Given below is one of the strands of a DNA segment : 3′ CGTACGTACGTACGTA 5′ (a) Write its complementary strand

Question

Q. Given below is one of the strands of a DNA segment : 3' CGTACGTACGTACGTA 5'

(a) Write its complementary strand.

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

CBSECBSE Class XII Board 2020Subjective· 2mImportance★★★★★

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Concept understanding — Palindromic DNA Sequences

Imagine you are looking in a mirror. Your right hand becomes the reflection’s left hand, and your left becomes its right. The word “MALAYALAM” reads the same forwards and backwards. That mirror-like symmetry is the core idea behind a palindromic DNA sequence.

In everyday language, a palindrome is a word, phrase, or number that reads identically in both directions. DNA is a long, double-stranded molecule — think of it as a twisted ladder. Each rung of the ladder is made of two chemical letters (called bases) that pair up in a very specific way: A always pairs with T, and C always pairs with G.

Now, a palindromic DNA sequence is a stretch of DNA where the sequence of letters on one strand reads exactly the same as the sequence on the opposite strand, but in the opposite direction. Because the two strands run in opposite directions (biologists call this “antiparallel”), the palindrome is not just a simple mirror of letters — it is a mirror of the pairing.

Note

In textbooks, you will often see a palindromic sequence written like this:

5' – GAATTC – 3'

3' – CTTAAG – 5'

Notice that if you read the top strand left to right (GAATTC) and then read the bottom strand right to left (also GAATTC), you get the same sequence. That is the palindrome.

Why does this matter? Because nature uses these sequences as recognition sites. Special proteins — especially restriction enzymes — are designed to find these exact palindromic stretches and cut the DNA at that precise spot. This is the foundation of genetic engineering:

Restriction enzymes act like molecular scissors. They only cut at their specific palindromic sequence.
Because the sequence is the same on both strands, the cut produces either “blunt” ends or “sticky” ends (short, single-stranded overhangs). Sticky ends are particularly useful because they can easily join with another piece of DNA that has the complementary sticky end — like two puzzle pieces.
This allows scientists to cut DNA from one organism and paste it into the DNA of another, creating recombinant DNA.

Important

The NCERT textbook (Class 12 Biology, Chapter 11) explicitly states: “Restriction enzymes cut the strand of DNA a little away from the centre of the palindromic site, but between the same two bases on the opposite strands.” This means the cut is not perfectly in the middle — it is offset, which creates the sticky ends. …

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The complementary DNA strand runs antiparallel with A pairing to T and C pairing to G; RNA transcription replaces T with U and reads the template strand in the 3′ to 5′ direction.

DNA exists as a double helix, two strands wound around each other in opposite directions—what we call antiparallel orientation. One strand runs 5′ to 3′, the other 3′ to 5′. The strands are held together by hydrogen bonds between nitrogenous bases, following strict pairing rules: adenine (A) always pairs with thymine (T), and cytosine (C) always pairs with guanine (G). This complementary base pairing is the foundation of DNA replication and transcription.

When you are given one strand of DNA, you can immediately deduce the other by applying these pairing rules and remembering that the strands run in opposite directions. The given strand reads 3′ CGTACGTACGTACGTA 5′. Notice the repeating CGTA motif—this is a palindromic sequence, meaning it reads the same in both directions when you account for complementarity. Such sequences are common recognition sites for restriction enzymes.

(a) The Complementary DNA Strand

To construct the complementary strand, pair each base according to the rules and write the new strand in the opposite direction:

C pairs with G
G pairs with C
T pairs with A
A pairs with T

Starting from the 5′ end of the new strand (which corresponds to the 3′ end of the original), the complementary strand is:

5′ GCATGCATGCATGCAT 3′

The two strands now sit antiparallel to each other, forming the characteristic double helix.

Note

The 5′ and 3′ labels refer to the carbon atoms in the sugar (deoxyribose) backbone. The 5′ end has a free phosphate group on the fifth carbon, while the 3′ end has a free hydroxyl group on the third carbon. This directionality matters enormously in replication and transcription.

(b) A Possible RNA Strand

Transcription is the process by which RNA polymerase synthesizes an RNA molecule using one DNA strand as a template. The enzyme reads the template strand in the 3′ to 5′ direction and builds the RNA strand in the 5′ to 3′ direction. RNA differs from DNA in two key ways: it uses uracil (U) instead of thymine (T), and its sugar is ribose rather than deoxyribose. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2026Set ANNUAL1 mark
$Q. The sequence of base pairs in a double-stranded DNA or RNA molecule that reads the same in forwards and backward on each strand is called _________ nucleotide sequence.$
›Reveal solutionSolution
The blank is palindromic — a base-pair sequence that reads the same in the 5'→3' direction on both strands, which is the recognition site for restriction endonucleases.

In the CBSE/NCERT Biotechnology: Principles and Processes chapter, restriction enzymes cut DNA at specific palindromic sequences. For example:

5' G A A T T C 3' 3' C T T A A G 5'

…
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CBSE 2019Set HE1 mark
$Q. Match the following: Column A item 'Palindromic' with the correct term from Column B:$
(1) Big-Bang
(2) D.N.A.
(3) Tansley
(4) Green revolution
(5) Male-sterility.
›Reveal solutionSolution

'Palindromic' matches with 'D.N.A.' — palindromic sequences are specific nucleotide sequences found within DNA.

A palindromic sequence in DNA is a sequence of base pairs that reads the same in the 5′→3′ direction on both strands (e.g., GAATTC / CTTAAG). Restriction endonucleases recognise such short palindromic sequences within a DNA molecule and cleave the DNA at or near them, which is fundamental …

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CBSE 2017Set ANNUAL1 mark
$Q. Sequences of base pairs in DNA that reads the same on both the strands when the orientation of reading is kept the same are called ________ sequences.$
›Reveal solutionSolution

A DNA sequence that reads identically on both strands (in the same 5'→3' direction) is called a palindromic sequence — the sites recognised and cut by restriction endonucleases.

In ordinary usage a palindrome is a word/phrase that reads the same forwards and backwards (e.g. 'MALAYALAM'). In molecular biology, a palindromic sequence in double-stranded DNA is a base-pair sequence that reads the same on both strands when each is read in the 5'→3' direction — for example:

5'-GAATTC-3'

3'-CTTAAG-5'

…

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Q.Given below is one of the strands of a DNA segment : 3′ CGTACGTACGTACGTA 5′

(a) The Complementary DNA Strand

(b) A Possible RNA Strand

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