Genomic evolution of primates
C.-K. James Shen, Institute of Molecular Biology Academia Sinica, Nankang, Taipei,
Taiwan 11529, Republic of China
Many labs have studied the evolution of primates by sequence
and expression analyses of specific genomic loci. In the case of the adult globin region,
the analysis has revealed promiscuously high rate of nucleotide substitutions at the
replacement sites of the baboon a globin genes. While the two
duplicated units containing the adult a 2/a
1 globin genes have undergone efficient gene correction, numberous DNA insertion and
genomic rearrangement events have occurred. Interestingly, the latter two processes are
tightly coupled, suggesting the existence of hot spots of DNA recombination at specific
sequence motifs, and/or chromatin structures.
More recently, we have discovered that in somatic cells, the
human DNA (5-cytosine)-methyltransferase gene, Dnmt1, encodes two alternatively spliced
forms of enzyme, instead of one as previously thought. The 48 bp extra exon in one of
these isoforms is actually derived from an Alu family repeat. The same scheme is also
utilized in chimpanzee. Furthermore, the mouse Dnmt1 gene also encodes two mRNAs differing
by 3 nt at the same position as the primate Dnmt 1 transcripts. However, no such
alternative splicing occurs in the somatic cells of the Old World Monkeys. The possible
implications of these observations are discussed in relation to the functions of the
enzymes and to the primate evolution.
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