a2

def get_length(dna):
    """ (str) -> int

    Return the length of the DNA sequence dna.

    >>> get_length('ATCGAT')
    6
    >>> get_length('ATCG')
    4
    """
    return len(dna)

def is_longer(dna1, dna2):
    """ (str, str) -> bool

    Return True if and only if DNA sequence dna1 is longer than DNA sequence
    dna2.

    >>> is_longer('ATCG', 'AT')
    True
    >>> is_longer('ATCG', 'ATCGGA')
    False
    """
    return len(dna1) > len(dna2)

def count_nucleotides(dna, nucleotide):
    """ (str, str) -> int

    Return the number of occurrences of nucleotide in the DNA sequence dna.

    >>> count_nucleotides('ATCGGC', 'G')
    2
    >>> count_nucleotides('ATCTA', 'G')
    0
    """
    return dna.count(nucleotide)

def contains_sequence(dna1, dna2):
    """ (str, str) -> bool

    Return True if and only if DNA sequence dna2 occurs in the DNA sequence
    dna1.

    >>> contains_sequence('ATCGGC', 'GG')
    True
    >>> contains_sequence('ATCGGC', 'GT')
    False

    """
    return dna1.count(dna2) > 0

def is_valid_sequence(dna):
    """
    (str) -> bool

    Return True if and only if the DNA sequence is valid

    >>> is_valid_sequence('ACCGT')
    True
    >>> is_valid_sequence('ACCDT')
    Fale

    """
    for nucleotide in dna:
        if nucleotide not in "ATCG":
            return False
    return True

def insert_sequence(dna1, dna2, loc):
    """
    (str, str, int) -> str

    Return the DNA sequence obtained by inserting the second DNA sequence into
    the first DNA sequence at the given index.
    precondition: You can assume that the index is valid.

    >>> insert_sequence('ATGC', 'GG', 2)
    'ATGGGC'
    >>> insert_sequence('ATGCCGTA', 'CCGGAA', 5)
    'ATGCCCCGGAAGTA'

    """
    return dna1[0:loc] + dna2 + dna1[loc:]

def get_complement(nucleotide):
    """
    (str) -> str

    Return the nucleotide's complement.

    >>> get_complement('A')
    'T'
    >>> get_complement('a')
    'a'

    """
    if nucleotide == 'A':
      return 'T'
    elif nucleotide == 'T':
      return 'A'
    elif nucleotide == 'G':
      return 'C'
    elif nucleotide == 'C':
      return 'G'
    return nucleotide

def get_complementary_sequence(dna):
    """
    (str) -> str

    Return the DNA sequence that is complementary to the given DNA sequence.

    >>> get_complementary_sequence('TACG')
    'ATGC'
    >>> get_complementary_sequence('AtGD')
    'TtCD'

    """
    sequence = ''
    for nucleotide in dna:
        sequence += get_complement(nucleotide)
    return sequence