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Dna. Essay

1038 words - 4 pages

Extracting DNA from the Bacterium Escherichia coliIntroduction Deoxyribonucleic acid is contained in allcells. The structure of DNA makes gene transmission possible. Since genes are segments of DNA, DNAmust be able to make exact copies of itself to enable thenext generation of cells to receive the same genes.The DNA molecule looks like a twisted ladder. Each"side" is a chain of alternating phosphate anddeoxyribose sugar molecules. The "steps" are formed bybonded pairs of purine-pyrimidine bases. DNAcontains four such bases the purines adenine (A) and guanine(G) and the pyrimidines cytosine (C) andthymine (T). The RNA molecule, markedly similar to DNA, usually consists of a single chain. The RNA chaincontains ribose sugars instead of deoxyribose. In RNA, the pyrimidine uracil (U) replaces the thymineofDNA. DNA and RNA are made up of basic units called nucleotides. In DNA, each of these is composed of aphosphate, a deoxyribose sugar, and either A, T, G, or C. RNA nucleotides consist of a phosphate, aribosesugar, and either A, U, G, or C. Nucleotide chains in DNA wind around one another to form a complete twist,or gyre, every tennucleotides along the molecule. The two chains are held fast by hydrogen bonds linking A toT and C to GA always pairs with T (or with U in RNA); C always pairs with G. Sequences of the paired basesare thefoundation of the genetic code. Thus, a portion of a double-stranded DNA molecule might read:A-T C-G G-CT-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 beingtranscribed, or copied, from an unzipped segýent of DNA, RNA nucleotides temporarily pair theirbaseswith those of the DNA strand. In the preceding example, the left hand portion of DNA would transcribe astrand 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 fromthe proper sequence of amino acids.Genes transmit their protein-building instructions by transcribing a special type of RNA calledmessengerRNA (mRNA). This leaves the cell nucleus and moves to structures in the cytoplasm calledribosomes,where protein synthesis takes place. Cell biologists believe that DNA also builds a type of RNA calledtransfer RNA, which floatsfreely through the cell cytoplasm. Each tRNA molecule links with a specificamino acid. When needed forprotein synthesis, the amino acids are borne by tRNA to a ribosome. TheGenetic Code Experimental evidence indicates that the genetic code is a "triplet" code; that is, each seriesofthree nucleotides along the DNA molecule orders where a particular amino acid should be placed in agrowing protein molecule. Three-nucleotide units on an mRNA strand for example UUU, UUG, and GUU arecalledcodons. The codons, transcribed from DNA, are strung out in a sequence to form mRNA. According to thetriplet theory, tRNA...

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