Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 16 The Molecular Basis of Inheritance

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: Life’s Operating Instructions In 1953, James Watson and Francis Crick shook the world – With an elegant double-helical model for the structure of deoxyribonucleic acid, or DNA Figure 16.1

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings DNA, the substance of inheritance – Is the most celebrated molecule of our time Hereditary information – Is encoded in the chemical language of DNA and reproduced in all the cells of your body It is the DNA program – That directs the development of many different types of traits

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 16.1: DNA is the genetic material Early in the 20th century – The identification of the molecules of inheritance loomed as a major challenge to biologists

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Search for the Genetic Material: Scientific Inquiry The role of DNA in heredity – Was first worked out by studying bacteria and the viruses that infect them

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Evidence That DNA Can Transform Bacteria Frederick Griffith was studying Streptococcus pneumoniae (A bacterium that causes pneumonia in mammals) He worked with two strains of the bacterium – A pathogenic strain [Smooth strain (S); encapsulated with a polysaccharide coat] and a nonpathogenic strain [Rough strain ( R); not encapsulated].

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Griffith found that when he mixed heat-killed remains of the pathogenic strain – With living cells of the nonpathogenic strain, some of these living cells became pathogenic Bacteria of the “S” (smooth) strain of Streptococcus pneumoniae are pathogenic because they have a capsule that protects them from an animal’s defense system. Bacteria of the “R” (rough) strain lack a capsule and are nonpathogenic. Frederick Griffith injected mice with the two strains as shown below: Griffith concluded that the living R bacteria had been transformed into pathogenic S bacteria by anunknown, heritable substance from the dead S cells. EXPERIMENT RESULTS CONCLUSION Living S (control) cells Living R (control) cells Heat-killed (control) S cells Mixture of heat-killed S cells and living R cells Mouse dies Mouse healthy Mouse dies Living S cells are found in blood sample. Figure 16.2

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Griffith called the phenomenon transformation – Now defined as a change in genotype and phenotype due to the assimilation of external DNA by a cell

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Evidence That Viral DNA Can Program Cells Additional evidence for DNA as the genetic material Came from studies of a virus (T2 bacteriophage) that infects bacteria (Escherichia coli, enteric bacteria)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Viruses that infect bacteria, bacteriophages – Are widely used as tools by researchers in molecular genetics Figure 16.3 Phage head Tail Tail fiber DNA Bacterial cell 100 nm

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Alfred Hershey and Martha Chase – Performed experiments showing that DNA is the genetic material of a phage known as T2

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Hershey and Chase experiment In their famous 1952 experiment, Alfred Hershey and Martha Chase used radioactive sulfur and phosphorus to trace the fates of the protein and DNA, respectively, of T2 phages that infected bacterial cells. Radioactivity (phage protein) in liquid Phage Bacterial cell Radioactive protein Empty protein shell Phage DNA Centrifuge Pellet (bacterial cells and contents) Radioactive DNA Centrifuge Pellet Batch 1: Phages were grown with radioactive sulfur ( 35 S), which was incorporated into phage protein (pink). Batch 2: Phages were grown with radioactive phosphorus ( 32 P), which was incorporated into phage DNA (blue) Agitated in a blender to separate phages outside the bacteria from the bacterial cells. Mixed radioactively labeled phages with bacteria. The phages infected the bacterial cells. Centrifuged the mixture so that bacteria formed a pellet at the bottom of the test tube. Measured the radioactivity in the pellet and the liquid Phage proteins remained outside the bacterial cells during infection, while phage DNA entered the cells. When cultured, bacterial cells with radioactive phage DNA released new phages with some radioactive phosphorus. Hershey and Chase concluded that DNA, not protein, functions as the T2 phage’s genetic material. RESULTS CONCLUSION EXPERIMENT Radioactivity (phage DNA) in pellet Figure 16.4

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Additional Evidence That DNA Is the Genetic Materia Prior to the 1950s, it was already known that DNA – Is a polymer of nucleotides, each consisting of three components: a nitrogenous base, a sugar, and a phosphate group Sugar-phosphate backbone Nitrogenous bases 5 end O–O– O P O CH O–O– H H O H H H 3 1 H O CH 3 N O N H Thymine (T) O OP O O–O– CH 2 H H O H H H H N N N H N H H Adenine (A) O O P O O–O– CH 2 H H O H H H H H H H N N N O Cytosine (C) O O P O CH O–O– H O H H 3 1 OH 2 H N N N H O N N H H H H Sugar (deoxyribose) 3 end Phosphate Guanine (G) DNA nucleotide 2 N Figure 16.5

