Given below is one of the strands of a DNA segment :

(a) Write its complementary strand.

(b) Write a possible RNA strand that can be transcribed from the above DNA molecule formed.

Correct Answer:
(a) The complementary DNA strand is: 5'-ATGCATGCATGCATGC-3'
(b) The transcribed RNA strand is: 5'-AUGCAUGCAUGCAUGC-3'

Step-by-Step Explanation:

1. Analysis of the Given DNA Strand (from Image 0):
From the first image (image_0.png), the sequence of the given single strand of DNA is:
3'-TACGTACGTACGTACG-5'
Here, the polarity of this template strand runs from the 3' end to the 5' end, with a rightward arrow indicating the direction of replication/transcription underneath the first repeating unit.

2. Part (a): Writing the Complementary DNA Strand (from Image 1):
According to Watson-Crick base-pairing rules for DNA:
• Adenine (A) pairs with Thymine (T) and vice-versa.
• Cytosine (C) pairs with Guanine (G) and vice-versa.
• The complementary strand runs in an antiparallel orientation (5' to 3' direction).

By matching each base in the template strand (3'-TACGTACGTACGTACG-5'):
• T pairs with A
• A pairs with T
• C pairs with G
• G pairs with C

Replicating this pattern across the entire sequence yields the complementary strand shown in the second image (image_1.png):
5'-ATGCATGCATGCATGC-3' (allocated as polarity = ¹/₂ mark, nucleotide sequence = ¹/₂ mark).

3. Part (b): Writing the Transcribed RNA Strand (from Image 2):
During transcription, the template DNA strand (3'-TACGTACGTACGTACG-5') is read by RNA polymerase to synthesize a complementary RNA strand running in the 5' to 3' direction.
The base-pairing rules for RNA synthesis are identical to DNA, except that Uracil (U) is incorporated in RNA instead of Thymine (T) to pair with Adenine (A):
• Thymine (T) on the DNA template pairs with Adenine (A) in the RNA.
• Adenine (A) on the DNA template pairs with Uracil (U) in the RNA.
• Cytosine (C) on the DNA template pairs with Guanine (G) in the RNA.
• Guanine (G) on the DNA template pairs with Cytosine (C) in the RNA.

Applying these transcription rules to the template DNA strand (3'-TACGTACGTACGTACG-5'):
• T → A
• A → U
• C → G
• G → C

This yields the transcribed RNA sequence shown in the third image (image_2.png):
5'-AUGCAUGCAUGCAUGC-3' (allocated as polarity = ¹/₂ mark, nucleotide sequence = ¹/₂ mark).