If you have a strand of DNA that is 21 nucleotides long, it will be transcribed by RNA polymerase into a strand of mRNA containing 21 nucleotides as well. Since each codon consists of 3 nucleotides, this strand will contain:
\[ \text{Number of codons} = \frac{\text{Total nucleotides}}{3} = \frac{21}{3} = 7 \text{ codons.} \]
If it begins with a start codon (AUG) and ends with a stop codon (which can be UAA, UAG, or UGA), this strand will be translated by ribosomes into a polymer, specifically a polypeptide (protein).
Considering the protein translation process, one of the codons is a start codon, and one is a stop codon, which means only 5 codons will be translated into amino acids (the start and end codons are not included in the final count of amino acids in the polymer). Therefore, the polymer will consist of:
\[ \text{Number of amino acids} = \text{Number of codons - 2} = 7 - 2 = 5 \text{ amino acids.} \]
In summary:
- The DNA will be transcribed by RNA polymerase into a strand containing 7 codons.
- This strand will be translated by ribosomes into a polymer that has 5 amino acids.