DNA discovery

• For nearly four billion years, the double-stranded DNA molecule has served as the bearer of genetic information
• DNA was present in the earliest single-celled organisms and in every other organism that has existed since
• Over that long period of time, the structure of the DNA molecule itself has not changed
• Evolution has honed and vastly expanded the programs of genetic information that the DNA molecule stores, expresses, and transmits from one generation to the next
• Under special conditions of little or no oxygen, DNA can withstand a wide range of temperature, pressure, and humidity and remain relatively intact for hundreds, thousands, even tens of thousands of years
• Molecular sleuths have retrieved a 38,000-year-old DNA from a Neanderthal skeleton
• This ancient DNA still carries readable sequences, shards of decipherable information that act as time machines for the viewing of genes in this long-vanished species
• Comparisons with homologous DNA segments from living people make it possible to identify the precise mutations that have fueled evolution
• Comparisons of Neanderthal and human DNA have helped anthropologists settle a long-running debate about the genetic relationship of the two
• The evidence shows that Neanderthals and our own species, Homo sapiens, last shared a common ancestor between 600,000 and 800,000 years ago
• Neanderthal ancestors migrated to Europe about 400,000 years ago while our own ancestors remained in Africa
• The two groups remained out of contact until 40,000 years ago, when Homo sapiens first arrived in Europe
• Within a few millennia, the Neanderthals were extinct
• Neanderthal's recently recovered DNA suggests that during the 10,000 years that Neanderthals shared Europe with Homo sapiens, some interbreeding took place
• One to four percent of the genomes of modern non-Africans can be traced to Neanderthals
• Francis Crick wrote that "almost all aspects of life are engineered at the molecular level, and without understanding molecules, we can only have a very sketchy understanding of life itself"
• DNA's genetic functions flow directly from its molecular structure
• All of DNA's genetic functions depend on specialized proteins that interact with it and read the information it carries, because DNA itself is chemically inert
• DNA's lack of chemical reactivity makes it an ideal physical container for long-term maintenance of genetic information in living organisms, as well as their nonliving remains
• Genetic material


  Must carry out two jobs: duplicate itself and control the development of the rest of the cell in a specific way
• DNA is the only molecule that fulfills these requirements
• At the beginning of the 20th century, geneticists did not know that DNA was the genetic material
• It took a cohesive pattern of results from experiments performed over more than 50 years to convince the scientific community that DNA is the molecule of heredity
• Nuclein


  A weakly acidic, phosphorus-rich material extracted from the nuclei of human white blood cells by Friedrich Miescher in 1869
• Nuclein's major component turned out to be DNA, although it also contained some contaminants
• DNA


  Deoxyribonucleic acid
• DNA


  • One of its constituents is a sugar known as deoxyribose
  • It is found mainly in cell nuclei
  • It is acidic
• After purifying DNA from the nuclein by chemical means, researchers established that it contains four distinct chemical building blocks linked in a long chain
• Nucleotides


  The four individual components that make up DNA
• Phosphodiester bonds


  The bonds joining one nucleotide to another
• Polymer


  The linked chain of building block subunits that make up DNA
• In 1902, English physician Archibald Garrod was the first to link inherited disease and protein
• Garrod noted that people who had certain inborn errors of metabolism did not have certain enzymes
• Other researchers added evidence of a link between heredity and enzymes from other species, such as fruit flies with unusual eye colors and bread molds with nutritional deficiencies
• Both organisms had absent or malfunctioning specific enzymes
• Feulgen reaction


  A procedure first reported in 1923 that made it possible to discover where in the cell DNA resides
• The Feulgen reaction relies on a chemical (Schiff's reagent) which stains DNA red
• In a preparation of stained cells, the chromosomes redden, while other areas of the cell remain relatively colorless
• The reaction shows that DNA is localized almost exclusively within chromosomes
• Typical eukaryotic chromosomes also contain an even greater amount of protein by weight