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Gene Mutations and Proteins

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Gene Mutations and Proteins

A cell uses DNA instructions to build proteins. In this investigation, compare gene sequences to find out how mutations can change a protein. Use the codon key below; treat each DNA triplet as a coding-strand codon. Write the matching mRNA codon by replacing T with U.

Codon Key

Use the DNA coding triplet to find the corresponding mRNA codon, then use the key to identify the amino acid. A stop codon ends translation.

DNA coding triplet → mRNA → amino acid

ATG → AUG → Met (methionine)

GAA → GAA → Glu (glutamic acid)

TGT → UGU → Cys (cysteine)

TGA → UGA → STOP

CAA → CAA → Gln (glutamine)

Decode the Gene

A research team compares versions of a short gene that helps make a fictional cell enzyme. A protein's amino-acid order is its primary structure; that order can affect how the protein folds and works.

Version

DNA coding strand

Original

ATG GAA TGT CAA

Variant A

ATG GAG TGT CAA

Variant B

ATG GAA TGA CAA

Variant C

ATG GAA ATG TCA A

1.Which statement best describes a gene mutation? Choose one.

A change in the DNA sequence
A change in the shape of a cell
A change in the number of proteins in a cell
A change that always harms an organism

2.Translate the Original gene sequence. Write its amino-acid chain using the abbreviations in the codon key.

3.Variant A changes the DNA triplet GAA to GAG. Both produce the same amino acid in this example. What kind of mutation is this, and what happens to the protein's amino-acid sequence?

4.Variant B changes TGT to TGA. Identify the mutation's effect on translation and predict what happens to the protein.

5.Match each mutation type to its most typical effect. Write the letter beside each number.

1. Silent
2. Missense
3. Nonsense
4. Frameshift

B. No amino-acid change
D. The reading groups shift, often changing later amino acids
C. A premature stop shortens the protein
A. One amino acid is substituted

6.Variant C has one extra A inserted after the first six DNA bases. Explain why this insertion can affect several amino acids rather than just one. Use the idea of reading groups of three bases.

7.Which level of protein structure is directly changed first if a mutation changes an amino acid? Choose one.

Primary structure—the amino-acid sequence
Secondary structure—the local coils and folds
Tertiary structure—the overall shape
Quaternary structure—the joining of multiple protein chains

8.A protein's shape helps determine its function. If a mutation changes its amino-acid sequence, explain one possible chain of effects from the gene to the cell.

9.A scientist finds a mutation in a gene but observes no difference in the protein made. Give one reasonable explanation for this result.

10.Mutations do not always have the same outcome. In two or three sentences, explain why one mutation might have little effect while another has a large effect on an organism.