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The History of DNA, A Blazing Recap in 5 Minutes

history of dna, DNA History

 

James Watson and Francis Crick scrawled their names in scientific history books with their 1953 depiction of DNA’s long-illusive structure, while the name Rosalind Franklin was lost to background noise. Franklin was the researcher responsible for fine-tuning the x-ray crystallography technology, essentially capturing high definition photographs of DNA which served as the structural underpinnings for Watson and Crick’s double-helix model. Few acknowledge Franklin’s contribution to the field, as her critical work was intercepted by her condescending colleague, Maurice Wilkins, and shared with Watson without her express permission. She hovered at the periphery of the limelight while Watson, Crick, and Wilkins shared the glory- as well as the Nobel Prize which was issued after Franklin’s death.

The History of DNA 

Regardless of its controversial back-story, the Watson and Crick model was indeed revolutionary. But even Watson and Crick could not fully grasp the far-reaching implications of their discovery. Their monumental leap was only a tiptoe forward into the expansive realm of DNA intricacies that were yet to be discovered.

Learning How DNA Replicates

Unbeknownst to Watson and Crick, their model provided the foundation for DNA replication. Yet the secret would lie fallow until the Meselson-Stahl experiment of 1958. Many leading scientists believed that both strands of the DNA molecule were copied independently, and the newly manufactured strands bonded to form the next generation DNA molecule; this was known as the conservative model. Matthew Meselson and Franklin Stahl, however, ultimately debunked this prominent theory and instead confirmed the validity of a semi-conservative model.

Their procedure was simple yet understatedly brilliant, piggybacking on the basic principles of density.  Since cells incorporate dietetic nitrogen into their DNA structures, the researchers cultivated a generation of bacteria within an environment of heavy nitrogen- an isotopic form of nitrogen-containing a higher number of protons. Midway through, they transitioned the same bacteria into an arena of regular nitrogen which was lighter and therefore less dense. Afterward, they subjected the bacterial DNA to centrifugation, so that varying densities would separate out into a gradient. According to the conservative model, the DNA from the parent generation exposed to heavy nitrogen would display a uniform high density, while the replicated generation grown within regular nitrogen would exhibit the uniform regular density of its surrounding culture. Bands of intermediate DNA density formed instead, suggesting that the fledging DNA comprised one heavy nitrogen parent strand and one newly synthesized strand containing regular nitrogen. Solidifying this semi-conservative model of DNA replication was a pivotal development in DNA understanding because it would allow scientists to imitate the replication process for the sake of further study. [1]

Be Fruitful and Multiply: Amplifying DNA

The basic comprehension of DNA replication techniques eventually paved the way for synthetic replication. Developed in 1976, the Polymerase Chain Reaction (PCR) allows for rapid amplification of particular DNA segments through mimicry of the natural replication process.  A DNA pool is heated to an unnaturally high temperature which dissolves the hydrogen bonds connecting the strands and forces separation. The isolated template strand is connected to a primer, a tiny DNA sequence which permits the attachment of DNA construction enzymes. Taq polymerase is typically the enzyme of choice; discovered in the heat-loving bacterium Thermus aquaticus, Taq polymerase is sustainable at the extreme temperatures required for artificial DNA replication. Once Taq links to the primer starter sequence, it travels along the template strand, matching up nucleotides according to the principles of complementary base pairing. PCR creates high volumes of genetic material available for analytic purposes. Perfection of this process was a milestone for DNA researchers, and a precursor to one of today’s most popular gene sequencing methods: the Sanger method. [2]

Sanger Cracks the Sequence

During PCR, the Taq polymerase uses free floating nucleotides called dNTPs to synthesize the new complementary strand. Sometimes, dNTPs are fragmented in such a way that they claim their own slot in the DNA sequence but preclude the attachment of further dNTPs, effectively halting the polymerase’s progress. Termed dideoxynucleotides (ddNTPs), these slightly broken particles can appear as adenine, guanine, cytosine, or thymine. Each form of ddNTP will interrupt the sequence at its respective slot in the DNA nucleotide sequence. When a DNA segment is amplified by PCR and independently subjected to each base-specific ddNTP, a researcher can study the resulting fragments to determine various base positions and segment lengths. ddNTPs can actually be tinted with individualized dyes, so modern results are conveniently readable. With the concretization of the Sanger method, DNA sequences finally became tangible entities- ones that could be meticulously analyzed, depicted, and recorded. [3][4]

Internal Regulation

DNA sequencing continuously increased in popularity, as scientists were intent on identifying the segments responsible for particular protein products. Yet an instrumental paper published by Andrew Fire and Craig Mello in 1998 focused instead on RNA interference which interrupted the typical transmission of DNA code to messenger RNA to the protein product. The researchers discovered that the normally single-stranded RNA molecule could sometimes adopt a double-stranded form. These unusual RNA units stimulate the cell to degrade any regular mRNA with matching genetic code. This interference acts like a genetic muffler, preventing the progression of protein synthesis. Essentially, the findings recorded in Fire and Mello’s paper revealed but one instance of internal cellular regulation, like a checks and balances system to manipulate genetic expression. [5]

Forward Momentum

The vast field of DNA research is rolling forward like a snowball down a hill, continuously accumulating greater mass and speed. DNA’s brief but blazing history reveals inexorable advancement and increasing sophistication. If the history of DNA is any indicator of the future, our understanding of DNA will increase exponentially in years to come.

Sources:

[1] http://www.visionlearning.com/en/library/Inside-Science/58/Meselson-and-Stahl/187
[2] http://www.ncbi.nlm.nih.gov/genome/probe/doc/TechPCR.shtml
[3] http://www.genetic-inference.co.uk/blog/2009/04/basics-sequencing-dna-part-1/
[4] https://www.thermofisher.com/us/en/home/life-science/sequencing/sanger-sequencing/sanger_sequencing_method.html

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    By: Avigail Goldberger

    Avigail Goldberger is an avid science devotee, with a particular interest in biology, genetics, and neuroscience. Her love of STEM subjects is equally matched by a passion for literature and writing, so she hopes that her eventual profession will synthesize her multifaceted academic drives.

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