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Erwin Chargaff analyzed the base composition of DNA – From a number of different organisms In 1947, Chargaff reported – That DNA composition varies from one species to the next; i.e. species-specific, in every species he studied, there was a regularity in base ratios. – The number of adenine (A) equals the number of thymines (T), and the number of guanines (G) equal the number of cytosines (C). – the A=T and G=C equalities became known as Chargaff’s rules. This evidence of molecular diversity among species – Made DNA a more credible candidate for the genetic material

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Building a Structural Model of DNA: Scientific Inquiry Once most biologists were convinced that DNA was the genetic material – The challenge was to determine how the structure of DNA could account for its role in inheritance

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Maurice Wilkins and Rosalind Franklin – Were using a technique called X-ray crystallography to study molecular structure Rosalind Franklin – Produced a picture of the DNA molecule using this technique (a) Rosalind Franklin Franklin’s X-ray diffraction Photograph of DNA (b) Figure 16.6 a, b

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 16.7a, c C T A A T C G GC A C G A T A T AT T A C T A 0.34 nm 3.4 nm (a) Key features of DNA structure G 1 nm G (c) Space-filling model T Watson and Crick deduced that DNA was a double helix – Through observations of the X-ray crystallographic images of DNA

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Franklin had concluded that DNA – Was composed of two antiparallel sugar- phosphate backbones, with the nitrogenous bases paired in the molecule’s interior The nitrogenous bases – Are paired in specific combinations: adenine with thymine, and cytosine with guanine. They were pointed toward the interior because they are hydrophobic.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings O –O–O O OH O –O–O O O H2CH2C O –O–O O O H2CH2C O –O–O O O O O O T A C G C A T O O O CH 2 O O–O– O O 5 end Hydrogen bond 3 end G P P P P O OH O–O– O O O P P O–O– O O O P O–O– O O O P (b) Partial chemical structure H2CH2C 5 end Figure 16.7b O

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Watson and Crick reasoned that there must be additional specificity of pairing – Dictated by the structure of the bases Each base pair forms a different number of hydrogen bonds – Adenine and thymine form two bonds, cytosine and guanine form three bonds

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings From X-ray photo of DNA taken by Rosalind Franklin, Watson and Crick deduced that : DNA is a helix with a uniform width of 2nm suggesting it has two strands. Purine and pyrimidine bases are stacked 0.32 nm apart. The helix makes one full turn every 3.4 nm along its length. There are 10 layers of nitrogenous base pairs in each turn of the helix.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Based on that, they put the base-pairing rule: – A must pair with T; Likewise, C must pair with G; There amount in a given DNA molecule will be nearly the same. – Bases from specific pair on one strand complement that on the other. – The sequence of bases can be highly variable which makes it suitable for coding genetic material.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings N H O CH 3 N N O N N N NH Sugar Adenine (A) Thymine (T) N N N N Sugar O H N H N H N O H H N Guanine (G) Cytosine (C) Figure 16.8 H

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 16.2: Many proteins work together in DNA replication and repair The relationship between structure and function – Is manifest in the double helix

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Basic Principle: Base Pairing to a Template Strand Since the two strands of DNA are complementary – Each strand acts as a template for building a new strand in replication

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In DNA replication – The parent molecule unwinds, and two new daughter strands are built based on base- pairing rules (a) The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C. (b) The first step in replication is separation of the two DNA strands. (c) Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand. (d) The nucleotides are connected to form the sugar-phosphate backbones of the new strands. Each “daughter” DNA molecule consists of one parental strand and one new strand. A C T A G A C T A G A C T A G A C T A G T G A T C T G A T C A C T A G A C T A G T G A T C T G A T C T G A T C T G A T C Figure 16.9 a–d

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Models of DNA replication DNA replication is semiconservative – Each of the two new daughter molecules will have one old strand, derived from the parent molecule, and one newly made strand – Question… is that true for all DNA replication product?

