what is the molecular weight of a mRNA that codes for a protein of molecular weight 75000, the molecular weight of a RNA residue is 320, the average mw of an amino acid is 120
what is the molecular weight of a mRNA that codes for a protein of molecular weight 75000, the molecular weight of a RNA residue is 320, the average mw of an amino acid is 120
The Correct Answer and Explanation is:
To find the molecular weight of an mRNA that codes for a protein of molecular weight 75,000 Da, we follow these steps:
Step 1: Estimate number of amino acids in the protein
We are told the average molecular weight of an amino acid is 120 Da. Number of amino acids=Protein molecular weightAverage MW of amino acid=75,000120≈625 amino acids\text{Number of amino acids} = \frac{\text{Protein molecular weight}}{\text{Average MW of amino acid}} = \frac{75,000}{120} \approx 625 \text{ amino acids}
Step 2: Estimate number of codons
Each amino acid is coded by a codon made up of 3 RNA nucleotides. So: Number of nucleotides in coding region=625×3=1,875 nucleotides\text{Number of nucleotides in coding region} = 625 \times 3 = 1,875 \text{ nucleotides}
Step 3: Account for additional regions in mRNA
mRNA contains not only the coding sequence, but also untranslated regions (UTRs) and other structural elements like:
- 5′ cap
- 5′ UTR
- 3′ UTR
- Poly-A tail
A conservative estimate is that coding region makes up ~70–80% of total mRNA length. So we can estimate total mRNA length as: Total nucleotides≈18750.75≈2500 nucleotides\text{Total nucleotides} \approx \frac{1875}{0.75} \approx 2500 \text{ nucleotides}
Step 4: Calculate molecular weight of mRNA
Given the average molecular weight of an RNA residue is 320 Da: Molecular weight of mRNA=2500×320=800,000 Da\text{Molecular weight of mRNA} = 2500 \times 320 = 800,000 \text{ Da}
✅ Final Answer:
800, 000 Daltons (or g/mol)\boxed{800,\!000 \text{ Daltons (or g/mol)}}
Explanation
Messenger RNA (mRNA) is a single-stranded nucleic acid that carries genetic instructions from DNA to the ribosome for protein synthesis. To determine the molecular weight of an mRNA that codes for a protein, we start by estimating how many amino acids make up the protein. Given that the protein has a molecular weight of 75,000 Daltons (Da) and the average molecular weight of an amino acid is 120 Da, the protein contains approximately 625 amino acids.
Each amino acid in a protein is encoded by a codon—a sequence of three nucleotides. Thus, a coding region for 625 amino acids requires 625 × 3 = 1,875 nucleotides. However, this is only the portion of the mRNA that directly translates into protein. In reality, mRNAs include additional regions essential for stability and regulation, such as the 5′ untranslated region (UTR), 3′ UTR, 5′ cap, and poly-A tail. These non-coding regions typically make up 20–30% of the mRNA sequence. Therefore, to account for these extra sequences, we estimate the total length of the mRNA to be about 2,500 nucleotides.
Given that each RNA nucleotide has an average molecular weight of approximately 320 Da, the total molecular weight of the mRNA becomes 2,500 × 320 = 800,000 Da. This includes both the coding and non-coding portions, providing a more realistic estimate of the total molecular weight of the full mRNA molecule. This calculation is essential in molecular biology and biotechnology for tasks like RNA synthesis, purification, and quantification.
