分子生物学(英文版)..doc

Chapter 3 Nucleic Acid 1. Physical and chemical structure of DNA Double-stranded helix Major groove and minor groove Base pairing The two strands are antiparallel G+C content (percent G+C) Satellite DNA Satellite DNA consists of highly repetitive DNA and is so called because repetitions of a short DNA sequence tend to produce a different frequency of the nucleotides adenine, cytosine, guanine and thymine, and thus have a different density from bulk DNA - such that they form a second or satellite band when genomic DNA is separated on a density gradient. 2. Alternate DNA structure Two bases have been extruded from base stacking at the junction. The white line goes from phosphate to phosphate along the chain. O is shown red, N blue, P yellow and C grey. 3. Circular and superhelical DNA DNA can also form a double-stranded, covalently-closed circle. These circular molecules are often coiled into a superhelix, the formation of which is catalyzed by enzymes called topoisomerases. 4. Denaturation of DNA Denaturation: A transition from the native to the denatured state DNA denaturation: also called DNA melting, is the process by which double-stranded DNA unwinds and separates into single-stranded strands through the breaking of hydrogen bonding between the bases. Hyperchromicity / Hyperchromic effect: the striking increase in absorbance of DNA (A260) caused by the denaturation of the double-stranded DNA molecule Melting temperature (Tm) : the temperature at which half of the DNA strands are in the double-helical state and half are denatured. The melting temperature depends on both the length of the molecule, and the specific nucleotide sequence composition of that molecule. Factors Affecting Tm G-C content of sample reagents that increase the solubility of the bases (anything that disrupts H-bonds or base stacking) Salt concentration pH Length 5. Renaturation Strands can be induced to renature (anneal) under proper conditions. Factors to consider: Tem