Question

directions: type your answer in the box.
a researcher sequenced an mrna strand and found the sequence 5 ucagga 3. what is the corresponding sequence of the dna strand from which the mrna was transcribed?
3
5

Explanation:

Step1: Recall base pairing rules

In transcription, mRNA is synthesized from a DNA template strand. The base pairing rules are: A (adenine) in DNA pairs with U (uracil) in mRNA, T (thymine) in DNA pairs with A (adenine) in mRNA, G (guanine) in DNA pairs with C (cytosine) in mRNA, and C (cytosine) in DNA pairs with G (guanine) in mRNA. Also, the direction of the DNA strand (template) will be antiparallel to the mRNA strand. The mRNA is \(5'\) UCAGGA \(3'\), so the DNA template strand will be \(3'\) to \(5'\), and we need to find the complementary sequence.

Step2: Determine each base in DNA

Let's go through each base of the mRNA sequence (\(5'\) U C A G G A \(3'\)) and find the corresponding DNA base:

• U (in mRNA) pairs with A (in DNA)
• C (in mRNA) pairs with G (in DNA)
• A (in mRNA) pairs with T (in DNA)
• G (in mRNA) pairs with C (in DNA)
• G (in mRNA) pairs with C (in DNA)
• A (in mRNA) pairs with T (in DNA)

So starting from the \(3'\) end (since DNA is \(3'\) to \(5'\)) corresponding to mRNA \(5'\) to \(3'\):
mRNA: \(5'\) U C A G G A \(3'\)
DNA: \(3'\) A G T C C T \(5'\)

Wait, let's check the direction again. The mRNA is \(5'\) to \(3'\), the DNA template (the one we are finding) is the non - coding strand? Wait, no. The mRNA is transcribed from the DNA template strand, so the DNA template strand has a sequence complementary to the mRNA, and antiparallel. So mRNA sequence: \(5'\) U C A G G A \(3'\). The DNA template strand (the one that was transcribed) will be \(3'\) to \(5'\), and its sequence is complementary to mRNA. So for each base in mRNA (from \(5'\) to \(3'\)), the DNA base (from \(3'\) to \(5'\)) is:

U (mRNA) → A (DNA)

C (mRNA) → G (DNA)

A (mRNA) → T (DNA)

G (mRNA) → C (DNA)

G (mRNA) → C (DNA)

A (mRNA) → T (DNA)

So putting it in order from \(3'\) to \(5'\): A G T C C T. Let's verify the base pairing. mRNA \(5'\) U C A G G A \(3'\) and DNA \(3'\) A G T C C T \(5'\). The base pairs are U - A, C - G, A - T, G - C, G - C, A - T. And the directions are antiparallel (mRNA \(5'\to3'\), DNA \(3'\to5'\)), which is correct for transcription.

Answer:

AGT CCT