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DNA

Combining Cells

The purpose of sex is to create a new generation. Its methodology is the transmitting of genetic material. This genetic material is contained within the deoxyribonucleic acid (DNA) in the nucleus of our cells. Within DNA are regions called genes. These genes carry the code for manufacturing the various proteins that allow the cells to function and make us what we are. The sex cells, sperm and ova, contain only one copy of the DNA (some of which was from the mother and some from the father in each sex cell). When these cells combine, they provide the double strand of DNA required to make new cells with a unique genetic code.

Learning the Process

In the early 1950s very little was known about genes and how they function. The major achievement of the decade was determining the structure of DNA. With that knowledge, scientists determined that DNA contained the code for genes. Three molecular biologists made discoveries in the 1950s that permanently altered scientists' perception of life.

DNA Structure

DNA was first isolated in 1869, but its significance was unknown. In the early 1950s Rosalind Franklin, working in the lab of Irish scientist M. H. F. Wilkins (who had studied uranium isotopes for use in the atomic bomb during the Manhattan Project), used X-ray diffraction to study DNA in its pure form. In this technique X-rays are bombarded into the molecule and reflected onto a photographic screen (see Women in Science). The Wilkins group discovered that DNA was helical and that the molecule had a constant width down its backbone. The American biochemist James D. Watson was working in England with English biophysicist Francis H. C. Crick when they learned about these findings. They tried various models, but found only one that would agree with the Wilkins data: the double helix. They posited that DNA is formed by two helixes wound around each other.

Complementarity

Together these scientists found that DNA is composed of four bases (adenine, thymidine, guanine, and cytosine) attached to a sugar-phosphate backbone. The two strands of DNA interact so that an adenine always bonds to a thymidine, and a guanine to a cytosine; this assures that the two strands always have what in scientific jargon is called the property of complementarity. It was found that the double helix could unwind to allow the structure to duplicate itself exactly. An adenine on one side will only bind to a thymidine and vice versa. Similarly, a guanine on one side of the molecule will only bind to a cytosine. The cytosine on the other side will only bond a guanine. As a result of the property of complementarity the molecule can reproduce itself with the accuracy required for reproduction.

Nobel Prize

These discoveries gave birth to the field of molecular biology, in which it can be determined exactly what genes cause certain characteristics in living organisms, including humans. Wilkins, Watson, and Crick were awarded the Nobel Prize in 1962 for their pioneering work on DNA.