Recombinant DNA, genetically engineered DNA prepared in vitro by cutting up DNA molecules and splicing together specific DNA fragments; usually uses DNA from more than one species of organism.

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Recombinant DNA,  genetically engineered DNA prepared in vitro by cutting up DNA molecules and splicing together specific DNA fragments; usually uses DNA from more than one species of organism  

Genetic Engineering Genetics: Genetic Research After Mendel DNA Carries Heredity 

Every living system has a blueprint for replication, or making copies of itself. This blueprint is commonly called heredity. The key structure of the hereditary process is the long, spiral DNA molecule. DNA consists of two complementary strands coiled around each other to form a twisting ladder called a double helix (see Genetics). The strands are made up of varying sequences of chemical groups which are called nucleotides. A nucleotide consists of a sugar and a phosphate group plus either of two purine bases--adenine (A) and guanine (G)--or either of two pyrimidine bases--thymine (T) and cytosine (C).  

   DNA contains the genetic code for making proteins from smaller molecules called amino acids. Each base on a strand of DNA pairs only with its complement on the other strand; that is, A pairs only with T, and G pairs only with C. Moreover, each set of three bases on a strand, such as AAA, AGC, GGG, or CGT, codes for a specific amino acid (or in the case of a few triplets, for an end to the protein-making process). Thus, a base triplet corresponds to a particular amino acid in the same way that a unit of the Morse telegraph code corresponds to an alphabet letter. In this manner, DNA directs the sequencing of the amino acids that grow into proteins.  

   In many organisms, DNA is restricted to the cell nucleus, while protein synthesis goes on at the endoplasmic reticulum, a system of membrane-lined tubes in the cytoplasm. Ordinarily attached to the endoplasmic reticulum are the ribosomes, "workbenches" for protein construction. Since the ribosomes are away from the nucleus, the building code must somehow be communicated from DNA to the ribosomes. This is done through ribonucleic acid (RNA). RNA is closely related to DNA and can carry genetic messages. First, DNA unwinds and separates its strands so that complementary strands of RNA can be assembled on them. A strand of so-called messenger RNA (mRNA) then travels out of the nucleus to the ribosomes, where protein synthesis begins.  

   The mRNA strand, like its DNA "parent," contains the total genetic information needed for sequencing amino acids into a particular protein. Imagine a protein containing only the two amino acids A and B strung out in this unvarying sequence: A--B--A--B--A--B (the sequence is deliberately shortened because proteins usually contain several hundred amino acids). A strand of mRNA has the series of complementary base triplets that codes for this sequence. However, another type of RNA called transfer RNA (tRNA) must carry the amino acids to the ribosome for assembly. When the mRNA code calls for amino acid A, the appropriate tRNA carries it in a form ready for peptide bonding with the next amino acid in line. In a peptide bond, the tail-end carbon atom of one amino acid is linked to the nitrogen atom of the next. When the code calls for it, another tRNA carries amino acid B. Bit by bit, the polypeptide chain grows to the desired length, guided by the mRNA directions. At the end of the operation, the newly formed protein is kicked off the ribosome. The protein instantly folds up in the most stable way. Synthesis proceeds at a fast pace. A protein containing 400 amino acids can be synthesized in about 20 seconds. (For more information about the role of DNA in protein synthesis, see Genetics.)  

   Of all the molecules that DNA could direct to be built, one might wonder why the information encoded in DNA is limited solely to the manufacture of protein. The reason is that so long as DNA can direct the making of protein enzymes, no other direction is necessary because enzymes aid in the building of all other cell molecules.  

   Most of the details of protein synthesis have been omitted from this discussion so that key events could be stressed. However, one procedure merits mention. Before an amino acid can be assembled into a polypeptide chain, it must first be modified to a so-called acyl amino acid, which is more reactive than an unmodified one. This important acyl conversion is powered by the energy stored in a molecule called adenosine triphosphate (ATP).  Nucleic Acids--The Key to Heredity 

The structure of DNA makes gene transmission possible. Since genes are segments of DNA, DNA must be able to make exact copies of itself to enable the next generation of cells to receive the same genes.  

