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BIOL 2400

[3.1] DNA STRUCTURE General Overview – DNA (deoxyribonucleic acid) + RNA (ribonucleic acid) are called nucleic acids – building block of DNA is deoxyribonucleoside 5’ triphosphate (dNTP) – deoxyribonucleotides are composed of 3 components: What kind of glycosidic linkage is o ribose (deoxyribose) – simple monosaccharide formed when an aromatic base drawn above or below the plane? o aromatic base – adenine (A), guanine (G), cytosine (C), thymine (T), β – up (above plane) [uracil (U) for RNA]  purines (G and A) α – down (below plane)  pyrimidines (C or T) [C or U in RNA] o phosphate group  nucleotides are phosphate esters of nucleosides – w/ P groups joined by the 5’ hydroxy group  when nucleotides contain 3 P residues = deoxynucleoside triphosphates (dNTP)  RIBOSE + PHOSPHATE = backbone of DNA b/c it is invariant – nucleoside is ribose with a purine/pyrimidine linked to 1’ carbon in a β-N-glycosidic linkage o A-ribose = adenosine, G-ribose = guanosine, C-ribose = cytidine, T-ribose = thymidine, U-ribose = uridine Polynucleotides – nucleotides are covalently linked by phosphodiester bonds b/w 3’ hydroxy group + 5’ phosphate group – oligonucleotide – polymer of several nucleotides; polynucleotide – polymer of many nucleotides – sequence of nucleotides in the chain are indicated from 5’  3’ direction Watson-Crick Model of DNA Structure *According to the Watson-Crick model, cellular DNA is a right-handed double helix held together by H-bonds b/w bases. – DNA in the nucleus is double-stranded (ds) w/ two long polynucleotide chains H-bonded together in antiparallel orientation o 5’ end of one chain is paired with the 3’ end of the other o H-bonding is specific (A is H-bonded to T, G is H-bonded to C), ∴ H-bonded pair consists of a purine + a pyrimidine  GC pair is held together by 3 H-bonds; AT pair by 2 H-bonds – binding of 2 complementary strands of DNA into a double-stranded structure is annealing or hybridization; whereas separation of strands is melting, or denaturation o T –mtemperature at which a solution of DNA molecules is 50% melted – DNA is also coiled o DNA strands form a right-handed double helix which is stabilized by van der Waals interactions b/w bases – bases are stacked 3.4 angstroms (Å) apart from each other; helix pattern repeats once every 34 Å, or every 10 base pairs – genome = sum total of an organism’s genetic info Humans have 46 chromosomes, o eukaryotic genomes is composed of large pieces of linear ds-DNA called 23 from each parent a chromosome o prokaryotic (bacterial) genomes are composed of a singular circular chromosome  DNA gyrase is a mechanism in prokaryotes which uses ATP to make it more compact and sturdy  gyrase breaks DNA and twists 2 sides of the circle around each other resulting in a twisted circle  twists created by DNA gyrase = supercoils o since eukaryotes have more DNA in their genome than prokaryotes, eukaryotic genome requires denser packaging  eukaryotic DNA is wrapped around globular proteins called histones  once wrapped, DNA has the appearance of beads on a string  DNA wrapped around a group of 8 (octamer) histones are called nucleosomes  string b/w the beads
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