The Ribonuclease Gene Human pancreatic ribonuclease has 128...

The Ribonuclease Gene Human pancreatic ribonuclease has 128 amino acid residues. (a) What is the minimum number of nucleotide pairs required to code for this protein? (b) The mRNA expressed in human pancreatic cells was copied with reverse transcriptase to create a "library" of human DNA. The sequence of the mRNA coding for human pancreatic ribonuclease was determined by sequencing the complementary DNA (cDNA) from this library that included an open reading frame for the protein. Use the nucleotide database at NCBI (www.ncbi.nlm.nih.gov/nucleotide) to find the published sequence of this mRNA. (Search for accession number D26129.) What is the length of this mRNA? (c) How can you account for the discrepancy between the size you calculated in (a) and the actual length of the mRNA?

The Ribonuclease Gene Human pancreatic ribonuclease has 128 amino acid residues.
(a) What is the minimum number of nucleotide pairs required to code for this protein?
(b) The mRNA expressed in human pancreatic cells was copied with reverse transcriptase to create a "library" of human DNA. The sequence of the mRNA coding for human pancreatic ribonuclease was determined by sequencing the complementary DNA (cDNA) from this library that included an open reading frame for the protein. Use the nucleotide database at NCBI (www.ncbi.nlm.nih.gov/nucleotide) to find the published sequence of this mRNA. (Search for accession number D26129.) What is the length of this mRNA?
(c) How can you account for the discrepancy between the size you calculated in (a) and the actual length of the mRNA?

Key Concepts

Genetic Code and Codons

The genetic code is the set of rules by which information encoded in nucleotide sequences is translated into proteins. In this code, each set of three nucleotides, known as a codon, corresponds to a specific amino acid. Since proteins are made of sequences of amino acids, knowing the number of amino acids allows us to calculate the minimum number of codons—and therefore the minimum number of nucleotides—required for protein synthesis.

Triplet Nature of the Codon

Because codons are composed of three nucleotides, translating a sequence of amino acids into a nucleotide sequence involves multiplying the number of amino acids by three. This triplet nature forms the basis of calculating the minimal coding sequence length; however, additional nucleotides may be present to signal the start and stop of translation.

Open Reading Frame (ORF)

An open reading frame is a continuous stretch of codons that begins with a start codon and ends with a stop codon. It represents the portion of the mRNA that is actually translated into protein. Understanding the concept of an ORF is essential when analyzing cDNA sequences since only the ORF encodes the protein product.

mRNA Structure and Untranslated Regions (UTRs)

mRNA molecules consist not only of the coding sequence (the ORF) but also of untranslated regions (UTRs) at both the 5’ and 3’ ends. These UTRs do not encode protein but play key roles in the regulation of translation, stability, and localization of the mRNA. Their presence explains why the length of an mRNA is typically greater than the length required merely to encode the protein.

cDNA Libraries and Reverse Transcriptase

A cDNA library is a collection of cloned complementary DNA (cDNA) fragments created by reverse transcribing mRNA molecules. This process, facilitated by the enzyme reverse transcriptase, converts the expressed mRNA into DNA, thereby preserving the coding information including the ORF and the UTRs. The study of cDNA helps researchers understand gene expression and the complete mRNA architecture.

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