The collagen gene: Evidence for its evolutionary assembly by amplification of a DNA segment containing an exon of 54 bp

We have determined the size and sequence of 8 exons in the gene that specifies chick type I α2 collagen. These 8 exons represent three different segments of the gene, but all encode information for the helical portion of the protein. Seven exons have a length of 54 bp, and the 8th has a size of 99 bp. The sequences within these exons vary except for the glycine codons, which occur every third triplet. Each exon begins with a glycine codon and ends with a triplet that precedes a glycine codon. The size and the sequences of the introns do not show any homology except at their ends. Of 7 introns examined the first six bases at the 5′ end of 5 of these are identical. The sequences at the 3′ end of the introns also show homologies. Our results imply that the ancestral gene for collagen arose by multiple duplications of a single genetic unit containing a 54 bp coding segment. The sequences within these exons drifted by successive point mutations and in some cases by additions or deletions of 9 bp or multiples of 9 bp.