Genetic Code and Sickle Cell Anemia

Introduction:
Sickle cell anemia is a genetic disorder that affects the structure and function of red blood cells. It is caused by a mutation in the gene that codes for a protein called hemoglobin, which is responsible for carrying oxygen in the blood. In sickle cell anemia, a single nucleotide change in the DNA sequence results in the substitution of glutamic acid with valine in the hemoglobin protein.

Genetic Code:
The genetic code is the set of rules by which information encoded in DNA or RNA is translated into proteins. It is based on the sequence of nucleotides in the DNA or RNA molecule, where three nucleotides, called codons, encode for a specific amino acid.

Codon for Valine:
The codon for valine is GUG. Each codon is composed of three nucleotides, and each nucleotide can be one of four possible bases: adenine (A), guanine (G), cytosine (C), or thymine (T). In the case of RNA, thymine is replaced by uracil (U).

Explanation:
In the given question, we are asked to identify the codon that codes for valine, which replaces glutamic acid in sickle cell anemia.

- Option A: AAG (adenine-adenine-guanine) does not code for valine.
- Option B: GAA (guanine-adenine-adenine) does not code for valine.
- Option C: GGG (guanine-guanine-guanine) does not code for valine.
- Option D: GUG (guanine-uracil-guanine) is the correct codon that codes for valine.

Therefore, option D (GUG) is the correct answer.

Summary:
In sickle cell anemia, the amino acid glutamic acid is replaced by valine due to a mutation in the genetic code. The codon GUG codes for valine, and it is the correct answer in this case.