Questions: Part A The average length of a transcription unit along a eukaryotic DNA molecule is about 27,000 nucleotide pairs, whereas an averaged-sized protein is about 400 amino acids long. What is the best explanation for this fact? Each amino acid in a protein is encoded by a triplet of nucleotides. Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides that are not translated. Many genes are subject to alternative RNA splicing. Submit Request Answer

The answer is the second one: Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides that are not translated.

Explanation for each option:

1. Each amino acid in a protein is encoded by a triplet of nucleotides.
   This statement is true, as each amino acid is indeed encoded by a codon, which is a sequence of three nucleotides. However, this does not explain the discrepancy between the length of the transcription unit and the size of the protein. The triplet nature of the genetic code is a basic principle of translation but does not account for the large size difference.

2. Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides that are not translated.
   This is the correct explanation. Eukaryotic genes often contain introns, which are noncoding sequences that are transcribed into RNA but are spliced out before translation. This results in a much longer initial RNA transcript compared to the final mRNA that is translated into protein. The presence of these noncoding regions accounts for the large size of the transcription unit relative to the protein.

3. Many genes are subject to alternative RNA splicing.
   While alternative splicing is a process that can produce different protein variants from a single gene, it does not directly explain the average length of a transcription unit. Alternative splicing involves the inclusion or exclusion of certain exons, but the presence of introns and noncoding regions is the primary reason for the length discrepancy.

In summary, the presence of long noncoding stretches in eukaryotic genes is the best explanation for the difference in length between transcription units and the proteins they encode.