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Structure of DNA Unit 5B.1 AP Biology 2007-2008 Genetic information is stored and passed to subsequent generations through DNA molecules and, in some cases, RNA molecules. Prokaryotes, viruses and eukaryotes contain plasmids, which are small extra chromosomal, double stranded circular DNA molecules AP Biology Proof that DNA is the carrier of genetic information involved a number of important historical experiments AP Biology Hershey & Chase Performed an experiment using radioactively labeled viruses that infect bacteria. Viruses were made of only protein and DNA AP Biology Procedure Hershey and Chase labeled the virus DNA with a radioactive isotope and the virus protein with a different isotope. They then watched the virus infect bacteria cells and followed the radioactive isotopes. AP Biology Results When the viruses infected bacteria, only the DNA entered the bacteria cell Evidence that DNA is the genetic material of a cell DNA caused bacteria to produce more viruses. AP Biology Franklin & Wilkins Used X-ray crystallography to take pictures of DNA helped lead to the discovery of the shape of DNA AP Biology Watson & Crick Double helix shape DNA is made of two strands of DNA that twist together like a spiral staircase AP Biology Avery-MacLeod-McCarty Proved that DNA was the transforming agent that caused nonpathogenic bacteria to turn pathogenic and kill mice Found that enzymes that destroy DNA did not transform and that enzymes that destroyed only proteins could transform AP Biology STRUCTURE OF NUCLEIC ACIDS Arrow from: http://www.harrythecat.com/graphics/b/arrow48d.gif Built from NUCLEOTIDE SUBUNITS NITROGEN BASES CAN BE: ADENINE GUANINE CYTOSINE THYMINE URACIL Sugar can be DEOXYRIBOSE (DNA) RIBOSE (RNA) Image by: Riedell DNA has no URACIL RNA has no THYMINE PURINES (A & G) have 2 RINGS PYRIMIDINES (T, C, & U) have 1 RING http://student.ccbcmd.edu/courses/bio141/lecguide/unit6/genetics/DNA/DNA/fg4.html http://student.ccbcmd.edu/~gkaiser/biotutorials/dna/fg29.html Directionality of DNA You need to PO4 nucleotide number the carbons! it matters! N base 5 CH2 This will be IMPORTANT!! O 4 3 AP Biology 1 ribose OH 2 The DNA backbone Made of phosphates and deoxyribose sugars 5 PO4 base 5 CH2 O Phosphate on 5’ carbon attaches to 3’ carbon of next nucleotide 4 1 C 3 O –O P O O 5 CH2 2 base O 4 1 2 3 OH AP Biology 3 Double helix structure of DNA “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic AP Biology material.” Watson & Crick Anti-parallel strands Nucleotides in DNA backbone are bonded from phosphate to sugar between 3 & 5 carbons 5 3 3 5 DNA molecule has “direction” complementary strand runs in opposite direction AP Biology Bonding in DNA 5 hydrogen bonds 3 covalent phosphodiester bonds 3 5 ….strong or weak bonds? AP Biology How do the bonds fit the mechanism for copying DNA? Base pairing in DNA Purines adenine (A) guanine (G) Pyrimidines thymine (T) cytosine (C) Pairing A:T 2 bonds C:G 3 bonds AP Biology CHARGAFF’s RULES Erwin Chargaff analyzed DNA from different organisms and found A = T G=C Now know its because: A always bonds with T G always bonds with C A Purine always bonds to a Pyrimidine AP Biology Chromosome Structure in Prokaryotes Approximately 5 million base pairs 3,000 genes Chromosome E. coli bacterium Bases on the chromosome DNA molecule in bacteria single DOUBLE STRANDED circular loop © Pearson Education Inc, publishing as Pearson Prentice Hall. All rights reserved AP Biology