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The Chemical Nature of the Gene Base Structure and Topology Copyright, ©, 2002, John Wiley & Sons, Inc., Karp/CELL & MOLECULAR BIOLOGY 3E The structure of DNA • Base composition – pyrimidines (cytosine [C], thymine [T]) – purines (adenine [A], guanine [G]) – backbone of alternating sugar & phosphate groups – joined by 3'-5'-phosphodiester bond – Nucleotide (& polymer) is polarized - 5' phosphate & 3'-OH Copyright, ©, 2002, John Wiley & Sons, Inc., Karp/CELL & MOLECULAR BIOLOGY 3E The structure of DNA • Some terminology – nucleoside (deoxyribose, nitrogenous base) – nucleotide (add phosphates) – RNA nucleotides are often employed in energy metabolism like ATP, GTP • X-ray diffraction revealed dimensions – 3.4 Å between nucleotides in stack – large structural repeat every 34 Å (3.4 nm) Copyright, ©, 2002, John Wiley & Sons, Inc., Karp/CELL & MOLECULAR BIOLOGY 3E The structure of DNA • Erwin Chargaff (Columbia, 1950) – Found relative base amounts varied – not always 1:1:1:1, but always constant within species – Purines equaled pyrimidines: [A] = [T]; [G] = [C] – A:G ratio of Btub = 0.4; human DNA - 1.56 – Chargaff gave DNA molecule specificity & individuality Copyright, ©, 2002, John Wiley & Sons, Inc., Karp/CELL & MOLECULAR BIOLOGY 3E Watson - Crick Model • Composed of 2 nucleotide chains • 2 chains spiral as right-handed helices – clockwise path moving away from observer • 2 chains of double helix antiparallel • sugar-phosphate-sugar-phosphate—outside • bases project toward center Copyright, ©, 2002, John Wiley & Sons, Inc., Karp/CELL & MOLECULAR BIOLOGY 3E Copyright, ©, 2002, John Wiley & Sons, Inc., Karp/CELL & MOLECULAR BIOLOGY 3E Figure 10.10a & b Watson - Crick Model • Planes of bases stack on top of each other – Hydrophobic interactions & van der Waal forces add stability • Chains held together by H bonds • radius 10 Å (1 nm); DNA width 20 Å (2 nm) • Pyrimidine pairs with purine Copyright, ©, 2002, John Wiley & Sons, Inc., Karp/CELL & MOLECULAR BIOLOGY 3E Watson - Crick Model • A with T; G with C (matches Chargaff's Rules) – Nitrogen atoms on cytosine C4 & adenine C6 are mostly in amino (NH2) not imino (NH) form – Oxygens on guanine C6 & thymine C4 mostly in keto (C=O), not enol (C—OH) form – These structural restrictions on base configurations are responsible for pairing restrictions Copyright, ©, 2002, John Wiley & Sons, Inc., Karp/CELL & MOLECULAR BIOLOGY 3E Copyright, ©, 2002, John Wiley & Sons, Inc., Karp/CELL & MOLECULAR BIOLOGY 3E Figure 10.10a & b Watson - Crick Model • 2 grooves of different width – Wider major groove; more narrow minor groove – DNA binding proteins have domains that fit into grooves • 10 residues (3.4 nm) per turn • complementarity (one strand complement to the other) Copyright, ©, 2002, John Wiley & Sons, Inc., Karp/CELL & MOLECULAR BIOLOGY 3E Utility of DNA Duplex • Primary functions of genetic material – Storage of genetic information - determines heritable characteristics; amino acid sequence of all proteins in organism must be contained within the DNA structure – Self-duplication and inheritance – DNA must contain the information for its own replication – Expression of genetic message - genes usually encode proteins; DNA must direct protein synthesis Copyright, ©, 2002, John Wiley & Sons, Inc., Karp/CELL & MOLECULAR BIOLOGY 3E Copyright, ©, 2002, John Wiley & Sons, Inc., Karp/CELL & MOLECULAR BIOLOGY 3E Figure 10.11 Utility of DNA Duplex • Watson - Crick model explains: – Information resides in DNA base sequence; change DNA sequence —> change protein coded for – Chains can separate as H bonds break & serve as template for synthesis of new strand —> at the end, of replication get 2 strands identical to each other & the original DNA molecule • According to the proposal, each new DNA double helix would contain one strand from original DNA molecule & one newly synthesized strand • A semiconservative mode of replication Copyright, ©, 2002, John Wiley & Sons, Inc., Karp/CELL & MOLECULAR BIOLOGY 3E DNA supercoiling • Jerome Vinograd et al. (Caltech, 1963) – found that 2 closed DNA circles of identical molecular mass could show very different sedimentation rates – Fast sedimenting DNA had more compact shape because molecule was twisted (supercoiled) upon itself (rubber band, phone cord) – Occupies less volume; moves more rapidly in response to centrifugal force or electrical field Copyright, ©, 2002, John Wiley & Sons, Inc., Karp/CELL & MOLECULAR BIOLOGY 3E DNA supercoiling • Relaxed: 10 base pairs per turn – Fuse two ends of strands to form circle without twisting: relaxed – DNA is underwound = negatively supercoiled – Circular DNA in nature is usually negatively supercoiled – Positively supercoiled DNA is overwound Copyright, ©, 2002, John Wiley & Sons, Inc., Karp/CELL & MOLECULAR BIOLOGY 3E DNA supercoiling • Supercoiling also occurs in eukaryotic DNA – Linear DNA around protein cores (nucleosomes) also supercoils – Negative supercoiling allows compaction – Long DNA’s fit into microscopic cell nucleus – Underwinding helps to separate the 2 strands of the helix – Aids both replication & transcription Copyright, ©, 2002, John Wiley & Sons, Inc., Karp/CELL & MOLECULAR BIOLOGY 3E Copyright, ©, 2002, John Wiley & Sons, Inc., Karp/CELL & MOLECULAR BIOLOGY 3E Figure 10.13 Topoisomerases • Type I topoisomerases – – create transient break in one strand, & then – allows a controlled rotation, which relaxes the molecule – Prevents supercoiling “build-up” during replication, transcription Copyright, ©, 2002, John Wiley & Sons, Inc., Karp/CELL & MOLECULAR BIOLOGY 3E Topoisomerases • Type II topoisomerases – – create transient break in both duplex strands, then – pass double stranded DNA segment through break – reseal severed strands to freeze structure – can also tie DNA molecule into knots, untie knots, – interlink independent circles (catenation) or separate Copyright, ©, 2002, John Wiley & Sons, Inc., Karp/CELL & MOLECULAR BIOLOGY 3E Copyright, ©, 2002, John Wiley & Sons, Inc., Karp/CELL & MOLECULAR BIOLOGY 3E Figure 10.14a-c Topoisomerases • Topoisomerase II is required to unlink DNA molecules before duplicated chromosomes can be separated during mitosis • Human topoisomerase II is target for a number of drugs (etoposide, doxorubicin) used to kill rapidly dividing cancer cells; drugs bind to enzyme & keep cleaved DNA strands from resealing Copyright, ©, 2002, John Wiley & Sons, Inc., Karp/CELL & MOLECULAR BIOLOGY 3E