. Describe the structure of DNA.
The three-dimensional structure of DNA – the double helix – arises from the chemical and structural feature of its two polynucleotide chains. Because these two chains are held together by hydrogen bonding between the bases on the different strands, all the bases are on the inside of the double helix, and the sugar-phosphate backbones are on the outside. In each case, a bulkier two ring base is paired with a single-ring base; A always pairs with T, and G with C. In this arrangement, each base pair is similar width, thus holding the sugar-phosphate backbones an equal distance apart along the DNA molecule. To maximize the efficiency of base-pair packing, the two sugar-phosphate backbones wind around each other to form a double helix, with one complete turn every ten base pairs.
2. How does an organism’s genotype determine its phenotype? An organism’s genotype, its genetic makeup, is the sequence of nucleotide bases in its DNA. The phenotype, the organism’s physical traits, arises from the actions of a wide variety of proteins. Structural proteins help make up the body of an organism, and enzymes catalyze its metabolic activities. DNA specifies the synthesis of proteins. However, a gene does not build a protein directly, but rather dispatches instructions in the form of RNA, which in turn programs protein synthesis. This molecular chain of command is from DNA in the nucleus to RNA to protein synthesis in the cytoplasm. The two main stages are transcription, the transfer of genetic information from DNA into an RNA molecule, and translation, the transfer of the information from RNA into a protein.
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