Question

if half of a dna molecule is acg-tcc-gta, the complementary half would be ______ . a cta-ctt-acc b tgc-agg-cat c acg-tcc-gta d ugc-agg-cau

Explanation:

Step1: Recall DNA base pairing rules

In DNA, adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G).

Step2: Analyze each segment of the given DNA

- For the first segment "ACG":
- A pairs with T, C pairs with G, G pairs with C. So "ACG" pairs with "TGC".
- For the second segment "TCC":
- T pairs with A, C pairs with G, C pairs with G. So "TCC" pairs with "AGG".
- For the third segment "GTA":
- G pairs with C, T pairs with A, A pairs with T. Wait, correction: G pairs with C, T pairs with A, A pairs with T? No, A pairs with T, T pairs with A, G pairs with C, C pairs with G. So "GTA": G (pairs with C), T (pairs with A), A (pairs with T)? Wait no, "GTA" is G - T - A. So G pairs with C, T pairs with A, A pairs with T? No, A pairs with T, T pairs with A, G pairs with C, C pairs with G. So "GTA" (G, T, A) will pair with C (for G), A (for T), T (for A)? Wait no, no: A pairs with T, T pairs with A, G pairs with C, C pairs with G. So:

- First base: G (in GTA) pairs with C? No, wait the original is "ACG - TCC - GTA", so the first part is ACG (A - C - G), second TCC (T - C - C), third GTA (G - T - A).

So for GTA (G - T - A):

- G pairs with C? No, wait no: in DNA, the base pairing is A - T and C - G. So the complementary base for G is C, for T is A, for A is T? Wait no, A (adenine) pairs with T (thymine), T (thymine) pairs with A (adenine), G (guanine) pairs with C (cytosine), C (cytosine) pairs with G (guanine).

So let's re - do:

- For "ACG" (A, C, G):

- A pairs with T, C pairs with G, G pairs with C. So complementary is T, G, C → TGC.

- For "TCC" (T, C, C):

- T pairs with A, C pairs with G, C pairs with G. So complementary is A, G, G → AGG.

- For "GTA" (G, T, A):

- G pairs with C? No, wait G pairs with C? No, G (guanine) pairs with C (cytosine), T (thymine) pairs with A (adenine), A (adenine) pairs with T (thymine). Wait, no: A (adenine) in DNA pairs with T (thymine), T (thymine) pairs with A (adenine), G (guanine) pairs with C (cytosine), C (cytosine) pairs with G (guanine). So "GTA" is G (guanine), T (thymine), A (adenine). So their complements:

- G → C? No, wait no: the complementary base for G is C? Wait no, when forming the complementary strand, if one strand has G, the other has C. If one has A, the other has T. If one has T, the other has A. If one has C, the other has G.

So "GTA":

- G (first base) → C? No, wait no: the original strand is "ACG - TCC - GTA", so we are finding the complementary strand. So the first strand is 5' - ACG - TCC - GTA - 3', the complementary strand is 3' - TGC - AGG - CAT - 5' (since DNA is antiparallel, but for base pairing, we just need to find the complementary bases). Wait, "GTA" is G - T - A. So G pairs with C? No, wait G (guanine) pairs with C (cytosine), T (thymine) pairs with A (adenine), A (adenine) pairs with T (thymine). Wait, no: A (adenine) pairs with T (thymine), T (thymine) pairs with A (adenine), G (guanine) pairs with C (cytosine), C (cytosine) pairs with G (guanine). So "GTA" (G, T, A) will have complementary bases C (for G), A (for T), T (for A)? No, that's not right. Wait, no: "GTA" is G - T - A. So:

- G (base 1) → C? No, wait no: the complementary base for G is C? Wait, no, let's take an example. If a DNA strand is ATG, the complementary strand is TAC. Because A - T, T - A, G - C. So ATG (A, T, G) → T (A's complement), A (T's complement), C (G's complement) → TAC.

So applying that to "GTA" (G, T, A):

- G's complement is C? No, wait G is guanine, its complement is cytosine (C). T is thymine, complement is adenine (A). A is adenine, complement is thymine (T). Wait, but in the example above, ATG (A, T, G) → T (A→T), A (T→A), C (G→C) → TAC. So "GTA" is G, T, A. So:

- G → C? No, wait G is the first base, so its complement is C? Wait no, in the example, G was the third base, and its complement was C. So yes, G→C, T→A, A→T. Wait, but then "GTA" (G, T, A) would have complement C, A, T → CAT? Wait, no: G (first base) → C? No, wait no, the direction: the original strand is 5' to 3', the complementary strand is 3' to 5', but for base pairing, we just need to match A - T and C - G. So let's write the original strand as a sequence: A C G T C C G T A.

Now, the complementary strand will have, for each base:

- A → T

- C → G

- G → C

- T → A

- C → G

- C → G

- G → C

- T → A

- A → T

Wait, no, the original is "ACG - TCC - GTA", which is A - C - G - T - C - C - G - T - A.

Now, let's pair each base:

- A (1st) → T

- C (2nd) → G

- G (3rd) → C

- T (4th) → A

- C (5th) → G

- C (6th) → G

- G (7th) → C

- T (8th) → A

- A (9th) → T

Wait, but that gives T - G - C - A - G - G - C - A - T, which is TGC - AGG - CAT (grouping into three - base codons: TGC, AGG, CAT).

Let's check the options:

Option A: CTA - CTT - ACC → does not match.

Option B: TGC - AGG - CAT → matches our calculation.

Option C: ACG - TCC - GTA → same as original, not complementary.

Option D: UGC - AGG - CAU → U is in RNA, not DNA, so incorrect.

Answer:

B. TGC - AGG - CAT