- Basic Concepts
- DNA & RNA
- Simple Inheritance
- Modify Mendelian Ratios
As we have mentioned before, it is the protein products of genes that cause many phenotypes. In this animation, we will see how gene sequence and phenotype, in particular coat curliness in dogs, relate.
Proteins fulfill a variety of cellular roles. For example, the protein keratin is a major structural component in hair, wool, nails and porcupine quills. Other proteins are enzymes, which speed up chemical reactions in cells. The enzyme tyrosinase, for example, is required to produce pigment in animal hair.
No matter what its role in the cell, however, the structure of a protein is critical to its function, and that structure is determined by the sequence of its gene. Let's turn to the gene sequence in this animation to see how all this works. As you can see, the gene codes for a short polypeptide with the amino acid sequence cysteine alanine - threonine alanine cysteine. The sequence of amino acids is called the primary structure of a polypeptide.
Because the cysteines have sulfur groups, they can bind to one another via a chemical bond called a disulfide bond. In hair, disulfide bonds between cysteines in the keratin provide strength, and cause the hair to bend. The more disulfide bonds hair has, the curlier it will be.
In the second frame, we have changed the T in the first codon to an A. This DNA sequence now codes for the amino acid serine, which can no longer form a disulfide bond with cysteine. Thus, the protein becomes straighter, and we no longer have a curly-haired dog.
Other mutations can completely inactivate protein function. Albino animals result from mutations in the tyrosinase gene that render the tyrosinase enzyme inactive and thus prevent pigment formation.