence was originally 'ATTGGCC' and cleaved after ATT, the list 
looks like this: ['ATT', 'GGCC'].

Notes/Hints:
- Start by writing pseudocode on a piece of paper. Only once you're done with 
pseudocode should you start working on Python code in VS Code.
- The input sequence may have more than 1 cleavage site, or no sites.
- The input sequence and guide RNA are reverse complements.
- There may be matches with the guide RNA, but not containing the PAM. These 
should not be cleaved.
- You may find it useful to comment out the given DNA and RNA inputs and use 
your own "toy examples" while working on the code to test whether pieces of 
your code work.
- `replace` function may be useful (see more at: 
https://www.w3schools.com/python/ref_string_replace.asp)
'''

DNA_seq = "CTTTACGACAAGAGGTACGCTTTGTGAGCGCGCTCCGCCGGCCCTCGCGGACCCTTGCTACAAGCT\
    CGACAGGATGTCTCCTCAGCATGTCCTACGTTGGAGGACCGCAACAGCGCCTGGCGGTGCGCACACAGCGGTC\
    GACCGATTTGCGGAGTCCCAGGCCCGGCCGATCCCGTCCCGTACCCTGGTGGACCGTCTAGGGGTCACCTGCC\
    CCTCCGCGATGATTAGATTGCCAGCCCGTAGCGTTTGCGGGTCGCCTACGCGGTCTCTCGGCTGCGCCGGGGA\
    TTCGAGCTAGGCCTTTGTAGGCCTGCCGCTGCACAAGGGCCAAAACTCTAACCCTCCCGTTAGCGGGGCGTAC\
    CTGGGAGAGGCCATGCCCACGAATGCGCGCCCCGCGGTTGCCTTTTGGGGGCGGGGTCGGCCCTCACGCCGGG\
    GCCCCGTCGGCGCGTCTCTGCGACAAAGGACACCGCGCTGAGCCAAAAGGCTGGCGAGGAACCCGGGAAAAGG\
    GCCCTGGCCCGGTACAGCCCGGGGCGGGGGCTACCCGGGCCTGGAGACCAAATTCGACCACGACCTAGGCCCG\
    ACGCGCGGGGCCCATGGCGTAAGCCACAGCAGCTAGAACGCCTTCTCGATAGGCCCACGAATCTGATCGTTGC\
    TATAATAGACGACCTGGCCACCTGGGGAGAATTGAGTTGTTCATGCGCCCCCGGGAAAAGGGCCCTGGCCGTG\
    TACACTCCGTCTAGCCCGAACCGCGAGCCCCACACCCGACCTCGACATGGCCTGGTTGGGTGGGCGGGCGACG\
    GGCGTAGGCCGACCACCGCCGTGGCTGAGACGGCCCCAGAAGAACGTCGCGGGTCAGGCGTTTGAGTGTAACG\
    AGGAACGCTCTTGGTCCACTTGGGGGTTCCCAACAGGTCCAGCGCCCGAGGTGTCGAGATACTGGGGCGCGCG\
    TGGTTGCGGCTACTGGTATATGGATGCACTGGGAGTCCTCTTTGGCCCGGAATGGCTGCACAGCTAACTGGGG\
    GCTCTTGCCCCCATCGGAGCGCGGGAACGCGCCACCGCTGGCA"

gRNA_seq = "GGGCCCUUUUCCCGGG"

# Your code here