Here's a problem to solve: A DNA strand contains codons, which consist of a sequence of three nucleotides. Which best describes the importance of codons found in the sequence of DNA?

Among the 4 following options, which is the correct answer?

- A: They contain the code for messenger RNA.
 
- B: They signal the start of mitotic cell division.
 
- C: They ensure proper replication of the genetic code.
 
- D: They are used to assemble amino acids for protein synthesis.
 