Collagen is one of the main proteins found in animals and has a fibrous structure. Collagen can be found inside cells as well as forming bundles called collagen fibres which exist in the extra cellular matrix. It is known to have excellent tensile strength.
Collagen is secreted by osteoblasts as the precursor tropocollagen.
Tropocollagen is a monomer made up of three polypeptide strands called alpha chains which twist around each other to form a triple helix.
Within each of the alpha chains, there is a regular arrangement of amino acids. This arrangement often displays a repeating triplet sequence of Gly-X-Y where Gly represents glycine and X and Y can be either proline or hydroxyproline. Since glycine is a small non-branched amino acid, it is found at the centre of the triple helix of collagen where there is no space for a larger amino acid. Due to its abundance, collagen is found in many part of the body and there are 28 known types of collagen. However type I, type II, type III and type IV collagen are the dominant types of collagen present in the body. Type 1 Major component of bone, tendons and skin Type 2 Major component of cartilage and often linked with proteoglycans which form ground substance and is therefore useful for absorbing shock to the body. Type 3 Forms reticular fibres (often found with type 1) and found in blood vessels and skins. Type 4 Forms meshwork structure of basement membranes and acts a filter system. Type 1 is the most dominant type collagen and is involved in bone formation. This type of collagen is also made from 3 alpha chains which form a triple helical structure. Two of the alpha chains are called alpha-1 chains due to their identical structure whilst the third chain is called an alpha-2 chain. These chains and their triple helical structure are formed in the rough endoplasmic reticulum. COL1A1 (said to be the collagen, type1, alpha 1 gene) encodes type 1 collagen by forming two of the three alpha chains mentioned above whilst COL1A2 forms the third alpha chain. Outside the cell, the alpha chains joined together by enzymes to form tropocollagen monomers which join together to form long collagen fibrils which cross link to form strong type I collagen fibres. Collagen fibres are involved in bone formation because the small gaps between tropocollagen monomers act as nucleation sites where mineral crystals can be laid down to form bone . Collagen fibres also form the fibrous part of osteoid which is the extracellular matrix of bone.
Osteoid is made up of proteins secreted by osteoblasts and then mineralised to form new bone tissue. The presence of collagen in bone gives it its excellent tensile strength.

Osteogenesis Imperfecta (OI) is defined by the Oxford medical dictionary as "a congenital disorder of connective tissue formation that effects bone, teeth and soft tissue". The term itself is explanatory and suggests an imperfection of some sort in bone formation. Osteogenesis imperfecta is a genetic bone disease characterised by bones that break easily and is therefore sometimes known as brittle bone disease. It is caused by a genetic disorder which causes abnormalities in genes which are responsible for the production of collagen, a key protein found in bone, responsible for bone strength. Patients are born with either insufficient amounts of collagen or lack the ability to make the proper form of collagen. Both of these effects occur due to a mutation in one of the genes which codes for collagen. The most common mutation is one that affects the glycine amino acid in the alpha chains. These can occur in either
COL1A1 or COL1A2. Since the gaps between the subunits of collagen act as nucleation sites for deposition of minerals which form bone, a default in collagen means that that the bone is formed defectively.
Mutations in the base sequence of the alpha chains often cause a premature stop codon to be created.