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BIOLOGY I Chapter 16: THE MOLECULAR BASIS OF INHERITANCE (DNA Structure and Function) Evelyn I. Milian Instructor BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) DNA Is the Genetic Material • What is DNA and why is it so important for life? DNA (deoxyribonucleic acid) is a type of organic macromolecule; it is a nucleic acid composed of subunits called nucleotides (we will study its structure in detail). DNA is the genetic material of nearly all organisms. It is found in eukaryotic and prokaryotic cells. The “molecular instructions” in DNA direct the life of each cell in an organism. DNA stores genetic information regarding the development, structure, and metabolic activities of a cell. DNA also enables organisms, or cells within an organism, to transmit information accurately from one generation to the next. Evelyn I. Milian - Instructor 2 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) The Levels of Structure and Function of the Genome The genome is the sum total of genetic material of a cell. Although most of the genome exists in the form of chromosomes, genetic material can appear in nonchromosomal sites as well. For example, bacteria and some fungi contain tiny extra pieces of DNA called plasmids, and certain organelles of eukaryotes (mitochondria and chloroplasts) are equipped with their own genetic material. Evelyn I. Milian - Instructor 3 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) History: The Search for the Genetic Material How Did Scientists Discover that Genes are Made of DNA? • Early in the twentieth century, scientists knew that the genes are on the chromosomes, but they did not know the composition of genes. The identification of the molecules of inheritance was a major challenge to biologists. • DNA and proteins were the candidates for the genetic material, but proteins seemed stronger because of their complexity and variety. Moreover, little was known about nucleic acids. • Scientists knew that this genetic material must be: 1. able to store information that pertains to the development, structure, and metabolic activities of the cell or organism; 2. stable so that it can be replicated with high fidelity during cell division and be transmitted from generation to generation; 3. able to undergo rare genetic changes called mutations that provide the genetic variability required for evolution to occur. Evelyn I. Milian - Instructor 4 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) History: The Search for the Genetic Material • Previous knowledge about nucleic acids: 1869, Johann Friedrich Miescher: He removed nuclei from pus cells and found a chemical he called nuclein. It was rich in phosphorus and had no sulfur, properties that distinguished it from protein. In this way, he helped paving the way for the identification of DNA as the carrier of inheritance. Later, other chemists did further research with nuclein and said that it contained an acidic substance they called nucleic acid. However, for many years nobody thought about nucleic acids as the genetic material because their physical and chemical properties seemed far too uniform to account for the variety of inherited traits exhibited by every organism. Evelyn I. Milian - Instructor 5 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) The Search for the Genetic Material: How Did Scientists Discover that Genes are Made of DNA? • After several diligent experiments, by the mid-1950s researchers realized that DNA, not protein, is the genetic material. • Experiments with bacteria and viruses that infect bacteria (phages, or bacteriophages) provided the first strong evidence that the genetic material is DNA. • The discovery of how DNA carries life’s blueprints was one of the greatest achievements of 20th century biology. Evelyn I. Milian - Instructor 6 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) Evidence That DNA Can Transform Bacteria (Transformation) • 1928: Frederick Griffith was trying to make a vaccine to prevent bacterial pneumonia. He was working with two strains (variants) of the Streptococcus pneumoniae bacterium (pneumococcus), a pathogenic (disease-causing) strain (S) and a nonpathogenic (harmless) strain (R). The pathogenic S (smooth) strain had a resistant capsule not found in the nonpathogenic R (rough) strain. He found that when he killed the pathogenic bacteria with heat and then mixed the cell remains with living bacteria of the nonpathogenic strain, some of the living cells became pathogenic. Furthermore, this new trait of pathogenicity was inherited by all the descendants of the transformed bacteria. He concluded that some chemical component of the dead pathogenic cells caused this heritable change. Evelyn I. Milian - Instructor 7 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) 8 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) Evidence That DNA Can Transform Bacteria: Bacterial Transformation • Griffith’s experiments showed that some substance in the heat-killed S strain changed the living but harmless R strain into the deadly S strain. • He called this phenomenon transformation, now defined as a genetic change due to the assimilation of external DNA by a cell. • Griffith’s work set the stage for the search for the identity of the transforming substance by other scientists during the following years. Evelyn I. Milian - Instructor 9 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) Animation: Griffith’s Experiment – Transformation • ..