   The DNA molecule looks like a twisted ladder. Each "side" is a chain of alternating phosphate and deoxyribose sugar molecules. The "steps" are formed by bonded pairs of purine-pyrimidine bases. DNA contains four such bases--the purines adenine (A) and guanine (G) and the pyrimidines cytosine (C) and thymine (T).  

   The RNA molecule, markedly similar to DNA, usually consists of a single chain. The RNA chain contains ribose sugars instead of deoxyribose. In RNA, the pyrimidine uracil (U) replaces the thymine of DNA.  

   DNA and RNA are made up of basic units called nucleotides. In DNA, each of these is composed of a phosphate, a deoxyribose sugar, and either A, T, G, or C. RNA nucleotides consist of a phosphate, a ribose sugar, and either A, U, G, or C.  

   Nucleotide chains in DNA wind around one another to form a complete twist, or gyre, every ten nucleotides along the molecule. The two chains are held fast by hydrogen bonds linking A to T and C to G--A always pairs with T (or with U in RNA); C always pairs with G. Sequences of the paired bases are the foundation of the genetic code. Thus, a portion of a double-stranded DNA molecule might read: A-T C-G G-C T-A G-C C-G A-T. When "unzipped," the left strand would read: ACGTGCA; the right strand: TGCACGT.  

   DNA is the "master molecule" of the cell. It directs the synthesis of RNA. When RNA is being transcribed, or copied, from an unzipped segment of DNA, RNA nucleotides temporarily pair their bases with those of the DNA strand. In the preceding example, the left hand portion of DNA would transcribe a strand of RNA with the base sequence: UGCACGU.  Genes and Protein Synthesis 

A genetic code guides the assembly of proteins. The code ensures that each protein is built from the proper sequence of amino acids (see Protein).  

   Genes transmit their protein-building instructions by transcribing a special type of RNA called messenger RNA (mRNA). This leaves the cell nucleus and moves to structures in the cytoplasm called ribosomes, where protein synthesis takes place (see Cell).  

   Cell biologists believe that DNA also builds a type of RNA called transfer RNA (tRNA), which floats freely through the cell cytoplasm. Each tRNA molecule links with a specific amino acid. When needed for protein synthesis, the amino acids are borne by tRNA to a ribosome.  

   For years biologists wondered how amino acids were guided to fit together in the exact sequences needed to produce the thousands of kinds of proteins required to sustain life. The answer seems to lie in the way the four genetic "code letters"--A, T, C, and G--are arranged along the DNA molecule.  The Genetic Code 

Experimental evidence indicates that the genetic code is a "triplet" code; that is, each series of three nucleotides along the DNA molecule orders where a particular amino acid should be placed in a growing protein molecule. Three-nucleotide units on an mRNA strand--for example UUU, UUG, and GUU--are called codons. The codons, transcribed from DNA, are strung out in a sequence to form mRNA.  

   According to the triplet theory, tRNA contains anticodons, nucleotide triplets that pair their bases with mRNA codons. Thus, AAA is the anticodon for UUU. When a codon specifies a particular amino acid during protein synthesis, the tRNA molecule with the anticodon delivers the needed amino acid to the bonding site on the ribosome.  

   The genetic code consists of 64 codons. However, since these codons order only some 20 amino acids, most, if not all, of the amino acids can be ordered by more than one of them. For example, the mRNA codons UGU and UGC both order cysteine. Because mRNA is a reverse copy of DNA the genetic code for cysteine is ACA or ACG. Some codons may act only to signal a halt to protein synthesis.  

   To illustrate the operation of the genetic code, assume that one protein is responsible for the development of brown hair and that this protein is composed of three amino acid molecules arranged in linear sequence--for example, cysteine-cysteine-cysteine. (This is a much simplified example, since proteins actually incorporate from 100 to 300 amino acid molecules.) The gene (DNA segment) specifying formation of this protein reads: ACAACAACA. It produces the mRNA segment UGUUGUUGU. This segment then drifts to a ribosome. Three tRNA molecules, each with the cysteine-bearing anticodon ACA, line up in order on the ribosome and deposit their cysteine to make the brown-hair protein.  

