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Biology 1090 Textbook Notes (final exam)

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University of Guelph
BIOL 1090
Marc Coppolino

Biology 1090 Textbook Notes Only Final ReviewChapter 9 197214 DNA and Molecular Structure of ChromosomesFunctions of the Genetic Material Genetic material performs these three essential functions 1 The genotypic function replication The genetic material must store genetic info and accurately transmit that info from parents to offspring generation after generation 2 The phenotypic function gene expression The genetic material must control the development of the phenotype of the organism That is the genetic material must dictate the growth of the organism from the single celled zygote to a mature adult 3 The evolutionary function mutation The genetic material must undergo changes to produce variations that allow organisms to adapt to modifications in the environment so that evolution can occur Chromosomes are made of proteins and nucleic acids DNA and RNA nucleic acids Proof that Genetic Information is Stored in DNA The genetic information of most living organisms is stored in DNA In some viruses the genetic info is present in RNA DNA is in the chromosomes while RNA and proteins are in the cytoplasm Most somatic cells of diploid organisms contain twice the amount of DNA as the haploid germ cells gametes of the same species The molecular composition of DNA is the same in all cells of the organism but the RNA and proteins are variable DNA is more stable Proof that DNA mediates transformation When Griffith injected both heat killed Type IIIS bacteria virulent when alive and live type IIR bacteria avirulent into mice and many of them died and the live type IIIS cells were recovered from their bodies Transforming principle something from the heat killed cells had converted the live type IIIR cells to type IIIS cells The mice played no role in the transformation it is in the DNA In separate experiments highly purified DNA from type IIIS cells was treated with enzymes 1 deoxyribonuclease which degrades DNA 2 ribonuclease or 3 protease the DNA was then tested for its ability to transform type IIR to IIIS Only DNase treatment had an effect eliminated the transforming activity Proof that DNA carries the genetic information in bacteriophage T2 Results of Hershey and Chase genetic information of a particular bacterial virus bacteriophage T2 was present in DNA Viruses are the smallest living organism only living in a sense that their reproduction is controlled by genetic information stored in nucleic acids They can only reproduce with a host acellular parasites Dependent on the metabolic machinery of the host They showed that the DNA of a virus particle entered the cell where most of the protein stayed outside The genetic info necessary for viral reproduction was present in DNA Their experiments contained DNA with phosphorous but no sulfur and proteins vice versa They could then label the phage DNA by growth in a medium containing radioactive phosphorus or opposite Thus when put in a blender the S labeled phage particles were mixed with E coli and most of the proteins could be removed without affecting progeny phage production DNA however was not subject to removal by shearing Indicated that DNA of the virus enters the cell because P was found in the cells Proof that RNA stores the genetic information in some viruses RNA viruses store their genetic information in nucleic acids rather than proteins FraenkelConrat experiment was done with tobacco mosaic virus a small virus composed of a single molecule of RNA encapsuled in a protein coat Treated them with chemicals that dissociate the protein coats of the viruses from the RNA moleculs and separated the proteins from the RNA They mixed the proteins from one strain with the RNA molecules from another which they found infective viruses with proteins from one strain and RNA from the other When tobacco leaves were infected the progeny viruses were always pheno and genotypically identical to the parent strain from which the RNA had been obtained Thus genetic info is stored in RNA and not proteinStructures of DNA and RNA Nature of the Chemical Subunits in DNA and RNA Nucleic acids are macromolecules composed of a phosphate group and a five carbon sugar pentose and a cyclic nitrogen containing compound called a base Four different bases adenosine guanine cytosine and thymine uracil in RNA Adenine and guanine are purines and cytosine and thymine are pyrimidines RNA usually exists as a single stranded polymer composed of a long sequence of nucleotides DNA is a right handed double stranded helix discovered by Watson and Crick 5 to 3 end Xray diffraction and concentrations Each of the two polynucleotide chains in a double helix consists of a sequence of nucleotides linked together by phosphodiester bonds joining adjacent deoxyribose moieties The two polynucleotide stands are held together in their helical shape by hydrogen bonding between bases in opposing strands resulting base pairs are stacked between the two chains perpendicular to the axis of molecule A and T and G and C Adenine and thymine form two H bonds and guanine and cytosine form 3 H bonds Hydrogen bonding is not possible between other base pairs Once the sequence of bases in one strand of a DNA double helix is known the other strand is also known because of base pairing The complementarity of the two strands of the double helix makes DNA uniquely suited to store and transmit genetic information from generation to generationThe base pairs are stacked about 034nm apart with 10 base pairs per 360 degrees The sugarphosphate backbones of the two complementary strands are antiparallel Unidirectionally along a double helix the phosphodiester bonds in one strand go from 3 to 5 carbon of the adjacent nucleotide and the other is vice versa This opposite polarity is importantStability of the double helix is a result of the hydrogen bonding and the hydrophobic regions withinWatson and cricks DNA is called BDNA which is the conformation that DNA takes place under physiological conditions in aq solutions containing low concentrations