Chapter 8 DNA The Chemical Nature of the Gene 8 1 Genetic Material Possesses Several Key Characteristics A Genetic material must contain complex information 1 Must be capable of storing large amounts of information instructions for all traits and 2 The information must have the capacity to vary because species differ in their genetic functions of an organism makeup 3 The genetic material must be stable because most alterations are likely to be detrimental B Genetic material must replicate faithfully 1 Genetic material must have the capacity to be copied accurately 2 At each cell division the genetic instructions must be transmitted to descendant cells with great accuracy 3 When organisms reproduce the coding must be copied with reliability C Genetic Material must encode the phenotype 1 The genotype must have the capacity to code for the phenotypic traits 2 There must be a mechanism for genetic instructions to be translated into the amino acid sequence of a protein 8 2 All Genetic Information Is Encoded in the Structure of DNA A Early Studies of DNA 1 The physical basis of heredity lies in the nucleus a Chromatin consists of nucleic acids and proteins 2 Nucleotides linked repeating units containing a sugar a phosphate and a base 3 Chargaff s rules the amount of adenine is always equal to the amount of thymine A T and the amount of guanine is always equal to the amount of cytosine G C B DNA As the Source of Genetic Information 1 Genes reside on chromosomes which were known to contain both DNA and protein 2 Transforming principle acquiring genetic virulence that causes a permanent genetic change in the bacteria Some substance in the polysaccharide coat of the dead bacteria might be responsible Isotopes radioactive forms of DNA and proteins It can be used to identify the location of a specific molecule because any molecule containing the isotope is radioactive 3 4 Experiments confirmed that DNA not protein is the genetic material of phages C Watson and Crick s Discovery of the Three Dimensional Structure of DNA 1 X ray diffraction X rays beamed at a molecule are reflected in specific patterns that reveal aspects of the structure of the molecule 8 3 DNA Consists of Two Complementary and Antiparallel Nucleotide Strands That Form a Double Helix A The Primary Structure of DNA 1 Nucleotides i A string of nucleotides joined together by phosphodiester linkages a DNA is typically a very long molecule and is therefore termed a macromolecule It is a polymer a chain made up of repeating units linked together b c The repeating units are nucleotides each comprising three parts a sugar a phosphate and a nitrogen containing base d Sugar i The sugars are pentose sugars with five carbon atoms ii Ribose RNA s sugar has a hydroxyl group OH attached to the 2 carbon chain iii Deoxyribose DNA s sugar has a hydrogen sugar at this position and therefore contains one oxygen atom fewer overall e Nitrogenous base which may be two different types i Purine consists of a six sided ring attached to a five sided ring only 1 Both DNA and RNA contain two purines a Adenine and Guanine A and G which differ in the positions of their double bonds and in the groups attached to the six sided ring ii Pyrimidine a six sided ring 1 Three are common in nucleic acids a Cytosine C thymine T and uracil U b Cytosine is present in both DNA and RNA c Thymine is found in DNA d Uracil is only found in RNA iii Nucleoside a deoxyribose or a ribose sugar and a base together iv Phosphate Group consists of a phosphorus atom bonded to four oxygen atoms 1 They are found in every nucleotide and frequently carry a negative charge making DNA acidic It is always bonded to the 5 carbon atom 2 v Deoxyribonucleotides DNA nucleotides because there are four types of bases there are four different kinds of DNA nucleotides vi Ribonucleotides equivalent with RNA They sometimes contain additional rare bases which are modified forms of four common bases f Polynucleotide strands i Phosphodiester linkages nucleotides connected by covalent bonds which join the 5 phosphate group of one nucleotide to the 3 carbon atom of the next nucleotide ii Polynucleotide strand a series of nucleotides linked together by stron covalent bonds 1 An important characteristic is its direction or polarity 2 5 end a free phosphate group is attached to one 5 carbon atom of the sugar in the nucleotide 3 3 end a free OH group is attached to the 3 carbon atom B Secondary Structures of DNA a The double helix i Antiparallel two polynucleotide strands wound around each other in opposite directions meaning that the 5 end of one strand is opposite to the 3 end of the other strand ii Held together by hydrogen bonds that link the bases on opposite strands and an interaction between the stacked base pairs iii Complementary DNA strands the two polynucleotide stands of a DNA molecule are not identical but complementary b Different secondary structures i B DNA structure exists when plenty of water surrounds the molecule and there is no unusual base sequence in the DNA conditions that are likely to be present in cells 1 Most stable of random sequence of nucleotides 2 An alpha helix clockwise spiral with 10 b p ii A DNA exists if less water is present 1 An alpha helix short and wide iii Z DNA forms a left handed helix 1 The sugar phosphate backbone zigags 8 4 Large amounts of DNA Are Packed into a Cell A Supercoiling takes place when the DNA is subjected to strain by being overwound or underwound B Relaxed State The lowest energy state for B DNA w a Positive supercoiling overrotated molecules b Negative supercoiling underrotated molecules c Supercoiling takes place when the strain of overrotaing or underrotating cannot be compensated by the turning of the ends of the double helix which is he case if the DNA is circular d Topoisomerases enzymes that add or remove from the DNA helix by temporarily breaking nucleotide strand rotating ends around each other and then rejoining the broken ends i Can either induce or relive supercoiling C Most DNA found in cells is negatively supercoiled which has two advantages over nonsupercoiled DNA and transcription a Supercoiling makes the separation of the two strands of DNA easier during replication b Supercoiled DNA can be packed into a smaller space than relaxed DNA 8 5 A Bacterial Chromosome Consists of a Single Circular DNA Molecule A Bacterial DNA is not attached to histone proteins as is eukaryotic DNA but bacterial DNA is complexed to a number of proteins
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