   Since code transmission from DNA to mRNA is extremely precise, any error in the code affects protein synthesis. If the error is serious enough, it eventually affects some body trait or feature.  Mutations 

Certain chemicals and types of radiation can cause mutations--changes in the structure of genes or chromosomes. The simplest type of mutation is a change in the DNA or RNA nucleotide sequence. Mutations may also involve the number of chromosomes or the gain, loss, or rearrangement of chromosome segments. If a mutation occurs in parental sex cells, the change is passed on to the offspring. In humans, an extra chromosome in body cells (47 instead of 46) has been implicated in Down's syndrome, a serious mental abnormality.  

   Most mutations are considered harmful and are, therefore, eventually eliminated. Some, however, enable an organism to adapt to a changing environment. Biologists believe that mutations have caused the many genetic changes involved in the evolution of species. (See also Evolution.) Genetic Terms 

allele. One of the members of a gene pair, each of which is found on chromosomes; the pair of alleles determines a specific trait.  chromosome. A structure in the cell nucleus containing genes.  dominance. The expression of one member of an allelic pair at the expense of the other in the phenotypes of heterozygotes.  gene. One of the chromosomal units that transmit specific hereditary traits; a segment of the self-reproducing molecule, deoxyribonucleic acid.  genotype. The genetic makeup of an organism, which may include genes for the traits that do not show up in the phenotype.  heterozygous. Containing dissimilar alleles.  homozygous. Containing a pair of identical alleles.  phenotype. The visible characteristics of an organism (for example, height and coloration).  recessiveness. The masking of one member of an allelic pair by the other in the phenotypes of heterozygotes.  June 19, 1997--Scientists Discover DNA Outside of Cells 

According to a report issued by Australian forensic scientists, humans are leaving traces of their DNA on everything they touch. Scientists from the Victoria Forensic Science Center in Victoria, Australia, reported in the journal Nature, that they had found DNA on coffee mugs, pens, keys, and assorted everyday items, then traced it back to its various "owners." While the scientists were unclear as to the exact source of this "naked "DNA, or DNA that exists outside of cells, they suggested that DNA might have escaped from dying cells and remained on the surface of the skin. Previously, DNA had been recovered from blood and semen, and apparently, in the case of accused "Unabomber" Theodore Kaczynski, from traces of saliva collected from licked stamps. Police have used DNA as evidence in cases of murder, rape, robbery, extortion, and drug trafficking, and DNA has also been used to determine disputed paternity. The researchers found that people can pick up other people's DNA on their hands, which raised the possibility that DNA found at a crime scene might not belong to the criminal.     A new study published in the scientific journal Cell by a team from the University of Munich in Germany, Pennsylvania State University in the United States, and other institutions, indicated that great strides had been made in solving the puzzle. Working with ancient DNA extracted from a forearm bone of the first Neanderthal fossil found, in the Neander Valley in Germany in 1856, the scientists found enough differences between the DNA of Neanderthals and that of modern humans to conclude that Neanderthals diverged about 600,000 years ago from the line that eventually would become today's Homo sapiens. 

   The extraction of analyzable DNA from the Neanderthal fossil was a significant accomplishment. Other scientific papers had been published in recent years claiming successful extraction of DNA from such fossils as dinosaur bones or insects trapped in amber, but in many instances the results of such studies had not been confirmed by other scientists. In the case of the Neanderthal DNA analysis, scientists worked under sterile conditions in two different laboratories and took pains to distinguish the Neanderthal DNA from any possible contaminants. The DNA they extracted and analyzed came from the mitochondria, which are energy-producing organelles found in cells. Mitochondria have their own DNA, which is separate from that of the nucleus, and mitochondrial DNA is passed on to offspring only through the maternal line. Mitochondrial DNA mutates much more quickly than nuclear DNA and thus provides a record of change over time that is read more easily. Many studies of evolution and the movements of earlier humans across continents have been carried out by analyzing differences in mitochondrial DNA.  

