Normal DNA produces healthy round red blood cells, while sickle cell DNA results in crescent-shaped sickle cells.

Explanation:
We are comparing two DNA sequences: one from a normal person and one from someone with sickle cell disease.

Let’s look at the DNA bases (top row) and the corresponding mRNA codons (middle row, shown as colored shapes), then infer the amino acids (bottom circles), and finally see how the blood cell shape changes.

Step 1: Identify the DNA sequence difference.
- Normal DNA: G G A C T T C T T
- Sickle Cell DNA: G G A C A T C T T

Only the 5th and 6th bases differ:
- Normal: T T
- Sickle: A T

So the change is at position 5–6: TT → AT in the DNA.

Step 2: Remember that mRNA is made from the DNA template strand — but here, the top row appears to be the *coding* (non-template) strand, because the mRNA matches it except T→U.
Indeed, in both cases, the mRNA shows U where DNA has T — so yes, the top DNA strand is the coding strand, and mRNA is directly transcribed (T → U).

So for the normal DNA:
- DNA triplet 1: GGA → mRNA: GGU
- DNA triplet 2: CTT → mRNA: CUU
- DNA triplet 3: CTT → mRNA: CUU

For sickle cell DNA:
- DNA triplet 1: GGA → mRNA: GGU
- DNA triplet 2: CAT → mRNA: CAU
- DNA triplet 3: CTT → mRNA: CUU

So only the second codon changes:
- Normal mRNA codon: CUU
- Sickle mRNA codon: CAU

Step 3: Use the genetic code to find the amino acids.

Recall:
- CUU → Leucine (Leu)
- CAU → Histidine (His)

So the normal hemoglobin has Leucine at that position, but sickle cell hemoglobin has Histidine instead.

This single amino acid change causes hemoglobin to stick together under low oxygen, deforming red blood cells into a sickle shape.

The question likely asks: *What amino acid replaces leucine in sickle cell hemoglobin?*
Or possibly: *What is the amino acid in the mutated position?*

From the diagram, the three circles (amino acids) correspond to the three codons. The middle circle changes.

Normal: Leu
Sickle: His

So the answer is Histidine.

Let me double-check the codons:
- DNA CTT → mRNA CUU → Leu ✔
- DNA CAT → mRNA CAU → His ✔
Yes — correct.

Final Answer:
Histidine