\..\..\BIOLOGY-SOLOMON\BIOLOGY-SOLOMONImages\chapter12\Animations\griffith.html Evelyn I. Milian - Instructor 10 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) Evidence That DNA Can Transform Bacteria • 1944: Oswald Avery, Colin MacLeod, and Maclyn McCarty purified the transforming molecule and published a paper demonstrating that it is DNA. Their evidence included the following observations: 1. DNA from S strain bacteria causes R strain bacteria to be transformed. 2. Enzymes that degrade proteins (proteinases) cannot prevent transformation, nor does RNase, an enzyme that digests RNA (ribonucleic acid). 3. The molecular weight of the transforming substance is so great that it must contain about 1,600 nucleotides! Certainly, this is enough for some genetic variability. Evelyn I. Milian - Instructor 11 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) 12 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) Confirmation of DNA Function: Evidence That Viral DNA Can Program Cells • Additional evidence for DNA as the genetic material came from studies of a virus that infects bacteria, called a bacteriophage or phage. • Virus: A noncellular parasitic agent consisting of an inner core of nucleic acid (DNA or RNA) and an outer coat of protein (capsid). • To reproduce, a virus must infect a cell and take over the cell’s metabolic machinery. 13 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) Evidence That Viral DNA Can Program Cells • 1952: Alfred D. Hershey and Martha Chase performed experiments with phage T2 to determine which of the phage components—protein or DNA—entered bacterial cells and directed reproduction of the virus. T2 infects Escherichia coli, a bacterium that normally lives in the intestines of mammals. • Hershey and Chase relied on a chemical difference between DNA and protein to solve the mystery: DNA contains phosphorus but no sulfur; proteins contain sulfur but no phosphorus. They used radioactive phosphorus to label the DNA core of the phage and radioactive sulfur to label the protein capsid of the phage. In this way, they were able to trace the component that entered the bacterial cells, and they determined that it was DNA. Evelyn I. Milian - Instructor 14 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) Evelyn I. Milian - Instructor 15 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) The Watson and Crick Model for the Structure of DNA • 1953: James Watson and Francis Crick reported their molecular model for DNA: the double helix, for which they received a Nobel Prize in 1962. • Their model conformed to X-ray measurements (done by Rosalind Franklin and Maurice Wilkins) and what was then known about the biochemistry of DNA. Evelyn I. Milian - Instructor 16 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) The Structure of DNA, a Nucleic Acid • Nucleic acids (DNA and RNA) are large organic molecules composed of subunits called nucleotides. Nucleic acids contain carbon, hydrogen, oxygen, nitrogen, and phosphorus. • Nucleic acids store and transmit hereditary information, and are involved in the synthesis of proteins, molecules with many functions. • Nucleotides are compounds that contain these molecules: a phosphate group; a pentose (five-carbon) sugar: deoxyribose in DNA; ribose in RNA; a nitrogenous base: one of four possible bases which are: adenine, cytosine, guanine, and thymine in DNA or uracil in RNA. Nucleotides are linked together by covalent bonds between the phosphate of one nucleotide and the sugar of the next nucleotide. The numbers 1 to 5 are the carbons. Covalent bonds: sharing of two or more electrons (negatively charged atomic particles). Evelyn I. Milian - Instructor 17 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) The Nucleotides in Nucleic Acids • Nucleotides contain: 1. A phosphate group. 2. A pentose (five-carbon) sugar: deoxyribose in DNA; ribose in RNA. 3. A nitrogenous base: adenine, guanine, cytosine, and thymine in DNA or uracil in RNA. • A and G are purines, with a double ring. • T and C are pyrimidines, with a single ring. Evelyn I. Milian - Instructor 18 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) Evelyn I. Milian - Instructor 19 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) The Structure of DNA (Deoxyribonucleic Acid) • 1940s: Erwin Chargaff analyzed the amounts of the four nucleotides in DNA from diverse organisms. He provided additional evidence that DNA is the genetic material. • The result was Chargaff’s rules: 1. The amount of A, T, G, and C in DNA varies from species to species. 