Question

If one strand of a DNA has the sequence ATGCTTCA, sequence of the bases in complementary strand is:

• A. CATTAGCT
• B. TACGAAGT
• C. GTACTTAC
• D. ATGCGACT

Answer 1

The Correct Option is B

The problem asks for the sequence of the complementary strand of a DNA molecule, given the sequence of one strand as ATGCTTCA.

Concept Used:

The structure of a DNA molecule is a double helix, where two strands are held together by hydrogen bonds between complementary base pairs. According to the principle of complementary base pairing (Chargaff's rules):

• Adenine (A) always pairs with Thymine (T).
• Guanine (G) always pairs with Cytosine (C).

Therefore, to find the sequence of the complementary strand, we replace each base in the given sequence with its corresponding complementary base.

Step-by-Step Solution:

Step 1: Write down the sequence of the given DNA strand.

\[ \text{Given Strand: } 5'\text{-ATGCTTCA-}3' \]

Step 2: Apply the complementary base pairing rules to each base in the sequence.

• The complement of Adenine (A) is Thymine (T).
• The complement of Thymine (T) is Adenine (A).
• The complement of Guanine (G) is Cytosine (C).
• The complement of Cytosine (C) is Guanine (G).

Step 3: Determine the complementary base for each position in the given sequence.

• For A at position 1, the complement is T.
• For T at position 2, the complement is A.
• For G at position 3, the complement is C.
• For C at position 4, the complement is G.
• For T at position 5, the complement is A.
• For T at position 6, the complement is A.
• For C at position 7, the complement is G.
• For A at position 8, the complement is T.

Step 4: Assemble the complementary bases in order to form the sequence of the new strand.

Combining the complementary bases from Step 3, we get the sequence of the complementary strand. The two strands in DNA are antiparallel, so if the given strand is in the 5' to 3' direction, the complementary strand will be in the 3' to 5' direction.

\[ \begin{array}{rc} \text{Given Strand:} & 5' - \text{A T G C T T C A} - 3' \\ & \quad \ ||\ ||\ ||\ ||\ ||\ ||\ ||\ || \\ \text{Complementary Strand:} & 3' - \text{T A C G A A G T} - 5' \end{array} \]

Final Result:

The sequence of the bases in the complementary strand is TACGAAGT.

Answer 2

DNA base pairing rules state that adenine (A) pairs with thymine (T) via 2 hydrogen bonds, and cytosine (C) pairs with guanine (G) via 3 hydrogen bonds. Therefore, for the given sequence ATGCTTCA, the complementary sequence is determined as follows:

Original Strand: A T G C T T C A

Complementary Strand: T A C G A A G T

This follows from the complementary base pairing rules.