DNA and Molecular Structure of Chromosome PDF

Chemical Components of Cells. DNA and Chromosomes Principles of Genetics, 6th Ed., by Snustad & Simmons, 2012, Ch. 9 (pp. 192-211) Principles of Genetics, 6th Ed., by Snustad & Simmons, 2012, Ch. 8 (pp. 173-175)  Discoveries of Inheritance and Nuclein Gregor Mendel compl...

Chemical Components of Cells. DNA and Chromosomes Principles of Genetics, 6th Ed., by Snustad & Simmons, 2012, Ch. 9 (pp. 192-211) Principles of Genetics, 6th Ed., by Snustad & Simmons, 2012, Ch. 8 (pp. 173-175)  Discoveries of Inheritance and Nuclein Gregor Mendel completed his experiments with peas in 1864 and published in 1866 under the title “Experiments in Plant Hybridization,” endeavored to explain how the characteristics of organisms are inherited. In 1868, Johann Friedrich Miescher, a young Swiss medical student, became fascinated with an acidic substance that he isolated from pus. After the pepsin treatment (a proteolytic enzyme that he isolated from the stomachs of pigs), he recovered an acidic substance that he called “nuclein.” 1940s – found the existence of polynucleotide chains, the key component of the acidic material in Miescher’s nuclein. 1944 – found the role of nucleic acids in storing and transmitting genetic information. 1953 – found the double-helix structure of DNA  Several lines of indirect evidence suggested that DNA harbors the genetic information of living organisms Most of the DNA of cells is located in the chromosomes (early genetic studies established a precise correlation between the patterns of transmission of genes and the behavior of chromosomes during sexual reproduction, providing strong evidence that genes are usually located on chromosomes), whereas RNA and proteins are also abundant in the cytoplasm The molecular composition of the DNA is the same (with rare exceptions*) in all the cells of an organism, whereas the composition of both RNA and proteins is highly variable from one cell type to another DNA is more stable than RNA or proteins. Since the genetic material must store and transmit information from parents to offspring, we might expect it to be stable, like DNA. Although these correlations strongly suggest that DNA is the genetic material, they by no means prove it Colony phenotypes of the two strains of Streptococcus pneumoniae studied by Griffith in 1928. Griffith’s discovery of transformation in Streptococcus pneumoniae Sia and Dawson’s demonstration of transformation in Streptococcus pneumoniae in vitro – showing that the mice played no role in the transformation process Avery, MacLeod, and McCarty’s proof that the “transforming principle” is DNA Hershey and Chase’s demonstration that the genetic information of bacteriophage T2 resides in its DNA - 1952 Bacteriophage T2, which infects the common colon bacillus Escherichia coli, is composed of about 50 percent DNA and about 50 percent protein Prior experiments had shown that all bacteriophage T2 reproduction takes place within E. coli cells. Therefore, when Hershey and Chase showed that the DNA of the virus particle entered the cell, whereas most of the protein of the virus remained adsorbed to the outside of the cell, the implication was that the genetic information necessary for viral reproduction was present in DNA The basis for the Hershey–Chase experiment is that DNA contains phosphorus but no sulfur, whereas proteins contain sulfur but virtually no phosphorus The genetic material of tobacco mosaic virus (TMV) is RNA, not protein. TMV contains no DNA; it is composed of just RNA and protein Structures of the four common deoxyribonucleotides present in DNA Structure of a polynucleotide chain The four major players—Francis Crick, Maurice Wilkins, James Watson, and Rosalind Franklin (clockwise from top left)—in the discovery of the double-helix structure of DNA  DNA STRUCTURE: THE DOUBLE HELIX One of the most exciting breakthroughs in the history of biology occurred in 1953 when James Watson and Francis deduced the correct structure of DNA. Their double-helix model of the DNA molecule immediately suggested an elegant mechanism for the transmission of genetic information. Watson and Crick’s double-helix structure was based on two major kinds of evidence: 1. When Erwin Chargaff and colleagues analyzed the composition of DNA from many different organisms, they found that the concentration of thymine was always equal to the concentration of adenine and the concentration of cytosine was always equal to the concentration of guanine. Their results strongly suggested that thymine and adenine as well as cytosine and guanine were present in DNA in some fixed interrelationship. Their data also showed that the total concentration of pyrimidines (thymine plus cytosine) was always equal to the total concentration of purines (adenine plus guanine). 