What is a complementary strand?

DNA (deoxyribonucleic acid) is made up of pentose Sugar, nitrogenous bases, and phosphate. DNA has a backbone of sugar and phosphate. As we all know DNA is a double helical structure (Watson & crick model) and both the strand of DNA are complementary of each other.

So, what does it mean to be a complementary strand (cDNA)?

Suppose we have a DNA strand 5’ GGTACTTGCCAT 3’ for this strand of DNA the complementary strand is 3’ CCATGAACGGTA 5’. This is because we all know that there are 4 base pairs in DNA: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T), these base pairs bonds together these bonds follow the lock and key principle.
in DNA A=T and G≡C (where A, G are purines and T, C are pyrimidines)

In the process of DNA Replication, both of the strands are unzipped by an enzyme named helicase and both of them serve as a template strand for new DNA (semiconservative nature of DNA).

IUPAC Degeneracies

Base	Name	Bases Represented	Complementary Base
A	Adenine	A	T
T	Thymidine	T	A
U	Uridine (in RNA)	U	A
G	Guanidine	G	C
C	Cytidine	C	G
Y	pyrimidine	C T	R
R	purine	A G	Y

 

Let’s see the methods for solving this:
Method 1: using the concept of if-elif-else conditional statements.
Below is the implementation:

# method 1:
# Define a function for finding
# complementary strand of given
# DNA strand
def complementary_strand_find(dna_strand):

    # empty string define
    complementary_strand = ""

    # looping through the
    # given DNA strand one
    # by one character at
    # a time 
    for base in dna_strand:

        # using if-elif-else
        # conditional statement
        if base == "A" :
            # string concatenation
            complementary_strand += "T"

        elif base == "T" :
            complementary_strand += "A"

        elif base == "U" :
            complementary_strand += "A"

        elif base == "G" :
            complementary_strand += "C"

        elif base == "C" :
            complementary_strand += "G"

        elif base == "Y" :
            complementary_strand += "R"

        elif base == "R" :
            complementary_strand += "Y"

        else :
            print("Wrong input")

            # assigning None to the variable
            # if wrong input is given 
            complementary_strand = None
            
            # break out of the loop
            # if wrong input is given
            break

    # return final result
    return complementary_strand

# main code
if __name__ == "__main__":

    # input the DNA strand
    dna_strand = "GGTACTTGCCAT"

    print("DNA strand is:",
          dna_strand)

    # function call and printing
    # the result
    print("complementary strand is:",
          complementary_strand_find(dna_strand))

Output:

DNA strand is: GGTACTTGCCAT
complementary strand is: CCATGAACGGTA

Method 2: Using the concept of Dictionary.
Below is the implementation:

# Define a function for finding
# complementary strand of given
# DNA strand
def complementary_strand_find(dna_strand):

    # create a dictionary for
    # storing conversion pairs
    conversion_dictionary = {
        "A" : "T",
        "T" : "A",
        "U" : "A",
        "G" : "C",
        "C" : "G",
        "Y" : "R",
        "R" : "Y"
        }

    # empty string define
    complementary_strand = ""

    # looping through the
    # given DNA strand one
    # by one character at
    # a time    
    for base in dna_strand:

        # checking base is present in
        # dictionary or not
        # if not then error input
        # is given
        if base in conversion_dictionary:
            
            # string concatenation
            complementary_strand += conversion_dictionary[base]
            
        else :
            print("Wrong input")

            # assigning None to the variable
            # if wrong input is given 
            complementary_strand = None
            
            # break out of the loop
            # if wrong input is given
            break

    # return final result
    return complementary_strand
     
# main code
if __name__ == "__main__":

    # input the DNA strand
    dna_strand = "GGTACTTGCCAT"

    print("DNA strand is:",
          dna_strand)

    # function call and printing
    # the result
    print("complementary strand is:",
          complementary_strand_find(dna_strand))

Output:

DNA strand is: GGTACTTGCCAT
complementary strand is: CCATGAACGGTA