THE MAJOR COMPOSITIONAL TRANSITIONS IN THE VERTEBRATE GENOME

The vertebrate genome underwent two major compositional transitions, b etween therapsids and mammals and between dinosaurs and birds. These t ransitions concerned a sizable part (roughly one-third) of the genome, the gene-richest part of it, and consisted in an increase in GC level s (GC is the molar fraction of guanine + cytosine in DNA) which affect ed both coding sequences (especially third codon positions) and noncod ing sequences. These major transitions were studied here by comparing GC, levels (GC, is the GC of third codon positions) of orthologous gen es from Xenopus, chicken, calf, and man.