2. In each species, the amount of A equals the amount of T and the amount of G equals the amount of C. Evelyn I. Milian - Instructor 20 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) The Structure of DNA: The Watson and Crick Model • The structure of a DNA molecule consists of two long strands of nucleotides wrapped around each other to form a double helix, (like a twisted ladder, or spiral) and oriented in antiparallel (opposite) directions (the sugar-phosphate groups are oriented in different directions). • Within each strand, the sugar (deoxyribose) of one nucleotide is linked to the phosphate of the next nucleotide, forming a sugarphosphate “backbone” on each side of the double helix. • The two DNA strands are held together by hydrogen bonds between complementary nitrogenous base pairs (purine with pyrimidine) (like the rungs or steps of the ladder). A—T: Adenine (a purine,with a double ring) forms hydrogen bonds only with thymine (a pyrimidine, with one ring). G—C: Guanine (a purine) forms hydrogen bonds only with cytosine (a pyrimidine). Evelyn I. Milian - Instructor 21 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) The Structure of a DNA Strand • Each nucleotide monomer consists of a nitrogenous base (T, A, C, or G), the sugar deoxyribose (blue), and a phosphate group (yellow). The phosphate group of one nucleotide is attached to the sugar of the next, resulting in a “backbone” of alternating phosphates and sugars from which the bases project. • The polynucleotide strand has directionality, from the 5’ end (with the phosphate group) to the 3’ end (with the –OH group). 5’ and 3’ refer to the numbers assigned to the carbons in the sugar ring. Evelyn I. Milian - Instructor 22 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) Watson and Crick’s Model of DNA a. A space-filling model of DNA. b. The two strands of the molecule are antiparallel— that is, the sugar-phosphates are oriented in different directions: The 5’ end of one strand is opposite the 3’ end of the other strand. c. Diagram of DNA double helix shows that the molecule resembles a twisted ladder or a spiral. The bases are joined by hydrogen bonds. Evelyn I. Milian - Instructor 23 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) DNA, a Nucleic Acid • Nucleotides (top) are composed of a deoxyribose (in DNA) sugar molecule linked to a phosphate group and to a nitrogenous base. The two nucleotides shown here are linked by hydrogen bonds between their complementary bases. • The ladderlike form of DNA’s double helix (bottom) is made up of many nucleotides, with the repeating sugarphosphate combination forming the backbone and the complementary bases the rungs. Evelyn I. Milian - Instructor 24 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) The Structure of DNA: The Watson and Crick Model • The Watson and Crick model for DNA fit the mathematical measurements provided by X-ray data for the spacing between the base pairs (0.34 nm) and for a complete turn of the double helix (3.4 nm). • The model also agreed with Chargaff’s rules, which said that the amount of A equals the amount of T and the amount of G equals the amount of C. A is hydrogen-bonded to T and G is hydrogen-bonded to C. This socalled complementary base pairing means that a purine is always bonded to a pyrimidine. Only in this way will the molecule have the width (2 nm) dictated by its X-ray diffraction pattern, since two pyrimidines together are too narrow, and two purines together are too wide. Evelyn I. Milian - Instructor 25 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) The Structure of DNA: The Watson and Crick Model • In a DNA double-helix, the two sugar-phosphate chains run in opposite directions. This orientation permits the complementary bases to pair. • Strong covalent bonds link the units of each strand, while weaker hydrogen bonds hold one strand to the other by the pairs of nitrogenous bases. • Adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). • The A-T pair has two hydrogen bonds, the G-C pair has three. Evelyn I. Milian - Instructor 26 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) The Structure of DNA: The Watson and Crick Model • The two upright strands, composed of the sugar deoxyribose (D) and phosphate groups (P), are held together by hydrogen bonds between complementary bases. Adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). Each strand can thus provide the information needed for the formation of a new DNA molecule. The A-T pair has two hydrogen bonds, the G-C pair has three hydrogen bonds. • The DNA molecule is twisted into a double helix. The two sugarphosphate strands run in opposite directions (antiparallel). Each new strand grows from the 5’ (“five prime”) end toward the 3’ end. Evelyn I. Milian - Instructor 27 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) Evelyn I. Milian - Instructor 28 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) The DNA Double Helix • Note: The “ribbons” in this diagram represent the sugar-phosphate backbones of the two DNA strands. The helix is “right-handed”, curving up to the right. • Denaturation of DNA, the separation of the two strands of the double helix, occurs under extreme (noncellular) conditions of pH (acidity level), salt concentration, and temperature. • For example, when a double-stranded DNA molecule is heated, it denatures into two singlestranded molecules. The heat breaks the hydrogen bonds holding the bases together in the center of the molecule but does not affect the covalent bonds of the backbone. Evelyn I. Milian - Instructor 29 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) Animation: DNA Structure: Subunits • ..