DNA as Genetic Material DNA Deoxyribonucleic Acid DNA
DNA as Genetic Material • DNA • • Deoxyribonucleic Acid DNA is the molecule that contains our genetic information • Originally protein was thought to possibly be responsible for genes
DNA as Genetic Material • Frederick Griffith (1941) was the first to propose that cells could change their genotypes by taking in foreign material
DNA as Genetic Material • Frederick Griffith (1941) was the first to propose that cells could change their genotypes by taking in foreign material • Transformation – a change in genotype caused when cells take up foreign genetic material
DNA as Genetic Material • Oswald Avery (1944) interpreted Griffith’s experiment after Griffith’s death • Tried to destroy transformation using enzymes • Transformation was not stopped by protein destroying enzymes, but was stopped by DNAdestroying enzymes • Proved DNA was the molecule of heredity
DNA Structure • DNA is a polymer • molecule made of many repeating parts • made of monomers called nucleotides
DNA Structure • A Nucleotide is made up of: • A 5 -carbon sugar (deoxyribose) • A phosphate group • One of the four nitrogen-containing bases
DNA Structure
DNA Structure
DNA Structure • The four bases are (A) _____, (T) _____, (C) _____, and (G) _____
DNA Structure • The four bases are (A) adenine, (T) thymine, (C) cytosine, and (G) guanine • Erwin Chargaff (1949) noted that different organisms had different amounts of the four bases
DNA Structure • However, Chargaff notes three consistencies • A = T • C = G • A+G = T+C
DNA Structure • Purines • nitrogen bases made of two rings of carbon and nitrogen • adenine and guanine l Pyrimidines • nitrogen bases made of one ring of carbon and nitrogen • thymine and cytosine
DNA Structure Rosalind Franklin and Maurice Wilkins (1952) used X-ray diffraction to study the structure of DNA • Bounced X-rays of a molecule which are then scattered on a film •
DNA Structure • They studied the patterns on the film • • Like shining a light on an object and studying its shadow Determined that DNA was made of some type of helix
Watson-Crick Model • DNA molecule is double stranded molecule called a _____ (twisted ladder)
Watson-Crick Model • DNA molecule is double stranded molecule called a double helix (twisted ladder)
Watson-Crick Model • The sides (backbone) of the molecule are made of alternating groups of _____ and ____
Watson-Crick Model • The sides (backbone) of the molecule are made of alternating groups of sugar and phosphate
Watson-Crick Model • The rungs (steps) of the molecule are made of ______ _____ of which there are four kinds.
Watson-Crick Model • The rungs (steps) of the molecule are made of nitrogen containing bases of which there are four kinds. l A, C, T, & G
Watson-Crick Model • The bases are attached to the _____ of the backbone.
Watson-Crick Model • The bases are attached to the sugar of the backbone.
Watson-Crick Model 2 -nanometer diameter overall One nanometer = 1 billionth (10 -9) of a meter 0. 34 -nanometer distance between each pair of bases 3. 4 -nanometer length of each full twist of the double helix In all respects shown here, the Watson–Crick model for DNA structure is consistent with the known biochemical and x-ray diffraction data. The pattern of base pairing (A only with T, and G only with C) is consistent with the known composition of DNA (A = T, and G = C).
Watson-Crick Model DNA close up
Watson-Crick Model • Sugars and phosphates are connected to each other by covalent bonds
DNA Structure • The N containing bases are connected to the sugar by covalent bond
DNA Structure • Covalent bonds l bonds connecting atoms that share electrons
Draw arrows pointing to the covalent bonds.
DNA Structure • The N bases are linked to each other by hydrogen bonds
Watson-Crick Model DNA close up
DNA Structure • Hydrogen bonds l l weak chemical attractions between small positive and negative charges much weaker than covalent bonds
Draw an arrow pointing to the hydrogen bonds.
Pairing Between Bases • Base Pairs (A, T, C, G) • A always pairs with T • C always pairs with G • These are called complementary base pairs
H-Bonding Between Bases • 2 Hydrogen bonds occur between adenine and thymine • 3 Hydrogen bonds occur between cytosine and guanine
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