of salt DNA is not a static invariant molecule though and has many forms In high concentrations of salts or in the dehydrated state DNA exists as ADNA This DNA is a right handed double helix but has 11 nucleotides per turn instead It is also shorter and thicker and DNA molecules never exist as this in vivo Left handed double helixes ar ZDNA It has 12 nucleuotides per turn with a single deep groove DNA Structure Negative Supercoils In Vivo All the functional DNA molecules present in living cells display a characteristic that they are supercoiled supercoils are introduced into a DNA molecule when one or both strands are cleaved and when the complementary strands at one end are rotated or twister around each other with the other end held fixed in space and thus not allowed to spin This supercoiling causes DNA to collapse into a tightly coiled structure similar to a coiled telephone cord They are introduced and removed from DNA by enzymes It only occurs in DNA with fixed ends that are not free to rotate therefore obviously the ends of the circular DNA molecules present in most prokaryotic chromosomes and in eukaryotic organelles are fixed Introduce supercoil by taking a circular DNA molecule cleaving it then twisting it 360 degrees If we rotate the free end in the same direction as the DNA double helix a positive supercoil will be produced If we rotate the free end in the opposite direction a negative supercoil left handed underwound DNA will result DNA molecules of almost all organisms is negative supercoiled in vivo SummaryDNA double helix with the two strands held together by H bonds between complementary bases AT and GCComplemntarity of the two strands makes DNA uniquely suited to store and transmit genetic infoThe two strands of DNA double helix have opposite polarityRNA usually exists single standed with uracilFunctional DNA molecules in cells are negatively supercoiledChromosome Structure in Prokaryotes and Viruses In most viruses and prokaryotes the single set of genes monoploid is stored in a single chromosome which in turn contains a single molecule of nucleic acid RNA or DNA Folded genome is the functional state of bacterial chromosome The DNA molecules in prokaryotic and viral chromosomes are organized into negatively supercoiled domains Bacterial chromosomes contain circular molecules of DNA segregated into about 50 domains Chromosome Structure in Eukaryotes Most eukaryotes are diploid having two sets of genes one from each parent DNA is packaged into several chromosomes and each chromosome is present in two diploids or more polyploids copies Interphase chromosomes are usually not visible however chemical analysis ad xray diffraction studies of isolated chromatin the complex of the DNA chromosomal proteins and other chromosome constituents isolated from nuclei have provided info about structure When chromatin is isolated from interphase nuclei the individual chromosomes are not recognizable Instead you see an irregular aggregate of nucleoprotein Chromatin consists of DNA and proteins The proteins are basic proteins called histones and a heterogenous largely acidic group referred to as nonhistone chromosomal proteins Histones play a major structural role in chromatin Histone types H1 H2a H2b H3 and H4 are present in almost all cell types In some sperm histones are replaced by another class of small basic proteins called protamines DNA and histones are in equal amounts to eachother 12232 Four of the types are specifically complexed with DNA to produce basic structural subunits of chromatin small ellipsoidal beads called nucleosomes The expose NH3 groups of arginine and lysine allow histone to act as polycations The positive charged side groups on histones are important when interacting with DNA because the negative charged phosphate groups Histones important in chromatin structure and DNA packaging Chemical modifications of histones can alter chromosome structure which can enhance or decrease the level of expression of genes located in the modified chromatinNonhistone chromosomal proteins are likely candidates for roles in regulation the expression of specific genes or gene setsDuring metaphase of meiosis and mitosis the DNA is packaged in a chromosome with a small length Each chromosome contains a single giant molecule of DNA that extends from one end through the centromere all the way to the other end It is highly condensed coiled and foldedWhen isolated chromatin from interphase cells is studied it is found to consist of a series of ellipsoidal beads joined by thin threads Further evidence for a regular periodic packaginh has come from studies of digestion of chromatin with various nucleases Partial digestion of chromatin with these nucleases yielded fragments of DNA in a set of discrete sizes that were integral multiples of the smallest size fragment Chromatin has a repeating structure and the bead is packaged in a nuclease resistant form Nucleosomes are not attacked by nuclease linkers or interbead threads areEndonuclease cleaves DNA internally Nucleosome core nuclease resistant structure that remains after digestion Consists of two molecules of each histone and 146 nucleotide base DNA The histones protect the segment of DNA in the nucleosome core from cleavage by endocnucleasesThe complete chromatin subunit consists of the nucleosome core the linker DNA and the associated nonhistone chromosomal proteins all stabilized by the binding of one molecule of histone 1 to the outside of the structure Complete nucleosome contains two full turns of DNA superhelix on the surface of the histone octamer Stabilized by binding H1 The basic structural component of eukaryotic chromatin is the nucleosome The structure of nucleosomes in transcriptionally active regions of chromatin is known to differ from nucleosomes in transcriptionally inactive regions The tails of some of the histone molecules protrude from the nucleosome and are accessible to enzymes that add and remove chemical groups such as methyl and acetyl groups The addition of these groups can change the level of expression of genes packaged in nucleosomes containing the modified histones
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