Genetics: Analysis of Genes and Genomes: Keyword Hot Links for chapter 6

The human gene for DNA polymerase alpha, POLA, is a present in the short arm of the X chromosome. It is one of a small number of X-linked genes that violate "Ohno's law" that the set of genes present in the X chromosome is the same in all mammals. In marsupials, such as the kangaroo, and in monotremes, such as the platypus, POLA and the other exceptional genes are autosomal. In the evolution of mammals, the monotreme lineage branched off first, 150–170 million years ago, and the marsupial lineage branched off later, about 120–150 million years ago. The nonmarsupial branch evolved into the placental mammals, which includes ourselves and most of the furry creatures with which we are familiar. What does this evolutionary history imply about the evolution of the X chromosome? You can find out at this keyword site, and also learn which other genes violate Ohno's law.

Large-scale genome sequencing has resulted in the complete, or almost complete, genome sequences of a large number of bacteria, the yeast Saccharomyces cerevisiae, the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster, the human Homo sapiens—and a large number of other genomic sequencing projects are in progress. The keyword Sanger Center will connect you with one of the world's leading sequencing centers and provide access to a cornucopia of information about various genome projects. The Sanger Centre, located in Hinxton, Cambridgeshire, is named after Fred Sanger, who in 1980 shared a Nobel Prize in Chemistry for his contributions to the determination of base sequences in nucleic acids. (It was his second: the first was awarded in 1958 for his work on the determination of amino acid sequences in proteins.)

The Holliday model revolutionized thinking about recombination because it was the first model that suggested how recombination takes place at the molecular level and incorporated new concepts such as mismatch repair in heteroduplex regions. An animated version of the model can be found at this keyword site. Note the key stages of strand invasion and branch migration. Be sure to view the three-dimensional animation as well, because it shows why the two possible types of resolution of the Holliday-junction are topologically equivalent.