   The scientists focused on a certain region of mitochondrial DNA that is particularly subject to mutations and compared the Neanderthal version to modern human mitochondrial DNA from people on five continents. If Neanderthals were the evolutionary precursors of modern humans, their DNA sequences should be more prevalent in contemporary European populations than in, for example, Asians or Africans. Yet the study found that the Neanderthal DNA differed by exactly the same amount from human samples taken around the world. Those numbers "do not indicate that [Neanderthals are] more closely related to modern Europeans than to any other population of contemporary humans," the scientists wrote. Assuming that mutations in mitochondrial DNA occur at a relatively consistent rate over time, the difference suggests that Neanderthals diverged from the lineage that produced the modern human gene pool about four times earlier than the modern lineage began. Other studies have placed that date at around 125,000 years ago; the research published in Cell similarly estimates that Neanderthals and modern humans diverged about 550,000 to 690,000 years ago.     DNA "fingerprinting," a new detection and identification method, was announced in 1985 by the British geneticist Alec Jeffreys. DNA is the genetic material found in all body cells. Except for identical twins, the DNA code for each individual is as unique as fingerprints. Chemical examination of blood, semen, and other body fluids at a crime scene can render positive identification when compared with the DNA molecules of a suspect. The method has been used with good results in rape cases. Evidence gathered from this technique is accepted in British courts, but the procedure had not been uniformly adopted for use in American court systems by the late 1980s.  

   Another possible method of using DNA for the purpose of identification was announced in 1997 by Australian forensic scientists, who reported that humans leave traces of their DNA on everything they touch. While the scientists were unclear as to the exact source of this "naked" DNA, or DNA that exists outside of cells, they suggested that DNA might have escaped from dying cells and remained on the surface of the skin. The researchers also found that people can pick up other people's DNA on their hands, which raises the possibility that DNA found at a crime scene might not belong to the suspect in question.  GENETIC ENGINEERING.  Each human cell holds a vast storehouse of genetic information in some 100,000 genes, which code for individual biochemical functions, strung out along 46 chromosomes. Collectively, this storehouse forms the human genome. The techniques of genetic engineering allow scientists to identify specific genes, to remove any one of those genes from an organism's chromosome, to clone or make a large number of identical copies of that gene, to analyze a copy in detail, to modify it, and to reinsert it into the genetic material of the organism from which it was derived or into the genetic material of a similar or very different organism. (See also Genetics.)  

   The development of genetic engineering has had a great influence on science and business and has begun to radically alter medicine and agriculture. One of the first steps in shedding new light on human evolution and in controlling or altogether eliminating many diseases was taken in the early 1990s. Scientists mapped, or took apart, the smallest human chromosomes: the Y chromosome and chromosome 21. Breaking these chromosomes into small pieces allowed researchers to reproduce these segments in large quantities. Researchers believe that this, in turn, will lay the groundwork for mapping and eventually controlling all genes, including those that may be responsible for certain diseases.  

   Genetic engineering had its origins during the late 1960s and early 1970s in experiments with bacteria, viruses, and small, free-floating rings of deoxyribonucleic acid (DNA) called plasmids, found in bacteria. While investigating how these viruses and plasmids move from cell to cell, recombine, and reproduce themselves, scientists discovered that bacteria make enzymes, called restriction enzymes, that cut DNA chains at specific sites. The 1978 Nobel prize for physiology or medicine was shared by the discoverer of restriction enzymes, Hamilton O. Smith, and the first people to use these tools to analyze the genetic material of a virus, Daniel Nathans and Werner Arber.  

   The action of restriction enzymes is the crux of genetic engineering. DNA is made up of two long intertwined helices. The backbone of each helix is constructed from a chain of organic compounds called nucleotides. DNA is the carrier of genetic information; it achieves its effect by directing the synthesis of proteins. DNA is composed of four different nucleotides, repeated in specific sequences that form the basis of heredity. Restriction enzymes recognize particular stretches of nucleotides arranged in a specific order and cut the DNA in those regions only. Each restriction enzyme recognizes a different nucleotide sequence. Thus, restriction enzymes form a molecular tool kit that allows scientists to cut the chromosome into various desired lengths, depending on how many different restriction enzymes are used. Each time a particular restriction enzyme or set of restriction enzymes is used, the DNA is cut into the same number of pieces of the same length and composition. At least 80 restriction enzymes are now known.  

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   When restriction enzymes are used along with other enzymes that tie together loose ends of DNA, it becomes possible to remove a bit of DNA from one organism's chromosome and to insert it into another organism's chromosome. This allows scientists to produce new combinations of genes that may not exist in nature. For example, a human gene can be inserted into a bacterium or a bacterial gene into a plant.  

   So far, however, there are limits to this ability. Scientists are unable to start with only test tubes full of nucleotides to create a whole new ...

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