Over the years, rapidly advancing technology and new scientific theories have resulted in growth in many areas of scientific thought. Scientists of the past who had endless questions about the human anatomy and the interesting nature of genes have discovered that most of the answers lie in a molecule called deoxyribonucleic acid (DNA) which carries genetic instructions for the development, functioning, growth and reproduction of all known organisms. Due to its overwhelming complexity, the life-changing discovery of DNA’s structure took many years to fully understand and stemmed from the development of different models and scientific theories that had been created over an extended period of time.
DNA consists of molecules called nucleotides. Each nucleotide is joined to the next nucleotide to form a polynucleotide DNA strand. DNA is made up of two strands of polynucleotides, which are joined together by weak hydrogen bonds. The strands are antiparallel, which means they run in opposite directions and are therefore wound into a double helix. The nucleotides contain a sugar group, a phosphate group and a nitrogenous base, which can be one of four complimentary pairs; adenine (A), thymine (T), guanine (G) and cytosine (C). The order of these nitrogen bases are what determines the DNA’s unique genetic code and instruct the cell which polypeptides and proteins to make. The “backbone” of DNA’s double helix ladder is made up of phosphate and sugar (deoxyribose). The nitrogenous bases (A, G, C or T) point towards the centre of the helix, forming the rungs of the ladder. DNA can be found in different forms – it can be unwound, a form we call chromatin. DNA can also coil around proteins called histones, forming nucleosomes, which then supercoil to form chromosomes. Most DNA is found inside the nucleus of a cell, but some can also be found in the mitochondria.
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Before it was discovered that the molecule carried genetic traits, scientists didn’t think DNA held any real significance due to its simple structure – a sugar linked to a phosphate and any of four bases. Although many were ready to dismiss his theories at the time, Oswald Avery is recognized as the man who first showed that DNA could carry genetic information in 1944. In a simple experiment that involved using strains of Streptococcus pneumoniae (bacteria that causes pneumonia), Avery and his colleagues were able to show that DNA (not proteins) could transform the properties of cells, which clarified the chemical nature of genes and lead them to conclude that DNA was the “transforming principle.” This was a huge milestone in the understanding of DNA because prior to Avery’s discovery, geneticists thought that it was proteins, with their greater chemical complexity, that transmitted genetic traits.
Erwin Chargaff conducted a piece of research in 1950 that paved the way for the discoveries of DNA’s structure and the process in which it replicates. After conducting a series of tests on nucleic acids he noted an observation that DNA varies from species to species, further supporting Oswald Avery’s evidence that it was highly likely that DNA was genetic material. He also concluded that in any double-stranded DNA, the number of guanine and cytosine units were equal and so were the number of adenine and thyamine units. This documentation of 1:1 ratios in DNA’s bases provided evidence that helped Crick and Watson understand how the bases fit into the double helix and how DNA could replicate itself.
In 1952, Rosalind Franklin made a contribution to DNA research that was crucial to the understanding of its molecular structure. Using X-ray diffraction rays, Franklin was able to photograph the DNA molecule. Rosalind Franklin is best known for being the first person to photograph DNA. Her famous photo 51 is what lead Watson and Crick to discover the structure of the DNA molecule. Franklin’s work was central to Crick and Watson’s revelation that DNA was a double helix and gave them the information they needed to create the DNA model that we use today.
DNA research has helped us to understand more about ourselves than scientists of the past could ever have imagined. It has enabled us to understand, cure and prevent inherited diseases, uphold justice (DNA fingerprinting), establish paternity, and even alter our genes. On the more controversial side of things, it has allowed people to do things like genetically modify plants and create diseases. Another concern is that DNA ancestry companies are selling people’s genetic information to advertising companies, risking genetic privacy. Ethical or unethical, there is no doubt that the understanding of DNA has opened up many pathways in the science world that will no doubt continue to expand as research continues, especially with how quickly technology is advancing. In saying this, we as a civilisation should make sure this is sensibly monitored – lest things get out of control.