Practice Passage Test - 1 | Practice Passages for MCAT PDF Download

Osteogenesis imperfecta, also known as brittle bone disease, is primarily caused by mutations in collagen genes. Collagen, the most abundant protein in mammals, is a crucial component of connective tissue. Human collagen consists of more than 28 types, with type I collagen making up over 90% of the total collagen content. Type I collagen is present in various tissues such as bones, tendons, ligaments, and skin. It is composed of three polypeptide chains that intertwine to form a triple-helix structure, held together by covalent cross-linkages. One of the distinct features of collagen is the presence of a regular Gly-Pro-X arrangement in its subunits, with X representing any amino acid. Glycine's small side chain allows it to fit perfectly within the center of the triple helix, while the angle of the proline residue facilitates the folding of polypeptide chains into the triple helical structure.

Osteogenesis imperfecta affects approximately 6 to 7 individuals per 100,000 people worldwide. The most common form, type I, is caused by a dominant mutation in the COL1A1 gene, leading to reduced collagen production. Other types, including II, III, and IV, result from dominant mutations in either the COL1A1 or COL1A2 genes, causing structural alterations in type I collagen. Individuals with osteogenesis imperfecta experience frequent bone fractures due to the weakened structure of their bones.