DNA structure and function.docx

DNA is a polymer made up of nucleotides. Structure of nucleotide Pentose sugar Phosphate group Nitrogenous base (A, T, C, G) The sugar-phosphate backbone remains constant along the length of polymer. The sequence of bases within the nucleotides along the length of the polymer will vary. Thymine an...

DNA is a polymer made up of nucleotides. Structure of nucleotide Pentose sugar Phosphate group Nitrogenous base (A, T, C, G) The sugar-phosphate backbone remains constant along the length of polymer. The sequence of bases within the nucleotides along the length of the polymer will vary. Thymine and cytosine= pyrimidines Adenine and guanine= purines The structural difference between uracil and thymine is the CH3 attached to thymine. DNA RNA Deoxyribose Ribose A, C, T and G A, C, G and U Nucleosides vs nucleotides Nucleoside describes the base bonded to the sugar via beta-glycosidic linkage. Nucleotide is nucleoside joined to one or more phosphates via ester linkage. Nucleosides are: (deoxy)adenosine (deoxy)guanine (deoxy)cytidine Thymidine Deoxyribonucleoside triphosphates (DNTPs) are the building blocks of DNA. Nucleotides are: dATP (deoxyadenosine 5’-triphosphate) dGTP (deoxyguanosine 5’-triphosphate) dCTP (deoxycytidine 5’-triphosphate) dTTP (deoxythymidine 5’-triphosphate) Nucleotides are joined together via phosphodiester bonds which requires ATP. Sequence of bases in DNA goes from 5’ to 3’ because at one end of the polymer, there is a free OH group on 5’ carbon of the pentose and the other end has a free OH group on the 3’ carbon of the pentose sugar = condensation reaction between the 2 OH’s. Structure of DNA double helix DNA polymer forms a double helix. The strands are anti parallel- one strand is 5’ to 3’ and the other strand is 3’ to 5’. Sugar-phosphate backbone is on the outside. Bases are on the inside. Bases are almost perpendicular to the helical axis. Adjacent bases are ~3.4 angstrom apart. Helical structures repeat every 34 angstroms. Diameter of the helix is constant at about 20 angstroms. How can DNA accommodate different sequences of purines and pyrimidines and maintain a constant diameter? Achieved through complementary base pairing. A purine and pyrimidine pairs up within the double helix. Ratio of A:T and C:G is similar in nearly all organisms which proves the theory of complementary base pairing. Base pair are held by hydrogen bonds. A-T joins via 2 hydrogen bonds C-G joins via 3 hydrogen bonds Different structural forms of DNA B-DNA= found within the cells, highly hydrated A-DNA= less hydrated Z-DNA= left-handed twist in the helix which causes the phosphoryl groups to zigzag through the double helix. DNA packaging Some DNA molecules are circular which can be coiled and supercoiled which allows it to be packaged up in prokaryotes. In eukaryotic, DNA is wrapped around proteins called histones. Further folded DNA packages into chromosomes.