Practicing Success

Target Exam

CUET

Subject

Biology

Chapter

Molecular Basis of Inheritance

Question:

Read the passage given below and answer the following 5 questions .
The process of copying genetic information from one strand of the DNA into RNA is termed as transcription. Here also, the principle of complementarity governs the process of transcription, except the adenosine complements now forms base pair with uracil instead of thymine. A transcription unit in DNA is defined primarily by the three regions in the DNA. There is a convention in defining the two strands of the DNA in the structural gene of a transcription unit. Since the two strands have opposite polarity and the strand that has the polarity 3'→5' acts as a template, and is also referred to as template strand. The other strand which has the polarity (5'→3') and the sequence same as RNA (except thymine at the place of uracil), is displaced during transcription. Strangely, this strand (which does not code for anything) is referred to as coding strand. The promoter and terminator flank the structural gene in a transcription unit. It is a DNA sequence that provides binding site for RNA polymerase, and it is the presence of a promoter in a transcription unit that also defines the template and coding strands.

The Template Srand of the DNA in the structural gene of a transcription unit is given below. What will be the Coding Strand for the given Template Strand? (Question 1)
3'-ATGCATGCATGCATGCATGCATGC-5'

Options:

5'-ACGTACGTACGTACGTACGTACGT-3'

5'-TACGTACGTACGTACGTACGTACG-3'

3'-TACGTACGTACGTACGTACGTACG-5'

5'-AUGCAUGCAUGCAUGCAUGCAU-3'

Correct Answer:

5'-TACGTACGTACGTACGTACGTACG-3'

Explanation:

The coding strand is the strand of DNA that has the same sequence as the mRNA (except for T/U substitution). To obtain the coding strand from the given template strand, you need to replace each nucleotide with its complementary base:

Given Template Strand: 3'-ATGCATGCATGCATGCATGCATGC-5'

Complementary bases: A (adenine) pairs with T (thymine)

T (thymine) pairs with A (adenine)

G (guanine) pairs with C (cytosine)

C (cytosine) pairs with G (guanine)

Applying these rules to the template strand, you get the Coding Strand: 5'-TACGTACGTACGTACGTACGTACG-3'

Therefore, the correct option is: 5'-TACGTACGTACGTACGTACGTACG-3'

Transcription is the first step of DNA based gene expression, wherein a segment (or whole) of a gene's DNA segment is copied (transcribed) into RNA, with the help of a group of enzymes called RNA Polymerases. RNA Polymerases use a single-stranded DNA template to synthesize a complementary strand of RNA. Specifically, RNA polymerase builds an RNA strand in the 5' to 3' direction, adding each new nucleotide to the 3' end of the strand.