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The Influence of Malthus
Interestingly, Darwin and Wallace found their inspiration in economics. An English parson named Thomas Malthus (pictured) published a book in 1797 called Essay on the Principle of Population in which he warned his fellow Englishmen that most policies designed to help the poor were doomed because of the relentless pressure of population growth. When Darwin and Wallace read Malthus, it occurred to both of them that animals and plants should also be experiencing the same population pressure. -
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Initial Research Developing the Structure of DNA
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Early Research of Darwin and Wallace
Charles Darwin (pictured) began formulating his theory of natural selection in the late 1830s. He wanted to amass a wealth of evidence before publicly presenting his idea. During those years he corresponded with Alfred Wallace, who was exploring the wildlife of South America and Asia. He sent Darwin his theories concerning evolution in 1858, which, to Darwin's shock, nearly replicated Darwin's own. -
Theory of Natural Selection Announcement
Charles Darwin and Alfred Wallace (pictured) make announcement of the theory of natural selection-that members of a population who are better adapted to the environment survive and pass on their traits. -
Establishing Basic Laws of Inheritance
In 1865 Mendel delivered two long lectures that were published in 1866 as "Experiments in Plant Hybridisation." This paper established what eventually became formalized as the Mendelian laws of inheritance. Mendel's laws represent a theory of particulate inheritance that describes how the germ cells of most organisms transmit characteristics from one generation to the next. Discrete "factors" (as Mendel called them) represent, as it was ultimately understood, specific genes. -
Bridging the Gap Between Evolutionary Theory and Genetics
Thomas Morgan (pictured), an embryologist, began to extensively breed the common fruit fly. He hoped to discover large-scale mutations that would represent the emergence of new species. As it turned out his experimental and theoretical work inaugurated research in genetics and promoted a revolution in biology. Evidence he adduced from embryology and cell theory pointed the way toward a synthesis of genetics with evolutionary theory. -
A Molecule of Inheritance
Frederick Griffith (pictured) studied the effects of virulent, or virus-causing, bacteria and nonvirulent bacteria injected into mice. The mice that received the virulent bacteria died, while the ones that received the nonvirulent bacteria lived. When Griffith exposed mice to a mixture of virulent and nonvirulent bacteria, the mice still died. Griffith determined that some molecular "transforming" agent (inheritance molecule) must have been active when the mice were injected. -
The Role of DNA
By exploring Griffith's transformation experiments, Oswald Avery (pictured) discovered that DNA was in fact the inheritance molecule, serving as the molecular material of chromosomes and genes. In his experiments he destroyed the lipids, ribonucleic acids, carbohydrates, and proteins of the virulent pneumonia. Transformation still occurred after this. Next he destroyed the deoxyribonucleic acid. Transformation did not occur. Avery had found the inheritance molecule, DNA. -
Understanding the Structure of DNA
Two scientists named, Rosalind Franklin and Maurice Wilkins (pictured), initiated research to make a crystal of the DNA molecule. If they could get DNA to crystallize, then they could make an x-ray pattern, thus resulting in understanding how DNA works. These two scientists were successful and obtained an x-ray pattern. The pattern appeared to contain rungs, like those on a ladder between to strands that are side by side. It also showed by an “X” shape that DNA had a helix shape. -
Better Understanding DNA
Erwin Chargaff discovers regularity in proportions of DNA bases for different species. In all organisms he studies, the amount of adenine (A) approximately equals that of thymine (T), and guanine (G) equals cytosine (C). This research led to a better understanding of the DNA molecule and aided scientists that were trying to make a model to understand how it works and what it does. -
A Forgotten Pioneer in DNA Research
By using a picture of Franklin''s crystallized DNA, provided by Wilkins, Watson and Crick were able to put together the model of DNA. Some have speculated that they did not give Rosalind Franklin enough credit for her work. Watson and Crick did use the new information very quickly as it is shown by the fact that their paper showing the model of DNA was published in the same issue of Nature as Franklin's pictures showing DNA. -
Establishing the Structure of DNA
James Watson (pictured) and Francis Crick (pictured) recognize how two pairs of complementary bases (adenine-thymine and guanine-cytosine) would have identical shapes if held together by hydrogen bond. Two long chains of such base pairs would likely form a double helix. The DNA molecule, comprised of long strands of such base pairs in specific and varied sequences, could embed genetic information that, if the strands were separated, could be copied. -
Implications for Understanding DNA
Subsequent work made it abundantly clear that DNA, indeed a double helix, was the chemical substance of genes. In 1962 Crick and Watson were awarded the Nobel Prize in Physiology or Medicine, shared with Maurice Wilkins, whose work with Rosalind Franklin on X-ray crystallography had provided crucial evidence. Discovery of the structure of DNA was the keystone to a half-century of research that initiated a scientific revolution.
