Question

1 the diagram below shows a portion of a dna molecule. which sequence of bases do the question marks repres diagram of dna base pairing with g g a t a c c t g on top, c c t a t g ? ? ? on bottom, and options a) cac, b) gca, c) gac, d) tca

Explanation:

Step1: Recall DNA base pairing

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

Step2: Analyze the given DNA strands

Top strand bases: G, G, A, T, A, C, C, T, G
Bottom strand (known) up to the 6th base: C, C, T, A, T, G
Now, for the 7th base (top is C), so bottom should be G (since C pairs with G).
For the 8th base (top is T), so bottom should be A (since T pairs with A)? Wait, no, wait the top strand's 7th base is C (wait, top strand: positions 1:G, 2:G, 3:A, 4:T, 5:A, 6:C, 7:C, 8:T, 9:G. Wait bottom strand: 1:C, 2:C, 3:T, 4:A, 5:T, 6:G, 7:?, 8:?, 9:?.

Wait, let's list each position:
Position 1: Top G, Bottom C (C-G pair)
Position 2: Top G, Bottom C (C-G pair)
Position 3: Top A, Bottom T (A-T pair)
Position 4: Top T, Bottom A (T-A pair)
Position 5: Top A, Bottom T (A-T pair)
Position 6: Top C, Bottom G (C-G pair)
Position 7: Top C, Bottom? → C pairs with G, so bottom should be G? Wait no, wait top strand position 7 is C? Wait top strand: G G A T A C C T G → so position 7 is C? Wait no, let's count the top strand: first base G (1), G (2), A (3), T (4), A (5), C (6), C (7), T (8), G (9). So bottom strand position 7: top is C, so bottom is G? Wait no, wait the bottom strand's 6th base is G (matches top C at 6). Then position 7: top is C (position 7), so bottom should be G? Wait no, wait the top strand's 7th base is C? Wait no, top strand: G (1), G (2), A (3), T (4), A (5), C (6), C (7), T (8), G (9). So bottom strand:
1: C (pairs with G)
2: C (pairs with G)
3: T (pairs with A)
4: A (pairs with T)
5: T (pairs with A)
6: G (pairs with C)
7: Top is C (position 7), so bottom is G? Wait no, wait the options: let's check the options. Wait maybe I made a mistake. Wait the top strand's 7th base is C? Wait no, top strand: G G A T A C C T G → so positions: 1:G, 2:G, 3:A, 4:T, 5:A, 6:C, 7:C, 8:T, 9:G. So bottom strand:
1: C (G-C)
2: C (G-C)
3: T (A-T)
4: A (T-A)
5: T (A-T)
6: G (C-G)
7: Top is C (position 7), so bottom is G? Wait no, the options are A) CAC, B) GCA, C) GAC, D) TCA. Wait maybe I messed up the direction. Wait DNA is antiparallel, but the base pairing is A-T, C-G. Let's re-express:

Top strand (template): G G A T A C C T G
Bottom strand (complementary): C C T A T G? ? ?

Wait position 7: top is C (7th base: C), so bottom should be G (C-G). Position 8: top is T, so bottom is A (T-A)? No, T pairs with A. Wait position 8: top T, bottom A? But option A is CAC, B is GCA, C is GAC, D is TCA. Wait maybe I got the top and bottom reversed. Wait maybe the top is the complementary and bottom is the template? No, standard DNA double helix: each strand is complementary. Wait let's check the pairs again.

Wait the first six bases:
Top: G G A T A C
Bottom: C C T A T G

Yes, G-C, G-C, A-T, T-A, A-T, C-G. Perfect. Now 7th base top: C, so bottom: G (C-G). 8th base top: T, so bottom: A (T-A)? No, T pairs with A. Wait 8th base top: T, so bottom A. 9th base top: G, so bottom C (G-C). Wait but that would be G A C? Wait no, 7th: G, 8th: A, 9th: C? But option C is GAC. Wait 7th: G, 8th: A, 9th: C → GAC? Wait no, 7th base bottom: G, 8th: A, 9th: C? Wait top 7: C, bottom G; top 8: T, bottom A; top 9: G, bottom C. So bottom 7: G, 8: A, 9: C → GAC? Wait option C is GAC. Wait but let's check again.

Wait top strand: G (1), G (2), A (3), T (4), A (5), C (6), C (7), T (8), G (9)
Bottom strand: C (1), C (2), T (3), A (4), T (5), G (6),?, ?, ?

So position 7: top C, bottom G (C-G)
Position 8: top T, bottom A (T-A)
Position 9: top G, bottom C (G-C)

So the bottom strand's 7th, 8th, 9th bases: G, A, C → GAC. Which is option…

Answer:

C. GAC