def DNA_to_mRNA_List(DNA_string):
    """Takes in DNA sequence string and converts it to an mRNA list.
    
    
    Parameters
    ----------
    DNA_string : string
        String that contains letters/characters of a DNA string, e.g., 'a,' t,' 'c,' and 'g.'
        
        
    Returns
    -------
    mRNA_List : list
        List that converts each and every value in input to corresponding mRNA values.
    """
    
    # Creates an empty list to which values will be appended
    mRNA_List = [];
    
    # Loops through each character of DNA string input
    for char in DNA_string:
        
        if char == 'a' or char == 'A': 
            # Characters are appended to mRNA_List 
            mRNA_List.append('U')
            
        elif char == 't' or char == 'T':
            mRNA_List.append('A')
            
        elif char == 'c' or char == 'C':
            mRNA_List.append('G')
            
        elif char == 'g' or char == 'G':
            mRNA_List.append('C')
            
    # Output mRNA_List is returned by function 
    return mRNA_List