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 complementary to the DNA template strand. The base pairing rules are: A (adenine) pairs with U (uracil) in RNA, but in DNA, A pairs with T (thymine); C (cytosine) pairs with G (guanine); G pairs with C; U in RNA pairs with A in DNA. Also, the direction of the DNA strand should be opposite to the mRNA strand. The mRNA is \(5' - UCAGGA - 3'\), so the DNA template strand (from which mRNA is transcribed) will have a \(3' - 5'\) direction (since mRNA is synthesized in \(5' - 3'\) direction, and DNA template is read in \(3' - 5'\) direction).

Step2: Pair each 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 \(3' - 5'\)):

For mRNA sequence \(5' - U - C - A - G - G - A - 3'\), the DNA template strand ( \(3' - 5'\)) will be:

First base (corresponding to U in mRNA, position 1 of mRNA \(5'\) end) : U → A

Second base (C in mRNA) : C → G

Third base (A in mRNA) : A → T

Fourth base (G in mRNA) : G → C

Fifth base (G in mRNA) : G → C

Sixth base (A in mRNA) : A → T

Now, arrange them in \(3' - 5'\) direction. So the sequence is \(3' - AGTCCT - 5'\)? Wait, no, wait. Wait, the mRNA is \(5' - UCAGGA - 3'\), so let's write the mRNA bases in order: positions (from \(5'\) to \(3'\)): 1: U, 2: C, 3: A, 4: G, 5: G, 6: A.

The DNA template strand is complementary and antiparallel. So the DNA strand (template) has direction \(3' - 5'\), so the first base of DNA ( \(3'\) end) corresponds to the last base of mRNA ( \(3'\) end)? Wait, no. Wait, transcription: the DNA template strand is read in \(3' - 5'\) direction, and mRNA is synthesized in \(5' - 3'\) direction. So the mRNA sequence \(5' - UCAGGA - 3'\) means that the first nucleotide of mRNA ( \(5'\) end) is U, which is complementary to the last nucleotide of the DNA template strand ( \(3'\) end)? No, actually, the DNA template strand's \(3'\) end is adjacent to the mRNA's \(5'\) end. Wait, maybe a better way: the mRNA sequence is \(5' - U - C - A - G - G - A - 3'\). The DNA template strand (from which it is transcribed) will be complementary, so each base in mRNA is paired with its complementary base in DNA, and the direction of DNA is \(3' - 5'\) (opposite to mRNA's \(5' - 3'\)).

So let's reverse the mRNA sequence's direction to match the DNA's \(3' - 5'\) direction? No, no. Let's take each base in mRNA from \(5'\) to \(3'\) and find the complementary base in DNA, then write the DNA sequence in \(3' - 5'\) direction.

So:

mRNA: \(5'\) U C A G G A \(3'\)

DNA (template, \(3' - 5'\)): A G T C C T

Wait, let's check:

- U (mRNA) pairs with A (DNA)
- C (mRNA) pairs with G (DNA)
- A (mRNA) pairs with T (DNA)
- G (mRNA) pairs with C (DNA)
- G (mRNA) pairs with C (DNA)
- A (mRNA) pairs with T (DNA)

Now, arrange these DNA bases in the order from \(3'\) to \(5'\). So the first base ( \(3'\) end) is A, then G, then T, then C, then C, then T. So the DNA sequence is \(3' - AGTCCT - 5'\)? Wait, no, wait. Wait, the mRNA is \(5' - UCAGGA - 3'\), so the DNA template strand (the one being transcribed) has a sequence that is complementary and antiparallel. So the mRNA is \(5' - U - C - A - G - G - A - 3'\), so the DNA template strand ( \(3' - 5'\)) should be:

For each base in mRNA (from \(5'\) to \(3'\)):

U → A

C → G

A → T

G → C

G → C

A → T

Now, since the DNA is \(3' - 5'\), the sequence is written as \(3' - A G T C C T - 5'\)? Wait, no, let's write the bases in the order of \(3'\) to \(5'\). So the first base ( \(3'\) end) is A (complementary to U, which is the first base of mRNA \(5'\) end), then G (complementary to C), then T (complementary to A), then C (complementary to G), then C (complementary to G), then T (complementary to A). So the sequence is \(3' - AGTCCT - 5'\)? Wait, but let's check the direction again. The mRNA is \(5' - UCAGGA - 3'\), so the DNA template strand (the non - coding strand) that is transcribed has a \(3' - 5'\) direction, and the bases are complementary. So U (mRNA) ↔ A (DNA), C ↔ G, A ↔ T, G ↔ C, G ↔ C, A ↔ T. So putting it all together, the DNA sequence is \(3' - AGTCCT - 5'\)? Wait, no, wait, maybe I got the direction wrong. Let's think about the coding and template strands. The mRNA is complementary to the template DNA strand. The coding DNA strand (the one with the same sequence as mRNA, except T instead of U) has the same direction as mRNA ( \(5' - 3'\)), but the template strand is antiparallel. Wait, the question is about the DNA strand from which the mRNA was transcribed (the template strand). So mRNA is \(5' - UCAGGA - 3'\), template DNA strand is \(3' - AGTCCT - 5'\)? Wait, no, let's do it step by step.

Let's write the mRNA sequence as a string: "UCAGGA" (from \(5'\) to \(3'\)).

To find the DNA template strand ( \(3' - 5'\)):

We need to replace each U with A, C with G, A with T, G with C, and reverse the order? No, no. Wait, the mRNA is synthesized in the \(5' - 3'\) direction, using the DNA template strand which is read in the \(3' - 5'\) direction. So the first nucleotide of mRNA ( \(5'\) end) is complementary to the last nucleotide of the DNA template strand ( \(3'\) end)? No, that's not right. Let's take an example. Suppose DNA template strand is \(3' - TACG - 5'\), then mRNA is \(5' - AUGC - 3'\). So the DNA template strand ( \(3' - 5'\)): T (3' end) pairs with A (5' end of mRNA), A pairs with U, C pairs with G, G (5' end of DNA template? No, wait, DNA template is \(3' - TACG - 5'\)? No, DNA is always written with \(5' - 3'\) direction for the coding strand, but template is \(3' - 5'\). Wait, maybe a better approach: for each base in mRNA (from \(5'\) to \(3'\)), the corresponding base in the DNA template strand ( \(3' - 5'\)) is:

Position 1 ( \(5'\) end of mRNA): U → A (DNA)

Position 2: C → G (DNA)

Position 3: A → T (DNA)

Position 4: G → C (DNA)

Position 5: G → C (DNA)

Position 6 ( \(3'\) end of mRNA): A → T (DNA)

Now, since the DNA template strand is \(3' - 5'\), the first base we wrote (A) is at the \(3'\) end of the DNA, and the last base (T) is at the \(5'\) end of the DNA. So the DNA sequence is \(3' - A G T C C T - 5'\), which is \(3' - AGTCCT - 5'\). Wait, but let's check the base pairing again. U (mRNA) pairs with A (DNA), C with G, A with T, G with C, G with C, A with T. So the sequence is A (3' end), G, T, C, C, T (5' end). So the DNA strand is \(3' - AGTCCT - 5'\).

Answer:

\(3' - AGTCCT - 5'\) (or in the box, we can write "AGTCCT" since the box is between \(3'\) and \(5'\))