Evo-Ed: Integrative Cases in Evolution Education

Cases for Evolution Education

Molecular Genetics

Fur color in beach mice (Peromyscus polionotus) can be altered if the transmembrane protein, MC1R, has a mutation that disrupts its role in the synthesis of the eumelanin pigment. A functional MC1R protein binds to an α-melanocyte stimulating hormone that stimulates intracellular pigment production whereas a non-functional MC1R protein does not.

3d render of eumelanin pigment monomer

DNA, Central Dogma and the MC1R Protein

DNA is made up of long strings of nucleotides. There are four nucleotides in DNA: Adenine (A), Thymine (T), Guanine (G) and Cytosine (C). These nucleotides are held together by a phosphate-sugar “backbone” and are complemented by a parallel strand of nucleotides making a DNA molecule “double-stranded”. Parallel strands are held together by hydrogen bonds between bases; Adenine binds to Thymine, Cytosine binds to Guanine. Discrete sections of DNA carry sequences of nucleotides that, when processed, are responsible for proteins or functional RNA chains. These sections are called genes. When making a protein, the DNA is first transcribed into a single-stranded piece of messenger RNA (mRNA) using an enzyme called RNA polymerase. The newly made mRNA is then translated through a ribosome into a long chain of amino acids. This chain of amino acids folds up to make a functional protein. Proteins build the organism, including all measurable and observable characteristics.

The sequence of nucleotides that codes for the mRNA that produces the MC1R protein is shown below.

A single nucleotide change from C to T caused a single amino acid change from Arg to Cys.

The Mutation

A single nucleotide substitution at position # 199 (cytosine to thymine) in the MC1R gene results in a non-functional MC1R protein. This small difference causes a change in the amino acid chain produced by translation. Amino acid at position #67 within the polypeptide chain becomes a cysteine rather than an arginine. This single change within the protein results in a conformation change that does not allow the MC1R protein to bind extracellular melanocyte stimulating hormone. When this binding does not occur eumelanin is not effectively produced within melanocyte cells (see Cell Biology for mor details).