2. When X rays are focused through fibers of purified molecules, the rays are deflected by the atoms of the molecules in specific patterns, called diffraction patterns, which provide information about the organization of the components of the molecules. These X-ray diffraction patterns can be recorded on X-ray-sensitive film just as patterns of light can be recorded with a camera and light-sensitive film. Watson and Crick used X-ray diffraction data on DNA structure provided by Maurice Wilkins, Rosalind Franklin. Photograph of the X-ray diffraction pattern obtained with DNA Diagram of a DNA double helix Space-filling diagram of a DNA double helix. A visual definition of negatively supercoiled DNA  Chromosome Structure in Prokaryotes and Viruses The smallest known RNA viruses have only three genes, and the complete nucleotide sequences of the genomes of many viruses are known. For example, the single RNA molecule in the genome of bacteriophage MS2 consists of 3569 nucleotides and contains 4 genes. The smallest known DNA viruses have only 9 to 11 genes. Again, the complete nucleotide sequences are known in several cases. For example, the genome of bacteriophage X174 is a single DNA molecule 5386 nucleotides in length that contains 11 genes. The largest DNA viruses, like bacteriophage T2 and the animal pox viruses, contain about 150 genes. Bacteria like E. coli have 2500 to 3500 genes, most of which are present in a single molecule of DNA. Diagram of the structure of the functional state of the E. coli chromosome  CHEMICAL COMPOSITION OF EUKARYOTIC CHROMOSOMES Interphase chromosomes are usually not visible with the light microscope. However, chemical analysis, electron microscopy, and X-ray diffraction studies of isolated chromatin (the complex of the DNA, chromosomal proteins, and other chromosome constituents isolated from nuclei) have provided valuable information about the structure of eukaryotic chromosomes Chemical analysis of isolated chromatin shows that it consists primarily of DNA and proteins with lesser amounts of RNA. The proteins are of two major classes: (1) basic (positively charged at neutral pH) proteins called histones and (2) a heterogeneous, largely acidic (negatively charged at neutral pH) group of proteins collectively referred to as nonhistone chromosomal proteins. Histones play a major structural role in chromatin The histones of all plants and animals consist of five classes of proteins. These five major histone types, called H1, H2a, H2b, H3, and H4, are present in almost all cell types The five histone types are present in molar ratios of approximately 1 H1:2 H2a:2 H2b:2 H3:2 H4. Four of the five types of histones are specifically complexed with DNA to produce the basic structural subunits of chromatin, small ellipsoidal beads called nucleosomes  The chemical composition of chromatin as a function of the total nuclear content The DNA and histone contents of chromatin are relatively constant, but the amount of nonhistone proteins present depends on the procedure used to isolate the chromatin (dashed arrow) Electron micrograph (a) and low-resolution diagram (b) of the beads-on-a-string nucleosome substructure of chromatin isolated from interphase nuclei Diagrams of the gross structure of (a) the nucleosome core and (b) the complete nucleosome The nucleosome core contains 146 nucleotide pairs wound as 1.65 turns of negatively supercoiled DNA around an octamer of histones—two molecules each of histones H2a, H2b, H3, and H4 The complete nucleosome contains 166 nucleotide pairs that form almost two superhelical turns of DNA around the histone octamer. One molecule of histone H1 is thought to stabilize the complete nucleosome Electron micrograph (a) and cryoelectron micrographs (b) of the 30-nm chromatin fibers in eukaryotic chromosomes Electron micrograph of a human metaphase chromosome showing the presence of 30-nm chromatin fibers  Electron micrograph of a human metaphase chromosome from which the histones have been removed A huge pool of DNA surrounds a central “scaffold” composed of nonhistone chromosomal proteins Note that the scaffold has roughly the same shape as the metaphase chromosome prior to removal of the histones Also note the absence of ends of DNA molecules in the halo of DNA surrounding the scaffold  Diagram showing the different levels of DNA packaging in chromosomes The 2-nm DNA molecule is first condensed into 11-nm nucleosomes, which are further condensed into 30-nm chromatin fibers The 30-nm fibers are then segregated into supercoiled domains or loops via their attachment to chromosome scaffolds composed of nonhistone chromosomal proteins DNA separation by gel electrophoresis Pulsed-field gel electrophoresis (for very longDNAs) Schematic of electrophoretic separation of DNAtopoisomers Separation of relaxed and supercoiled DNA by gel electrophoresis Intercalation of ethidium into DNA Thank You