\..\..\BIOLOGY-SOLOMON\BIOLOGY-SOLOMONImages\chapter12\Animations\dna_subunits_adv.html Evelyn I. Milian - Instructor 30 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) 31 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) DNA REPLICATION • DNA replication is the process of copying a DNA molecule. Following replication, there is usually an exact copy of the DNA double helix. • DNA replication must occur before a cell can divide (Remember: During the S phase of the cell cycle). • As soon as Watson and Crick developed their double-helix model, they commented, “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” Evelyn I. Milian - Instructor 32 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) REPLICATION OF DNA • In a second paper, Watson and Crick stated their hypothesis for how DNA replicates: “Now our model for deoxyribonucleic acid is, in effect, a pair of templates, each of which is complementary to the other. We imagine that prior to duplication the hydrogen bonds are broken, and the two chains unwind and separate. Each chain then acts as a template for the formation onto itself of a new companion chain, so that eventually we shall have two pairs of chains, where we only had one before. Moreover, the sequence of pairs of bases will have been duplicated exactly.” * • * F. H. C. Crick and J. D. Watson, “The Complementary Structure of Deoxyribonucleic Acid.” Proc. Roy. Soc. (A) 223 (1954): 80. Evelyn I. Milian - Instructor 33 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) Proposed Models of DNA Replication • Conservative model: The parent molecule somehow re-forms after the replication (it is conserved); in other words, the two parental strands are rejoined. • Semiconservative model: Each of the two daughter molecules of DNA will have one old strand derived from the parent molecule, and one newly synthesized strand. • Dispersive model: Each strand of DNA following replication has a mixture of old and new DNA. Evelyn I. Milian - Instructor 34 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) Meselson & Stahl Experiment: Semiconservative DNA Replication • • • Semiconservative replication was experimentally confirmed by Matthew Meselson and Franklin Stahl in 1958. They showed that each daughter double helix contains an old strand and a new strand. Meselson and Stahl grew bacteria in heavy nitrogen (15N) medium to label the bases of DNA, making them more dense; and then transferred some of the cells to light nitrogen (14N, the ordinary one) so that the newly synthesized strands would be light. They isolated DNA from bacterial cells after one and two generations and centrifuged it to separate DNA into bands based on density. After one division in light nitrogen, DNA molecules are hybrid with intermediate density. After two divisions, DNA molecules separate into two bands—one for light DNA and one for hybrid DNA, consistent with the semiconservative model of replication. Evelyn I. Milian - Instructor 35 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) Semiconservative Replication of DNA • Semiconservative nature of DNA replication: During DNA replication, each parent strand remains intact. One new strand (gold) is assembled on each of the parent strands (blue). In other words, each of the two daughter molecules of DNA will have one old strand derived from the parent molecule, and one newly synthesized strand. Evelyn I. Milian - Instructor 36 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) * SUMMARY OF STEPS IN THE REPLICATION OF DNA * • During DNA replication, each old DNA strand of the parental molecule (original double helix) serves as a template (mold) for a new strand in a daughter molecule. • Replication requires the following steps or stages: 1. Uncoiling (unwinding). The old strands that make up the parental DNA molecule are unwound (or separated) and “unzipped” (i.e., the weak hydrogen bonds between the paired bases are broken). • 2. 3. • A special enzyme called helicase unwinds the molecule. Synthesis: complementary base pairing and elongation. New complementary nucleotides are positioned through the process of complementary base pairing. Joining and termination. The complementary nucleotides join the template strands to form new strands. Each daughter DNA molecule contains an old strand and a new strand (semiconservative replication). *** Steps 2 and 3 are carried out by an enzyme complex called DNA polymerase. Evelyn I. Milian - Instructor 37 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) A Simplified View of DNA Replication: The Basic Concept • In this simplification, a short segment of DNA has been untwisted into a structure that resembles a ladder. The rails of the ladder are the sugarphosphate backbones of the two DNA strands; the