10_16_lecture.pptx

Chapter 16 The Structural Basis of Cellular Information: DNA, Chromosomes, and the Nucleus Lectures by Kathleen Fitzpatrick © 2016 Pearson Education, Inc. Simon Fraser University 16.2 DNA Structure © 2016 Pearson Education, Inc. Chargaff’s Rules Reveal That A = T and G = C  Erwin Chargaff stu...

Chapter 16 The Structural Basis of Cellular Information: DNA, Chromosomes, and the Nucleus Lectures by Kathleen Fitzpatrick © 2016 Pearson Education, Inc. Simon Fraser University 16.2 DNA Structure © 2016 Pearson Education, Inc. Chargaff’s Rules Reveal That A = T and G = C  Erwin Chargaff studied base composition of DNA and quantify the relative amounts of the four bases  He showed that the DNA from different cells of a given species has the same percentage of each of the four bases  The base composition varies among species © 2016 Pearson Education, Inc. Chargaff’s Most Striking Observation  Chargaff observed that for all DNA samples examined, the amount of A = T, and the amount of G=C  These are called Chargaff’s rules  The significance was not understood until Watson and Crick proposed the double-helix model for DNA structure © 2016 Pearson Education, Inc. Watson and Crick Discovered That DNA Is a Double Helix  James Watson and Frances Crick built wire models to try to determine the structure of DNA that agreed with everything known about DNA  It was known that DNA had a sugar phosphate backbone with nitrogenous bases attached to each sugar  It was known that at physiological pH, the bases would be able to form hydrogen bonds with each other © 2016 Pearson Education, Inc. The Double Helix Model  The critical evidence came from X-ray diffraction data produced by Rosalind Franklin  It revealed that DNA was a long thin helical molecule  Based on this information and other observations, Watson and Crick produced the double helix model © 2016 Pearson Education, Inc. The Watson-Crick Model  There are 10nt pairs per turn, & 0.34 nm per nucleotide pair  The 2-nm diameter of the helix is just right for one purines and one pyrimidine © 2016 Pearson Education, Inc. The Double Helix Model (continued)  The purine-pyrimidine pairing is consistent with Chargaff’s rules  The two strands are held together by hydrogen bonding between bases on opposite strands  The hydrogen bonds fit within the helix only when they form between complementary bases: adenine with thymine and guanine with cytosine © 2016 Pearson Education, Inc. Replication of Genetic Information  The most important aspect of the double helix model was that it suggested a mechanism for replication of DNA  The two strands could separate so that each could act as a template to dictate synthesis of a new complementary strand  This observation was made in the original paper too © 2016 Pearson Education, Inc. Key Features of DNA Structure  Strands of DNA form a major groove and a minor groove  The phosphodiester bonds are oriented in opposite directions in the two DNA strands  This is called antiparallel orientation © 2016 Pearson Education, Inc. Measuring DNA Length  DNA length is measured in base pairs (bp)  Larger stretches are measured in multiples of a single base pair—for example, the kilobase (kb) is 1000 bp © 2016 Pearson Education, Inc. Structural Variants of DNA  The right-handed helix is called BDNA  Flexible, depending on nucleotide sequence; it is the main form of DNA  Z-DNA is a lefthanded helix;  its biological significance is not well understood © 2016 Pearson Education, Inc. DNA Can Be Interconverted Between Relaxed and Supercoiled Forms  The DNA double helix can be twisted upon itself to form supercoiled DNA  Twisted DNA in the same direction is positive supercoil  Twisted DNA in the opposite direction is negative supercoil © 2016 Pearson Education, Inc. Supercoiling  Supercoiling occurs in both linear and circular DNA molecules but is more easily studied in circular DNA  A DNA molecule can go back and forth between the supercoiled state and the nonsupercoiled, or relaxed, state  Extensive supercoiling helps make chromosomal DNA more compact © 2016 Pearson Education, Inc. Interconversion Between Relaxed and Supercoiled DNA  Topoisomerases both induce and relax supercoils  Type I topoisomerases: introduce single-strand breaks in DNA  Type II topoisomerases: introduce double-strand breaks; one example in bacteria is DNA gyrase © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. The Two Strands of a DNA Double Helix Can Be Denatured and Renatured  Strand separation (DNA denaturation or melting) can be induced experimentally by raising temperature or pH  All DNA absorbs light, with a maximum around 260 nm © 2016 Pearson Education, Inc. Renaturation  Reformation of the DNA double helix is called DNA renaturation (reannealing); it is accomplished by lowering the temperature to permit hydrogen bonds to reform © 2016 Pearson Education, Inc. 16.3 DNA Packaging  Very long molecules of DNA must be fit into the cell and, in the case of eukaryotes, into the nucleus  DNA packaging is a challenge for all forms of life © 2016 Pearson Education, Inc. Bacteria Package DNA in Bacterial Chromosomes and Plasmids  Bacterial chromosomes were once thought to be naked DNA  However, it is now known that the DNA is packaged somewhat similarly to the chromosomes of eukaryotes  The main bacterial genome is called the bacterial chromosome © 2016 Pearson Education, Inc. Bacterial Chromosomes  Bacteria have single, multiple, linear, or circular chromosomes depending on the species, but a single circular chromosome is most common  The DNA molecule is bound to small amounts of protein and localized to a region of the bacterial cell called the nucleoid  The bacterial DNA is negatively supercoiled and folded into loops  The loops of bacterial DNA are held in place by RNAs and proteins © 2016 Pearson Education, Inc. Bacterial Plasmids  Bacteria may contain one or more plasmids  Small, usually circular DNA molecules containing genes for their own replication  They may also carry genes for cellular functions © 2016 Pearson Education, Inc. Types of Plasmids  Virulence factors enhance the ability to cause disease by producing toxic proteins  Metabolic plasmids produce enzymes required for certain metabolic reactions  Cryptic plasmids have no known function © 2016 Pearson Education, Inc. Eukaryotes Package DNA in Chromatin and Chromosomes  In eukaryotes, there is more DNA per cell, and it interacts with more proteins  When bound to proteins, DNA is called chromatin  At the time of division, the chromatin fibers condense into a more compact structure, the chromosome © 2016 Pearson Education, Inc. Histones  Histones are a group of small basic proteins with high lysine and arginine content  The negatively charged DNA binds stably to the positively charged proteins  The mass of histones in a chromosome is approximately equal to the mass of the DNA © 2016 Pearson Education, Inc. Nucleosomes Are the Basic Unit of Chromatin Structure  When isolated from cells, chromatin fibers appear as a series of tiny particles attached by thin filaments (“beads-on-a-string”)  The “beads” are called nucleosomes © 2016 Pearson Education, Inc. A Histone Octamer Forms the Nucleosome Core © 2016 Pearson Education, Inc. Nucleosomes Are Packed Together to Form Chromatin Fibers and Chromosomes  Nucleosome formation is the first step in packaging of nuclear DNA  Isolated chromatin measures about 10 nm in diameter, but chromatin of intact cells measures about 30 nm (the 30-nm chromatin fiber)  Histone H1 facilitates formation of the 30-nm fiber © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. Further Packing of Chromatin  The 30-nm fiber seems to be packed together in an irregular, three-dimensional zigzag structure  These fibers fold into DNA loops 50,000–100,000 bp in length, stabilized by cohesin protein  The loops are spatially arranged through attachment to nonhistone proteins that form a chromosomal scaffold © 2016 Pearson Education, Inc.