Diagram of DNA double helix with complementary base pairing and nucleotide structure

Let’s look at the DNA strand shown. We see two strands connected by base pairs: A with T, G with C, and so on.

In DNA:
- Adenine (A) always pairs with Thymine (T)
- Guanine (G) always pairs with Cytosine (C)

Looking from top to bottom on the left strand, we have:

1. A → must pair with T (already shown on right)
2. T → must pair with A (already shown on right)
3. G → must pair with C (already shown on right)
4. C → must pair with G (already shown on right)
5. ? → this is blank on the left, but the right has a striped box — which matches the pattern of the second box from top (which was T paired with A). So if the right side has the same pattern as the T-A pair, then the left must be A.
6. ? → next one down, right side has dotted pattern — that matches the fourth box (C-G pair), so left must be G.

Wait — let’s double-check the patterns:

Top to bottom, left strand bases given: A, T, G, C, ?, ?

Right strand bases (from pairing rules):

- Opposite A → T
- Opposite T → A
- Opposite G → C
- Opposite C → G

Now for the fifth position: Right side shows a striped box — looking back, the second box (T on left) had a striped box on right? No — wait, let’s match the shading/patterns carefully.

Actually, in the diagram:

Left column (top to bottom):
Box 1: A (blue outline, light gray fill)
Box 2: T (blue outline, diagonal stripes)
Box 3: G (blue outline, dark gray)
Box 4: C (blue outline, dots)
Box 5: ??? (blue outline, diagonal stripes) ← same as Box 2’s right-side partner? Wait no — actually, each pair is shown together.

Better approach: Look at each horizontal pair.

Pair 1: Left = A, Right = ? → but it's drawn as matching shape — A fits into its complement. In standard DNA, A pairs with T. The right box here has a “notch” that fits A — that’s T’s shape.

Similarly:

Pair 2: Left = T, Right = ? → should be A. And indeed, the right box has a bump that fits T’s notch — that’s A.

Pair 3: Left = G, Right = ? → should be C. Shape matches.

Pair 4: Left = C, Right = ? → should be G. Shape matches.

Pair 5: Left = ???, Right = ??? → but we can see the right box has diagonal stripes — which matches Pair 2’s right box (which was A). So if right box is A, then left must be T.

Wait — no! Let’s reverse: In Pair 2, left was T, right was A (diagonal stripes). So if in Pair 5, the right box has diagonal stripes, that means it’s A → so left must be T.

But hold on — let’s list what’s visible:

From top:

Row 1: Left=A, Right= [shape that fits A] → that’s T
Row 2: Left=T, Right= [shape that fits T] → that’s A (and it has diagonal stripes)
Row 3: Left=G, Right= [fits G] → C
Row 4: Left=C, Right= [fits C] → G (dots)
Row 5: Left=?, Right= [diagonal stripes] → since diagonal stripes were used for A in Row 2, this right box is A → so left must be T
Row 6: Left=?, Right= [dots] → dots were used for G in Row 4 → so right is G → left must be C

So missing bases on left strand, row 5 and 6: T and C

Therefore, the two missing letters are T and C.

Final Answer:
T, C