rungs are the pairs of nitrogenous bases. Simple shapes symbolize the four kinds of bases. Dark blue represents DNA strands present in the parent molecule; light blue represents free nucleotides and newly synthesized DNA. Evelyn I. Milian - Instructor 38 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) DNA replication requires protein “machinery” • The replication of DNA by base pairing appears simple and straightforward, but it requires special proteins and enzymes (catalytic proteins that accelerate specific chemical reactions). Evelyn I. Milian - Instructor 39 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) DNA Replication: Basic Steps • After the DNA double helix unwinds (by helicase), each old strand serves as a template for the formation of the new strand. • Complementary nucleotides available in the cell pair with those of the old strand and then are joined together to form a new strand. • After replication is complete, there are two daughter DNA double helices. Each one is composed of an old strand and a new strand. • Each daughter double helix has the same sequence of base pairs as the parental double helix had before unwinding occurred. Evelyn I. Milian - Instructor 40 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) DNA Replication: A Closer Look (* Study the Figures *) • DNA replication begins at a site called an origin of replication and proceeds in both directions from that point (it’s bidirectional). The point where the two DNA strands separate is called the replication fork. The enzyme helicase untwists the double helix at replication forks. Single-strand binding proteins stabilize the unpaired DNA strands. • An RNA primer, a short polynucleotide complementary to the template strand, is synthesized (by primase) and enters the origin of replication. The RNA primer is required as a start point for the addition of nucleotides by DNA polymerase III; it can only add nucleotides to the 3’ end of an existing polynucleotide strand or RNA primer. • DNA polymerase then catalyzes the synthesis of the new DNA strand by adding new nucleotides in the 5’ 3’ direction. • Because of the 5’ 3’ direction of DNA polymerase, DNA replication is continuous in one strand and discontinuous in the other: The leading strand is synthesized continuously, and the lagging strand is synthesized in short segments, called Okazaki fragments. The fragments are joined together by DNA ligase. Evelyn I. Milian - Instructor 41 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) A Closer Look at DNA Replication Evelyn I. Milian - Instructor 42 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) Synthesis of leading and lagging strands during DNA replication 1. DNA polymerase III elongates new DNA strands in the 5’ 3’ direction (it adds nucleotides to the 3’ end of an existing strand). 2. One new strand, the leading strand, can elongate continuously in the 5’ 3’ direction as the replication fork progresses. 3. The other new strand, the lagging strand, must grow in an overall 3’ 5’ direction by addition of short segments, Okazaki fragments, that grow 5’ 3’ (numbered here in the order they were made). 4. DNA ligase joins Okazaki fragments by forming a bond between their free ends. This results in a continuous strand. Evelyn I. Milian - Instructor 43 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) Origins of DNA Replication in Bacteria and Eukaryotes • • Evelyn I. Milian - Instructor In the circular chromosome of E. coli and many other bacteria (prokaryotes), only one origin of replication is present. The parental strands separate at the origin, forming a replication bubble with two forks. Replication proceeds in both directions until the forks meet on the other side, resulting in two daughter DNA molecules. In each linear chromosome of eukaryotes, DNA replication begins when replication bubbles form at many origins along the giant DNA molecule. The bubbles expand as replication proceeds in both directions. Eventually the bubbles fuse and synthesis of the daughter strand is complete. 44 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) Some of the Proteins Involved in the Initiation of DNA Replication • Helicase unwinds and separates the parental DNA strands. • Topoisomerase breaks, swivels, and rejoins the parental DNA ahead of the replication fork, relieving the strain caused by unwinding. • Single-strand binding proteins stabilize the unwound parental strands. • Primase synthesizes RNA primers, using the parental DNA as a template. Evelyn I. Milian - Instructor 45 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) DNA REPLICATION: Incorporation of a Nucleotide into a DNA Strand • • DNA polymerase catalyzes the addition of a nucleoside triphosphate to the 3’ end of a growing DNA strand (a nucleoside triphosphate has a nitrogenous base, a pentose sugar, and three phosphate groups). When a nucleoside triphosphate bonds to the sugar in a growing DNA strand, it loses two phosphates. Hydrolysis of the phosphate bonds provides the energy for the reaction. Evelyn I. Milian - Instructor 46 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) Evelyn