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Q3:
If the sequence of one strand of DNA is written as follows: 5' -ATGCATGCATGCATGCATGCATGCATGC-3' Write down the sequence of complementary strand in 5'→3' direction.

Solution :

Step 1: Identifying the Polarity of the Given DNA Strand

The given sequence is provided as:
Template Strand ($S_1$): 5'- A T G C A T G C A T G C A T G C A T G C A T G C A T G C -3'
[Per the antiparallel nature of the DNA double helix, the complementary strand ($S_2$) must possess 3' to 5' orientation relative to $S_1$.]

Step 2: Applying Chargaff’s Base-Pairing Rules

According to the Watson-Crick base-pairing principle:

  • Adenine ($A$) pairs with Thymine ($T$) via two hydrogen bonds.
  • Thymine ($T$) pairs with Adenine ($A$) via two hydrogen bonds.
  • Guanine ($G$) pairs with Cytosine ($C$) via three hydrogen bonds.
  • Cytosine ($C$) pairs with Guanine ($G$) via three hydrogen bonds.
[Reference: Watson-Crick Model of DNA structure]

Step 3: Generating the Complementary Strand (3' to 5')

Aligning the nucleotides according to the pairing rules for the 3' to 5' strand:
Given 5': A T G C A T G C A T G C A T G C A T G C A T G C A T G C :3'
Comp 3': T A C G T A C G T A C G T A C G T A C G T A C G T A C G :5'

Step 4: Reorienting the Complementary Strand to 5' → 3'

The question requires the complementary strand to be written in the 5' → 3' direction. To achieve this, we reverse the sequence obtained in Step 3:
Original 3' → 5' sequence: $T-A-C-G-T-A-C-G-T-A-C-G-T-A-C-G-T-A-C-G-T-A-C-G-T-A-C-G$
Reversed 5' → 3' sequence: $G-C-A-T-G-C-A-T-G-C-A-T-G-C-A-T-G-C-A-T-G-C-A-T-G-C-A-T$

Visualization of Base Pairing

5' - A T G C A T G C A T G C A T G C A T G C A T G C A T G C - 3' 3' - T A C G T A C G T A C G T A C G T A C G T A C G T A C G - 5' Resulting 5' → 3' Strand: G C A T G C A T G C A T G C A T G C A T G C A T G C A T

Final Solution: The complementary strand in the 5' → 3' direction is: 5'- GCATGCATGCATGCATGCATGCATGCAT -3'


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