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DNA
Deoxyribonucleic acid (DNA) is a nucleic acid that acts as the blueprint for how to build a living organism or a virus. In living organisms, DNA usually consists of two molecules wrapped closely together, while for viruses, it is usually a single molecule. Every living cell has DNA molecules inside of it, which give instructions for the cell on how to develop and function. Certain pieces of the DNA segments carry information about how to construct proteins and RNA molecules, and these segments are called genes. A gene influences a particular characteristic in an organism.
In living organisms, DNA has a unique shape it consists of two long chains of polymers that wrap around each other like vines. This is called a double helix structure. The individual polymer chains are made from repeating pieces called nucleotides. DNA includes four kinds of nucleotides adenine (A), guanine (G), cytosine (C) and thymine (T). The direction of the nucleotides in one polymer strand is the opposite of the direction of the other strand, and because of this, this arrangement of DNA strands is called antiparallel. The different ends of one polymer chain are called the 3' end and the 5' end, based on kind which kind of carbon atom is at the end. The two nucleotides on opposite DNA strands that are connected are a called a base pair.
In addition to nucleotides, another important structural part of DNA is alternating pieces of phosphate and sugar. These make up the backbone of the DNA because they link the two polymer strands together, through hydrogen bonds.
Inside cells, DNA is usually packaged in an X-shaped structure called a chromosome. In eukaryotic cells (cells with nuclei), DNA is found in the cell nucleus, and some can also be found in the mitochondria. In prokaryotic cells (cells without nuclei), the DNA is stored in the cytoplasm. The set of chromosomes in a cell makes up its genome; for humans, this is 3 billion base pairs of DNA arranged into two sets of 23 chromosomes.
In order for a cell to read the “code” of a gene, it begins by copying a stretch of the DNA into RNA, a related nucleic acid. This is called transcription. The RNA copy is then read by a ribosome, which decodes the RNA sequence in a process called translation. The strand is divided into units of three nucleotides, which are called codons. Because there are four nucleotides in DNA, there are 64 possible codons for each three-nucleotide sequence (4 x 4 x 4). Each codon signifies one of the amino acids. Because there are only 20 standard amino acids, most amino acids have more than one codon. In addition, there are three codons that indicate the end of the coding region. These are called “stop” or “nonsense” codons. When translation is complete, the cell knows what kind of protein to make.
DNA can be damaged by exposure to ultraviolet light or x-rays, as well as oxidizing agents, alkylating agents. Damage can mean breaks in the double-strand structure, mutations, or the insertion or deletion of pieces. DNA damage isn’t unusual, and a typical human cell has about 150,000 bases that have suffered damage.
DNA is also involved in another important process replication. In order for an organism to grow, its cell must divide; and for a cell to divide, it needs to copy its DNA so the two resulting cells (daughter cells) contain the same information. The first step in replication is to separate the two strands of DNA, using an enzyme called DNA polymerase. The individual strands are copied, and then the old strands are reconnected.
Methods have been developed to modify DNA from organisms. In the laboratory, there are techniques for making “recombinant DNA,” which is a man-made DNA sequence that has been assembled from other DNA sequences. Genetically modified organisms can be useful in medical research or agriculture.
Since the 1980’s, scientists have been able to use DNA from blood or other bodily substances to identify the matching DNA of an individual. This technique, which is often used at crime scenes, is called DNA profiling. This method is a reliable technique for identifying a matching DNA, but identification can be complicated if the scene is contains DNA from several people.
DNA can also be use to look at modern family relationships, such as determining the father of a baby.
Many scientists contributed to the discovery of DNA, although James Watson and Francis Crick are the most well known. DNA was first isolated by the Friedrich Miescher, a Swiss doctor, in 1869. He called it nuclein. In 1919, Phoebus Levene identified several pieces of DNA’s structure, including the base, sugar and phosphate nucleotide unit. In 1937 William Astbury produced X-ray diffraction patterns that demonstrated for the first time that that DNA had a regular structure.
In 1953, James D. Watson and Francis Crick made a major breakthrough in our understanding of DNA by introducing the double-helix model of DNA structure. They also introduced the theory that DNA bases were paired. In 1962, Watson and Crick received the Nobel Prize in Physiology or Medicine.
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