I. Milian - Instructor 47 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) • Review the details for this figure in your book (Campbell & Reece, Biology). Evelyn I. Milian - Instructor 48 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) Enzymes and Other Proteins in Bacterial DNA Replication Evelyn I. Milian - Instructor 49 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) Proofreading and Repairing DNA • Mistakes in DNA can result in the altered or diminished function of encoded proteins and thus disrupt normal cell operations. • During DNA replication, DNA polymerases proofread newly made DNA, replacing any incorrect nucleotides. • Mismatched nucleotides sometimes evade proofreading by a DNA polymerase or arise after DNA synthesis is completed. Mismatch repair. Special repair enzymes recognize incorrectly paired nucleotides and remove them. DNA polymerases then fill in the missing nucleotides. Nucleotide excision repair. This repair mechanism is commonly used to repair DNA lesions caused by the sun’s ultraviolet radiation or by harmful chemicals. It involves three enzymes: nucleases cut out (excise) and replace damaged stretches of DNA. DNA polymerase adds the correct nucleotides, and DNA ligase closes the breaks. Evelyn I. Milian - Instructor 50 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) Nucleotide Excision Repair of DNA Damage • A team of enzymes detects and repairs damaged DNA. This figure shows DNA containing a thymine dimer, a type of damage often caused by ultraviolet radiation. A nuclease enzyme cuts out the damaged region of DNA and a DNA polymerase (in bacteria, DNA pol I) replaces it with a normal DNA segment. Ligase completes the process by closing the remaining break in the sugarphosphate backbone. Evelyn I. Milian - Instructor 51 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) Chromatin Packing in a Eukaryotic Chromosome • Diagrams and micrographs (done with a transmission electron microscope) depicting the progressive levels of DNA coiling and folding. Eukaryotic chromatin making up a chromosome is composed mostly of DNA, histones, and other proteins. The histones bind to each other and to the DNA to form nucleosomes, the most basic units of DNA packing. Histone tails extend outward from each bead-like nucleosome core. Additional folding leads ultimately to the highly condensed chromatin of the metaphase chromosome. In interphase cells, most chromatin is less compacted (euchromatin), but some remains highly condensed (heterochromatin). Histone modifications may influence the state of chromatin condensation. Evelyn I. Milian - Instructor 52 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) Animation: DNA REPLICATION • ..\..\..\BIOLOGY-SOLOMON\BIOLOGY-SOLOMONImages\chapter12\Animations\replicating_dna.html Evelyn I. Milian - Instructor 53 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) DNA STRUCTURE AND REPLICATION: Some Websites for Review • DNA structure interactive tutorial: http://www.umass.edu/molvis/tutorials/dna/ • DNA replicating song (creative and funny!): http://www.youtube.com/watch?v=dIZpb93NYlw • DNA structure: http://www.youtube.com/watch?v=qy8dk5iS1f0 • Molecular visualization of DNA: http://www.youtube.com/watch?v=4PKjF7OumYo • DNA replication brief videos: http://www.youtube.com/watch?v=teV62zrm2P0&feature=related http://www.youtube.com/watch?v=AGUuX4PGlCc&feature=related http://www.youtube.com/watch?v=z685FFqmrpo&feature=related Evelyn I. Milian - Instructor 54 BIOLOGY I – Chapter 16: The Molecular Basis of Inheritance (DNA) References • Audesirk, Teresa; Audesirk, Gerald & Byers, Bruce E. (2005). Biology: Life on Earth. Seventh Edition. Pearson Education, Inc.-Prentice Hall. NJ, USA. • Brooker, Robert J.; Widmaier, Eric P.; Graham, Linda E.; Stiling, Peter D. (2008). Biology. The McGraw-Hill Companies, Inc. NY, USA. • Campbell, Neil A.; Reece, Jane B., et al. (2011). Campbell Biology. Ninth Edition. Pearson Education, Inc.-Pearson Benjamin Cummings. CA, USA. • Cowan, Marjorie Kelly; Talaro, Kathleen Park. (2009). Microbiology A Systems Approach. Second Edition. The McGraw-Hill Companies, Inc. NY, USA. www.mhhe.com/cowan2e • Ireland, K.A. (2011). Visualizing Human Biology. Second Edition. John Wiley & Sons, Inc. NJ, USA. • Mader, Sylvia S. (2010). Biology. Tenth Edition. The McGraw-Hill Companies, Inc. NY, USA. • Martini, Frederic H.; Nath, Judi L. (2009). Fundamentals of Anatomy & Physiology. Eighth Edition. Pearson Education, Inc. – Pearson Benjamin Cummings. CA, USA. • Solomon, Eldra; Berg, Linda; Martin, Diana W. (2008). Biology. Eighth Edition. Cengage Learning. OH, USA. • Starr, Cecie. (2008). Biology: Concepts and Applications , Volume I. Thompson Brooks/Cole. OH, USA. • Tortora, Gerard J.; Derrickson, Bryan. (2006). Principles of Anatomy and Physiology. Eleventh Edition. John Wiley & Sons, Inc. NJ, USA. www.wiley.com/college/apcentral. • Tortora, Gerard J.; Funke, Berdell R.; Case, Christine L. (2010). Microbiology An Introduction. Tenth Edition. Pearson Education, Inc.-Pearson Benjamin Cummings; CA, USA. www.microbiologyplace.com. Evelyn I. Milian - Instructor 55