How Do Siamese Cats Get Their Color? Exploring the Role of Proteins in Molecular Genetics

By Todd, Amber; Kenyon, Lisa | The Science Teacher, January 2016 | Go to article overview

How Do Siamese Cats Get Their Color? Exploring the Role of Proteins in Molecular Genetics


Todd, Amber, Kenyon, Lisa, The Science Teacher


When asked about protein, students often mention meat, protein bars, and protein's role in building muscles. Many students are not aware of the most basic function of protein: linking genes and traits.

Because of its importance in molecular genetics, protein function is included in the life sciences section of the Next Generation Science Standards (NGSS Lead States 2013; see box, p. 35). Students should be able to explain how the structure of DNA determines the structure and function of proteins, including essential structural, signaling, transport, and catalytic activities in cells. Proteins' role in trait-producing mechanisms is challenging to grasp, as most students connect genotype and phenotype by explaining that genotypes "give" phenotypes (i.e., genes directly determine traits), completely bypassing the role of proteins in the process (Duncan and Reiser 2007; Lewis and Kattmann 2004).

This article describes a 10th-grade biology unit (six or seven instructional days) we developed on this topic that addresses the driving question: "How do Siamese cats get their coloration?" It asks students to make explicit connections among genes, proteins, and traits. The unit has been taught in both urban and suburban schools.

A molecular genetics framework

Researchers have created helpful scaffolds and learning progressions for teaching molecular genetics (Dougherty 2009; Duncan, Rogat, and Yarden 2009; Elmesky 2013; Roseman et al. 2006). A framework (Figure 1) based on a scaffold developed by Duncan and Reiser (2007) describes how DNA leads to observable traits. The first bracketed section (red and blue #2) describes the central dogma of molecular biology: how genes in DNA get transcribed to RNA and then translated to protein. The second bracketed section (blue #1) describes how the function of proteins leads to observable traits.

If classroom instruction focuses on describing only the central dogma (red), the unit concludes with protein synthesis, the creation of proteins as an end product. Although teachers typically discuss example roles for proteins (mainly enzymes), students often lack the second half of the framework (blue #1), how protein function leads to traits. When teachers move on to discussing patterns of inheritance, students are often unable to understand how proteins are involved in producing heritable traits or to explain how proteins carry out the essential functions of life in cells. Because of this, students often directly connect genes to traits (Lewis and Kattmann 2004).

Authors of two of the molecular genetics learning progressions (Duncan, Rogat, and Yarden 2009; Roseman et al. 2006) propose that the function of proteins should be taught before discussing the function of DNA. Although this order is the opposite of traditional classroom instruction, it allows students to see a purpose for proteins instead of seeing the creation of proteins as the final step. Thus, when the central dogma--how DNA gets transcribed to RNA and then translated to protein--is later introduced, students see DNA and genes as the instructions for making proteins, which carry out the functions of the cell, not as the direct instructions for traits. Our Siamese cat unit explains how proteins lead to observable traits and is designed to be taught before the introduction of DNA and the central dogma.

[FIGURE 1 OMITTED]

Unit design

You have probably seen a chemical reaction similar to the one that gives Siamese cats their color, although you may not know it. When apples brown in response to oxygen exposure, a molecule in the fruit (catechol) reacts with oxygen to form a dark-pigment molecule (benzoquinone), producing the brown color (Figure 2). In Siamese cats, a similar reaction occurs: Using dissolved oxygen carried to cells by the bloodstream, the amino acid tyrosine is oxidized to dopaquinone, using the enzyme tyrosinase (Figure 2). Dopaquinone then reacts with other molecules, producing the different forms of melanin--the pigment responsible for skin, hair, and fur coloration--causing the dark coat coloration. …

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