![]() Examination of cells by cytochemistry, after staining with dyes that bind specifically to just one type of biochemical, had shown that chromosomes are made of DNA and protein, in roughly equal amounts. This observation led to the proposal that genes are located in chromosomes and by the 1930s it was universally accepted that the chromosome theory was correct. How could the molecular nature of the genetic material be determined? Back in 1903, WS Sutton had realized that the inheritance patterns of genes paralleled the behavior of chromosomes during cell division. The starting point for the new molecular biology was to discover what genes are made of. These scientists were the first molecular biologists and the first to suggest that ‘life’ could be explained in molecular terms our current knowledge of how the genome functions stems directly from their pioneering work. In 1944, Erwin Schrödinger, more famous for the wave equation which still terrifies many biology students taking introductory courses in physical chemistry, published a book entitled What is Life?, which encapsulated a variety of issues that were being discussed not only by geneticists but also by physicists such as Niels Bohr and Max Delbrück. It was not until the 1930s that scientists began to ask more searching questions about genes. The chemical nature of genes was equally unknown, and indeed was an irrelevance for most geneticists, who in the years immediately after 1900, when Mendel's work was rediscovered, were able to make remarkable advances in understanding heredity without worrying about what genes were actually made of. The precise biological function of DNA was not known, and the supposition that it was a store of cellular phosphorus seemed entirely reasonable at the time. To make such a connection - to infer that genes are made of DNA - would have been quite illogical in the late 19th century or indeed for many decades afterwards. It is very unlikely that Miescher and Mendel were aware of each other's work, and if either of them had happened to read about the other's discoveries then they certainly would not have made any connection between DNA and genes. Mendel's paper in the Proceedings of the Society of Natural Sciences in Brno describes his hypothesis that inheritance is controlled by unit factors, the entities that geneticists today call genes. Three years before Miescher discovered DNA, Gregor Mendel had published the results of his breeding experiments with pea plants, carried out in the monastery gardens at Brno, a central european city some 550 km from Tübingen in what is now the Czech Republic. Miescher's chemical tests showed that DNA is acidic and rich in phosphorus, and also suggested that the individual molecules are very large, although it was not until the 1930s when biophysical techniques were applied to DNA that the huge lengths of the polymeric chains were fully appreciated. The first extracts that Miescher made from human white blood cells were crude mixtures of DNA and chromosomal proteins, but the following year he moved to Basel, Switzerland (where the research institute named after him is now located) and prepared a pure sample of nucleic acid from salmon sperm. First, however, we must understand the structure of DNA.ĭNA was discovered in 1869 by Johann Friedrich Miescher, a Swiss biochemist working in Tübingen, Germany. Later in this chapter we will examine how the human genome is constructed, some of this information dating from the old days when biologists studied genes rather than genomes, but much of it revealed only since the Human Genome Project was completed in the first year of the new millennium. We begin our journey with our own genome, which is quite naturally the one that interests us the most. It explains what genomes are (Part 1), how they are studied (Part 2), how they function (Part 3), and how they replicate and evolve (Part 4). Both types of cell have about 8000 copies of the mitochondrial genome, 10 or so in each mitochondrion. These are called somatic cells, in contrast to sex cells or gametes, which are haploid and have just 23 chromosomes, comprising one of each autosome and one sex chromosome. The vast majority of cells are diploid and so have two copies of each autosome, plus two sex chromosomes, XX for females or XY for males - 46 chromosomes in all. Each of the approximately 10 13 cells in the adult human body has its own copy or copies of the genome, the only exceptions being those few cell types, such as red blood cells, that lack a nucleus in their fully differentiated state.
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