Answer key: DNA to Protein Activity

For teachers. This page is hidden from search engines and not linked from student pages, but it is not password-protected, so please don’t share the link with students. Student activity: DNA to Protein Activity.

Goal: Transcribe DNA into mRNA, translate codons into amino acids with tRNA anticodons, and explain how one base substitution causes sickle-cell hemoglobin.

Connects to NGSS HS-LS1-1 (Explain how the structure of DNA determines the structure of proteins, which carry out the essential functions of life.)

Completed results table

Every value is the tool’s own readout for that setting, calculated with the same code as the DNA to mRNA converter.

Answers

1. The demo’s coding strand begins 5′-ATG GCC-3′. Write the matching mRNA and tRNA anticodons for these two codons, and name the amino acids.

   mRNA 5′-AUG GCC-3′ (copy the coding strand, with U in place of T). Anticodons 3′-UAC CGG-5′. AUG is methionine (Met, the start codon) and GCC is alanine (Ala).

2. How many codons does the 39-nucleotide demo contain, and why does its protein have only 7 amino acids?

   39 ÷ 3 = 13 codons, but the ribosome stops at the UGA stop codon, codon 8. A stop codon codes for no amino acid, so only codons 1–7 are translated: Met-Ala-Ile-Val-Met-Gly-Arg.

3. Compare codon 7 of normal β-globin and of sickle-cell β-globin. What is the base change, and how does it change the protein?

   Codon 7 changes from GAG to GUG, caused by an A → T substitution at nucleotide 20 of the DNA. GAG codes for glutamic acid (Glu) and GUG for valine (Val), so one amino acid changes: a missense mutation.

4. The sickle-cell protein is still 14 amino acids long here. Why does a substitution change only one amino acid, while deleting one base would change every codon after it?

   A substitution swaps one base but leaves every codon in its group of three, so only codon 7 changes and the chain stays 14 amino acids long. Deleting a base shifts the reading frame, so every later codon is read in a new set of three: a frameshift.

5. You typed the template strand 3′-TACGGACTT-5′. What mRNA and amino acid sequence did you get?

   mRNA 5′-AUG CCU GAA-3′ (each base pairs with its complement, with U opposite A). It codes for Met-Pro-Glu.

6. Challenge: valine is nonpolar, while glutamic acid is acidic and carries a negative charge. Suggest why this one change makes hemoglobin molecules stick together.

   The charged glutamic acid on the protein’s surface helps keep hemoglobin dissolved. Valine is nonpolar (hydrophobic), so when oxygen is low the new patch sticks to a hydrophobic pocket on a neighboring hemoglobin molecule. The molecules join into long fibers that bend red blood cells into a sickle shape.