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Lecture 3: Structure of DNA and RNA Part 1: DNA Structure (Deoxyribonucleic acid = DNA) Formation of nucleotide The bases of the DNA Naming Bases, Nucleosides, Nucleotides DNA is composed of polynucleotide chains - DNA is composed of two polynucleotide chains twisted around each other in the form of a double helix; - The sugar-phosphate “backbones” of the polynucleotide chains coil around the outside of the helix, and the nitrogenous bases (G, C, A, T) point toward the center; - Two polynucleotide chains (DNA strands) are held together by hydrogen bonds between the paired bases; - Two DNA strands run antiparallel to each other in the DNA molecule Sugar-Phosphate Backbone of DNA - Phosphodiester linkage: the phosphoryl group between the two nucleotide has one sugar esterified to it through a 3’-hydroxyl and a second sugar esterified to it through a 5’-hydroxyl. - Phosphodiester linkages create the repeating, that is sugar-phosphate backbone of the polynucleotide chain. - DNA chains have a free 5’-phosphate at one end and free 3’-hydroxyl at the other end. The convention is to write DNA sequences from the 5’ end to the 3’-end Base pairs of DNA 3 4 2 3 4 1 6 2 1 6 The helical structure of DNA The double helix has minor and major grooves Nonpolar hydrogen Hydrogen bond acceptor Hydrogen bond donor Methyl group -The major groove is rich in chemical information. - Chemical groups in the major groove specifies the identity of the base pairs. These patterns are important because they allow proteins recognize and bind to DNA The double helix exits in multiple conformations 11 bases/ helical turn 10 bases/ helical turn Left-handed helix Some DNA molecules are circles The chromosomes of eukaryotic cells each contain a single (extremely long) DNA molecule. However, most bacterial chromosomes are

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