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Gene Expression

Since the discovery about 30 years ago of a distinct form of collagen in cartilage, now known as Type II collagen, many other unique molecular species have been observed. Types I, II, III, V, and XI collagen are categorized as fiber-forming collagens. They all exhibit lengthy, uninterrupted collagenous domains and are first synthesized as biosynthetic precursors (procollagens).

Gene cloning experiments have demonstrated that the Group I collagen genes are evolutionarily related, for they share a common ancestral gene structure. Human chromosome number 17 contains the coding information for the α1 chain of Type I collagen, while chromosome 7 codes for its complementary α2 chain. A comparison of the five fibrillar collagens described shows that, with one exception [Types III and α2 (V) are located on chromosome 2], all other genes are located on different chromosomes.

The genes coding for fiber-forming collagens are large, about 10 times the size of the functional mRNA. Many of the exons (coding sequences) are 54 base pairs (bp) in length and are separated from each other by large intervening sequences (introns) that range in size from about 80 to 2,000 bp. The gene itself contains 38,000 bp and is very complex.

The finding that most exons of these genes have identical lengths suggests that the ancestral gene for collagen was assembled by multiple duplications of single genetic units containing an exon of 54 . It is likely that a primordial exon this size could have encoded for a gly-pro-pro tripeptide repeated six times (3 × 3 × 6). Such a polypeptide of 18 amino acids probably had the minimum length needed to form a stable triple helical structure.

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