...r, forming the rungs of the ladder.The nucleotides in one DNA strand have a specific association ith the corresponding nucleotides in the other DNA strand. Because of the chemical affinity of the bases, nucleotides containing adenine are alays paired ith nucleotides containing thymine, and nucleotides containing cytosine are alays paired ith nucleotides containing guanine. The complementary bases are joined to each other by eak chemical bonds called hydrogen bonds.In 1953 American biochemist James D. atson and British biophysicist Francis Crick published the first description of the structure of DNA. Their model proved to be so important for the understanding of protein synthesis, DNA replication, and mutation that they ere aarded the 1962 Nobel Prize for physiology or medicine for their ork.IIIPROTEIN SYNTHESIS DNA carries the instructions for the production of proteins. A protein is composed of smaller molecules called amino acids, and the structure and function of the protein is determined by the sequence of its amino acids. The sequence of amino acids, in turn, is determined by the sequence of nucleotide bases in the DNA. A sequence of three nucleotide bases, called a triplet, is the genetic code ord, or codon, that specifies a particular amino acid. For instance, the triplet GAC guanine, adenine, and cytosine is the codon for the amino acid leucine, and the triplet CAG cytosine, adenine, and guanine is the codon for the amino acid valine. A protein consisting of 100 amino acids is thus encoded by a DNA segment consisting of 300 nucleotides. Of the to polynucleotide chains that form a DNA molecule, only one strand, called the sense strand, contains the information needed for the production of a given amino acid sequence. The other strand aids in replication.Protein synthesis begins ith the separation of a DNA molecule into to strands. In a process called transcription, a section of the sense strand acts as a template, or pattern, to produce a ne strand called messenger RNA mRNA. The mRNA leaves the cell nucleus and attaches to the ribosomes, specialized cellular structures that are the sites of protein synthesis. Amino acids are carried to the ribosomes by another type of RNA, called transfer RNA tRNA. In a process called translation, the amino acids are linked together in a particular sequence, dictated by the mRNA, to form a protein.A gene is a sequence of DNA nucleotides that specify the order of amino acids in a protein via an intermediary mRNA molecule. Substituting one DNA nucleotide ith another containing a different base causes all descendant cells or viruses to have the altered nucleotide base sequence. As a result of the substitution, the sequence of amino acids in the resulting protein may also be changed. Such a change in a DNA molecule is called a mutation. Most mutations are the result of errors in the replication process. Exposure of a cell or virus to radiation or to certain chemicals increases the likelihood of mutations.IVREPLICATION In most cellular organisms, replication of a DNA molecule takes place in the cell nucleus and occurs just before the cell divides. Replication begins ith the separation of the to polynucleotide chains, each of hich then acts as a template for the assembly of a ne complementary chain. As the old chains separate, each nucleotide in the to chains attracts a complementary nucleotide that has been formed earlier by the cell. The nucleotides are joined to one another by hydrogen bonds to form the rungs of a ne DNA molecule. As the complementary nucleotides are fitted into place, an enzyme called DNA polymerase links them together by bonding the phosphate group of one nucleotide to the sugar molecule of the adjacent nucleotide, forming the side rail of the ne DNA molecule. This process continues until a ne polynucleotide chain has been formed alongside the old one, forming a ne double-helix molecule.VTOOLS AND PROCEDURES Several tools and procedures facilitate are used by scientists for the study and manipulation of DNA. Specialized enzymes, called restriction enzymes, found in bacteria act like molecular scissors to cut the phosphate backbones of DNA molecules at specific base sequences. Strands of DNA that have been cut ith restriction enzymes are left ith single-stranded tails that are called sticky ends, because they can easily realign ith tails from certain other DNA fragments. Scientists take advantage of restriction enzymes and the sticky ends generated by these enzymes to carry out recombinant DNA technology, or genetic engineering. This technology involves removing a specific gene from one organism and inserting the gene into another organism.Another tool for orking ith DNA is a procedure called polymerase chain reaction PCR. This procedure uses the enzyme DNA polymerase to make copies of DNA strands in a process that mimics the ay in hich DNA replicates naturally ithin cells. Scientists use PCR to obtain vast numbers of copies of a given segment of DNA.DNA fingerprinting, also called DNA typing, makes it possible to compare samples of DNA from various sources in a manner that is analogous to the comparison of fingerprints. In this procedure, scientists use restriction enzymes to cleave a sample of DNA into an assortment of fragments. Solutions containing these fragments are placed at the surface of a gel to hich an electric current is applied. The electric current causes the DNA fragments to move through the gel. Because smaller fragments move more quickly than larger ones, this process, called electrophoresis, separates the fragments according to their size. The fragments are then marked ith probes and exposed on X-ray film, here they form the DNA fingerprinta pattern of characteristic black bars that is unique for each type of DNA.A procedure called DNA sequencing makes it possible to determine the precise order, or sequence, of nucleotide bases ithin a fragment of DNA. Most versions of DNA sequencing use a technique called primer extension, developed by British molecular biologist Frederick Sanger. In primer extension, specific pieces of DNA are replicated and modified, so that each DNA segment ends in a fluorescent form of one of the four nucleotide bases. Modern DNA sequencers, pioneered by American molecular biologist Leroy Hood, incorporate both lasers and computers. Scientists have completely sequenced the genetic material of several microorganisms, including the bacterium Escherichia coli. In 1998, scientists achieved the milestone of sequencing the complete genome of a multicellular organisma roundorm identified as Caenorhabditis elegans. The Human Genome Project, an international research collaboration, has been established to determine the sequence of all of the three billion nucleotide base pairs that make up the human genetic material. An instrument called an atomic force microscope enables scientists to manipulate the three-dimensional structure of DNA molecules. This microscope involves laser beams...
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