Deoxyribonucleic Acid (DNA)

Prior to the discovery of the nucleic acids, the Austrian monk Gregor Mendel (1822-1884) worked out the laws of inheritance by the selective breeding of peaplants. As early as 1865 he proposed that "factors" from each parent were responsible for the inheritance of certain characteristics in plants. The Swiss biochemist Friedrich Miescher (1844-1895) discovered the nucleic acids in 1868 in nuclei isolated from pus cells scraped from surgical bandages. However, research on the chemical structure of nucleic acids lagged until new analytical techniques became available in the mid twentieth century. With the advent of these new methods came evidence that the nucleic acid we now know as DNA. DNA was present in the nuclei of all cells and evidence about the chemical structure of its nucleotide components.

Despite knowledge of the chemical structure of nucleotides and how they were linked together to form DNA, the possibility that DNA was the genetic material was regarded as unlikely. As late as the mid twentieth century, proteins were thought to be the molecules of heredity because they appeared to be the only cellular components diverse enough to account for the large variety of genes. In 1944, Oswald Avery (1877-1955) and his colleagues showed that non-pathogenic strains of pneumococcus, the bacterium that causes pneumonia, could become pathogenic (disease-causing) if treated with a DNA-containing extract from heat-killed pathogenic strains. Based on this evidence, Avery concluded that DNA was the genetic material. However, widespread acceptance of DNA as the bearer of genetic information did not come until a report by other workers in 1952 that DNA, not protein, enters a bacterial cell infected by a virus. This showed that the genetic material of the virus was contained in its DNA, confirming Avery's hypothesis.

Shortly afterwards in 1953, James Watson (1928-) and Francis Crick (1916-) proposed their double helix model for the three-dimensional structure of DNA. They correctly deduced that the genetic information was encoded in the form of the sequence of nucleotides in the molecule. With their landmark discovery began an era of molecular genetics in biology. Eight years later investigators cracked the genetic code. They found that specific trinucleotide sequences—sequences of three nucleotides—are codes for each of 20 amino acids, the building blocks of proteins.

In 1970 scientists found that bacteria contained restriction enzymes molecular "scissors" that recognize a particular sequence of 4-8 nucleotides and will always cut DNA at or near that sequence to yield specific (rather than random), consistently reproducible DNA fragments. Two years later it was found that the bacterial enzyme DNA ligase could be used to rejoin these fragments. This permitted scientists to construct "recombinant" DNA molecules; that is, DNA molecules composed of segments from two different sources, even from different organisms. With the availability of these tools, genetic engineering became possible and biotechnology began.

By 1984 the development of DNA fingerprinting allowed forensic chemists to compare DNA samples from a crime scene with that of suspects. The first conviction using this technique came in 1987. Three years later doctors first attempted to treat a patient unable to produce a vital immune protein using gene therapy. This technique involves inserting a portion of DNA into a patient's cells to correct a deficiency in a particular function. The Human Genome Project also began in 1990. The aim of this project is to determine the nucleotide sequence in DNA of the entire human genome, which consists of about three billion nucleotide pairs. In 2001, researchers announced the completion of the sequencing of a human genome, promising refinement by 2003.