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The Structure and Nature of DNA
DNA, or deoxyribonucleic acid, is the basic system upon which life on Earth is constructed. In a very real sense, DNA is a kind of program for life that cells use to replicate themselves and transmit information from generation to generation. Over eons, as life changes and adapts to new environmental conditions, that information is stored in the genetic code of all life on the planet as DNA molecules evolve and are altered to meet those changing conditions. The result is the myriad of different kinds of life that is now present on the planet, a variety that is all the more remarkable because it is based on the same fundamental piece of biological software: DNA. Incredibly DNA is a relatively simple chemical compound, so simple in fact that early researchers were dubious that it could be considered the molecule of life ("The Discovery of the Molecular Structure of DNA"). Nonetheless, despite its simplicity, DNA possesses a complex genetic code that contains all the instructions necessary for its own replication as well as the synthesis of necessary proteins within living cells themselves.
As such, it is crucial to our understanding of life on the planet that we understand the chemistry behind DNA. The interactions of the few basic chemicals that constitute DNA are responsible for all manifestations of life. Without straying too far into scientific reductionism, it is nonetheless apparent that a deeper understanding of the workings of DNA can provide new insights into way in which life arose and has developed over the past several billion years. Analysis of the chemistry of DNA is also relevant to furthering our understanding of the existing biological community and the place of humanity within that community. As such, this essay will examine some of the basic underpinnings of the structure and nature of DNA including, but certainly not limited to, replication, protein synthesis, ribosomes, and the genetic code.
As early as the 1940s, scientists began to suspect that DNA was the basic molecule of life through which traits were passed from one generation to the next. What eluded them, however, was a clear understanding of the structure of the DNA molecule itself. The mysteries that would be resolved by many scientists, including Francis and Crick included the fact that DNA was comprised of a "phosphate backbone [...] on the outside with bases on the inside" and that the spatial organization of these components was the double helix ("The Discovery of the Molecular Structure of DNA"). Though these chemical revelations seem simplistic and obvious to us today, at the time the scientific community lacked the understanding of genetic molecular mechanics necessary to understand the basic chemical interactions of which DNA is a critical part.
When Francis and Crick published their seminal paper on the structure of DNA in 1953, they had resolved this structure based on theirs and others previous understanding of the DNA molecule. In addition to being a double helix, DNA is comprised of different amounts of four bases: adenine, thymine, guanine, and cytosine. But importantly, Watson grasped the key concept that the amount of adenine in a given DNA molecule is always the same as the amount of thymine; the same is true of the amounts of guanine and cytosine. Further, the chemical bond between adenine and thymine is exactly the same length as the chemical bond between guanine and cytosine. That revelation meant that the double helix shape of the DNA molecule would take on a more elegant form as each 'rung' in the helix was equal in length meaning the "sugar-phosphate backbone would be smooth" ("The Discovery of the Molecular Structure of DNA").
Specifically, DNA is composed of two chemical strands that run in opposite directions. On the outside of the strand is a sugar-phosphate backbone with bases of adenine, thymine, guanine, and cytosine on the inside. The bases on the inside of the strands are always paired with bases on another strand, with adenine always bonded to thymine in a two-hydrogen bond, and guanine to cytosine in a three-hydrogen bond. This pairing structure significantly restricts the arrangements that can be created with two separate strands of DNA, a fact that is of the utmost importance during replication ("The Discovery of the Molecular Structure of DNA"). When DNA is being copied, it is separated along the base pairs that form the foundation for a new helix. When one DNA helix is unzipped in this fashion, then, the result will be the construction of two -- ideally identical -- strands of DNA. The copying of DNA into more DNA is known as replication and is the basis by which DNA can be propagated and spread throughout living organisms. Replication begins with the local separation of DNA molecules by a DNA polymerase enzyme, a process that allows the split halves to be available for enzymes within the cell that construct complementary copies from sugar-phosphates and bases within the cell ("Replication/Translation/Transcription").
One of the main purposes of the DNA molecule is the synthesis of specific proteins according to the plan laid out by the molecule itself. Protein synthesis occurs within ribosomes, organelles within all cells, that specifically exist to synthesize the proteins required by the cell for its functioning (Farabee). The exact information that is passed to the ribosome for the synthesis of proteins will vary from organism to organism and from DNA strand to DNA strand. It is the specific sequence of base pairs in the DNA molecule in which this information is stored. This information is known as an organism's genetic code and provides the instructions that other parts of the cell, including the ribosomes, use to take specific actions such as the synthesis of specific proteins. Once this information is in the ribosomes, proteins are synthesized there through a process known as transcription and translation. In both transcription and translation, proteins are synthesized; the difference in nomenclature is largely due to the location at which the synthesis occurs. Transcription is the synthesis of proteins within the nucleus, while translation is the synthesis of proteins within the cytoplasm of the cell ("Replication/Transcription/Translation").
Of the two protein synthesis types, transcription precedes translation. Transcription occurs within the nucleus as "DNA is transcribed into RNA to produce mRNA (messenger ribonucleic acid), rRNA (ribosomal RNA), or tRNA (transport RNA)" ("Replication/Transcription/Translation"). Enzymes that are produced by specific genes along the DNA molecule are responsible for this transcription, the end result of which is that the RNA information is transported outside of the nucleus walls and delivered to the ribosomes where it will become instrumental in the translation process of actually synthesizing proteins within the ribosomes. Specifically, RNA polymerase is responsible for opening, or unzipping, the strands of the DNA molecule that are being transcribed within the nucleus (Farabee). The transcription process relies on the existence of an RNA polymerase to break apart the original DNA molecule in a meaningful way in order to transcribe the parts of the molecule required for later protein synthesis.
Translation, thus, is a kind of product of transcription within the cell. Within the ribosomes in the cytoplasm of the cell, rRNA binds to a single strand of mRNA to transport relevant parts of the genetic code in DNA to the ribosomes of the cell. In addition, tRNA bind to specific amino acids in the cell, and transport them to the ribosomes where they will be coupled together to form a polypeptide. As always, it is the instructions within the genetic code that determines how many amino acids, of the twenty available, will be brought to the ribosomes by tRNA and in what sequence ("Replication/Transcription/Translation"). Importantly, then, it is the genetic code of the DNA molecule which outlines specific instructions that will be undertaken by other mechanisms within the cell and within the…[continue]
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