|
|
 |
 |
 |
 |
| Friday, 19 November 2004 |
 |
|
 |
Features | |
| News Business Features Editorial Security Politics World Letters Sports Obituaries
|
Reading the Book of Life through the Human Genome Project
Excerpts from the Plenary lecture delivered at the joint academic sessions of the Faculty of Science and the Faculty of Medicine, University of Colombo by Professor Eric H Karunanayake, Director, Institute of Biochemistry, Molecular Biology & Biotechnology, University of Colombo The elucidation of the double helical structure of Deoxyribo-Nucleic Acid (DNA) by James Watson and Francis Crick in 1953 has been a golden land mark leading to current status of molecular biology and genetics. Its discovery, however, was preceded by scintillating X-ray photographs of crystals of DNA obtained by Rosalind Franklin at King's College, London, and conclusive evidence that DNA is the genetic material by Maclyn McCarty, Oswald Avery and Collin Macleod. Although the initial response to Watson and Crick structure was lukewarm, it provided the foundation for understanding molecular damage and repair, replication and inheritance of genetic material, and the diversity and evolution of species. Jim Watson and Francis Crick with Maurice Wilkins were awarded the Nobel Prize for Physiology or Medicine in 1962. Double helix Most of us have grown up with the double helix, and yet it is still startling to consider how quickly DNA biology has progressed in just a lifetime. The elucidation of the double helix was followed by the provocative hypothesis, the central dogma, by Francis Crick and the prediction of the existence of an adaptor molecule by Jim Watson. This was soon followed by the cracking of the genetic code by Nirenberg, Khorana and Holley in 1966. This completed the picture of how the genetic language of DNA written in four letters (A, G, C and T), coded in triplet form in the messenger RNA is finally decoded to produce functional proteins. The scientific tools available at that time, however, did not facilitate detail analysis of gene structure. This was predominantly due to chemical and physical homogeneity of genes, the occurrence of most genes as single copies and non availability of biochemical methods to amplify a given gene with yields sufficient quantities for physico-chemical analysis. This impasse, was to be resolved by a series of fundamental discoveries and development of elegant biochemical and molecular tools that began to unravel around late 1960s and early 1970s. In fact it is not an exaggeration to conclude that this was made possible when decades of advances by thousands of scientists in genetics, biochemistry, cell biology, and physical chemistry came together in the laboratories of Paul Berg, Herbert Boyer, and Stanley Cohen to yield techniques for locating, isolating, preparing, and studying small segments of DNA from much larger chromosomes. Chemical scissors The discovery of chemical scissors, the restriction enzymes, that cuts DNA at specific sequences enabled the scientists to cut enormously large DNA molecules into sizable fragments. Stanley Cohen and Herbert Boyer then showed that such fragments produced from two different sources of DNA, could be joined together by using the enzyme ligase. This was followed by the ligation of a restriction enzyme cut DNA fragment to similarly cut bacterial plasmid. Thus a recombinant DNA molecule was produced. This recombinant plasmid when introduced into E. coli and cultured, the plasmid multiplied to yield unlimited amounts of fragment of DNA ligated to the plasmid. This was the birth of recombinant DNA technology (rDNA technology) or DNA cloning. Thus in 1972, Cohen and Boyer created the first recombinant plasmid capable of multiplying itself in bacterial system. This was yet another important land mark development towards our understanding of molecular basis of life. The scientific community was, however, deeply concerned with the impact of this technology on ethical, legal and safety of this technology. It was this concern among the scientific community that led to the now famous, Asilomar Conference, where leading scientists deliberated over safety issues, and in fact enforced a voluntary moratorium among themselves to hold further experiments until clearance. As usual Jim Watson, was rather disappointed. Clone genes The stage was then ready to clone genes and produce them in large quantities for biochemical analysis. The cloning technology enabled the construction of genomic libraries where an entire genome is cloned as individual recombinant clones. There was yet another tool not available. Two genes differ from each other by the sequence of four nucleotide bases, Adenine (A), Guanine (G), Cytosine (C), and Thymine (T), and number of these four chemical substances present in the gene of interest. Thus having produced large amount of the gene, the next stage is its sequencing, that is to determine the order in which these four letters are arranged in a given gene. This technology was not available. Fredrick Sanger, a former Nobel Laureate for protein sequencing, along with Allan Maxam and Walter Gilbert developed the technology, and were awarded Nobel Prize in Chemistry, been Sanger's second. Laboratories then started rapid generation of gene sequences. The GenBank was then established at the National Institute Genetic Engineering and Biotechnology, USA, for the deposition of these sequences which is freely accessible by the scientists. 1985 saw yet another unprecedented invention of a molecular technique the Polymerase Chain Reaction (PCR) by Karry Mullis. PCR technology, over the last two decades has undoubtedly revolutionized the study of molecular genetics. PCR provides for almost unlimited capacity for the amplification of a genetic sequence. The discovery of these basic technologies was soon followed by automation, further enhancing the efficiency and capacity of laboratories. Huntington's disease gene The first human disease gene to be cloned was the gene for Huntington's disease in 1985 followed by the gene for Duchenne Muscular Dystrophy in 1986.This was soon followed by the identification of the gene for Cystic Fibrosis, the gene for cystic fibrosis trans-membrane receptor (CFTR gene) in 1989. The ability to clone genes and sequence them, the construction of genomic libraries of bacterial pathogens, parasites and their vectors began to revolutionize medical diagnosis. The term 'DNA based' diagnosis, now replaced by the more fashionable term molecular diagnostics was thus born. DNA sequences specific for pathogens were identified from genomic libraries and so called DNA probes were developed and pathogens in human body fluids were detected by nucleic acid hybridization. The availability of sequences of genes when mutated account for genetic diseases, facilitates more precise, specific and sensitive diagnosis of genetic diseases, career detection and prenatal diagnosis using PCR. When these developments were taking place in advanced countries, Sri Lanka did not stay backward. Thanks to funding from Swedish agencies a laboratory for molecular biology and gene technology was established as far as 1986 at the University of Colombo. This was the only laboratory of its kind in Sri Lanka at that time and later gained international recognition. February, 2001 marks a glorious achievement of mankind, an achievement even more unprecedented that landing a man on the moon. An achievement by thousands of scientists at major centres of the world, and the completion of reading the secret book of life. The first draft sequence of the human genome was unveiled simultaneously in the world's two most prestigious journals Nature and Science in February, 2001. The sequence was 99.9 % accurate, an achievement unimaginable compared with 3 billion base-pair human genome. Final finished version of the human genome sequence was completed in 2003 and the HGP ended up with all goals achieved ! It is indeed a fitting tribute to Jim Watson, Francis Crick, Rosalind Franklin and Maurice Wilkins that Human Genome Sequence was completed on the 50th anniversary of the elucidation of its double helical structure. Jim Watson was interviewed by the Scientific American on the 50th Anniversary, Francis Crick was not available as he was indisposed. Protein codin genes Now we can read the book of life. It revealed that we carry around 33,000 to 35,000 protein coding genes. A gene is defined as the unit of genetic information. This number of genes was very much less than the number anticipated, around 80,000 to 100,000 genes. This was one of the unexpected revelations. The genes are also not uniformly distributed among the chromosomes. Some chromosomes are rich in genes while others carry less number of genes. (To be continued) |
|
News | Business | Features
| Editorial | Security
Produced by Lake House |