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure a–c Conservative model. The two parental strands reassociate after acting as templates for new strands, thus restoring the parental double helix. Semiconservative model. The two strands of the parental molecule separate, and each functions as a template for synthesis of a new, comple- mentary strand. Dispersive model. Each strand of both daughter mol- ecules contains a mixture of old and newly synthesized DNA. Parent cell First replication Second replication A B c

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Experiment performed by Meselson and Stahl – Supported the semiconservative model of DNA replication Matthew Meselson and Franklin Stahl cultured E. coli bacteria for several generations on a medium containing nucleotide precursors labeled with a heavy isotope of nitrogen, 15 N. The bacteria incorporated the heavy nitrogen into their DNA. The scientists then transferred the bacteria to a medium with only 14 N, the lighter, more common isotope of nitrogen. Any new DNA that the bacteria synthesized would be lighter than the parental DNA made in the 15 N medium. Meselson and Stahl could distinguish DNA of different densities by centrifuging DNA extracted from the bacteria.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure EXPERIMENT The bands in these two centrifuge tubes represent the results of centrifuging two DNA samples from the flask in step 2, one sample taken after 20 minutes and one after 40 minutes. RESULTS Bacteria cultured in medium containing 15 N Bacteria transferred to medium containing 14 N 2 1 DNA sample centrifuged after 20 min (after first replication) 3 DNA sample centrifuged after 40 min (after second replication) 4 Less dense More dense

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CONCLUSION Meselson and Stahl concluded that DNA replication follows the semiconservative model by comparing their result to the results predicted by each of the three models in Figure – The first replication in the 14 N medium produced a band of hybrid ( 15 N– 14 N) DNA. This result eliminated the conservative model. – A second replication produced both light and hybrid DNA, a result that eliminated the dispersive model and supported the semiconservative model.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings First replicationSecond replication Conservative model Semiconservative model Dispersive model Same bands New band generated Stays as one band

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings DNA Replication: A Closer Look The copying of DNA (DNA replication) – The process of DNA replication is; – A. Complex: the helical molecule must untwist while it copies its own strands. More than a dozen enzymes and other proteins participate in DNA replication – B. Very rapid: in bacteria 500 nucleotides are added/sec. In human cells, 50 nucleotides /sec are added. – C. Accurate: only about one in a billion nucleotides is incorrectly paired.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Getting Started: Origins of Replication The replication of a DNA molecule – Begins at special sites called origins of replication, have a specific sequence of nucleotides and where the two strands are separated. – specific proteins required to initiate replication. – The DNA double helix opens at the origin of replication and replication fork ( The Y-shaped region of replicating DNA molecules where new strands are growing ) spread in both directions, initiating a replication bubble. – Bacterial DNA have one origin of replication. – Eukaryotic DNA have hundreds or thousands of origins of replication.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A eukaryotic chromosome – May have hundreds or even thousands of replication origins Replication begins at specific sites where the two parental strands separate and form replication bubbles. The bubbles expand laterally, as DNA replication proceeds in both directions. Eventually, the replication bubbles fuse, and synthesis of the daughter strands is complete Origin of replication Bubble Parental (template) strand Daughter (new) strand Replication fork Two daughter DNA molecules In eukaryotes, DNA replication begins at many sites along the giant DNA molecule of each chromosome. In this micrograph, three replication bubbles are visible along the DNA of a cultured Chinese hamster cell (TEM). (b) (a) 0.25 µm Figure a, b

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Elongation of new DNA at a replication fork Synthesis of the new DNA strand is catalyzed by enzymes called DNA polymerases, which add nucleotides to the 3 end of a growing strand, grows in 5’—3’ direction. Each nucleotide that is added is actually a nucleoside (a sugar and a base) with three phosphate groups.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings New strandTemplate strand 5 end 3 end Sugar A T Base C G G C A C T P P P OH P P 5 end 3 end 5 end A T C G G C A C T 3 end Pyrophosphate 2 P OH Phosphate Elongating a New DNA Strand Elongation of new DNA at a replication fork needs energy Energy required for building new strands comes from the hydrolysis of nucleoside triphosphate. Nucleoside triphosphate DNA Polymerase Figure DNA polymerase catalysis the addition of a nucleoside triphosphate to the 3’ end of a growing DNA strand

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Antiparallel Elongation How does the antiparallel structure of the double helix affect replication?

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings DNA polymerases add nucleotides – Only to the free 3  end of a growing strand Along one template strand of DNA, the leading strand (the DNA strand which is synthesized as a single polymer in the 5’  3’ direction towards the replication fork) is synthesized. DNA polymerase III can synthesize a complementary strand (leading strand) continuously, moving toward the replication fork.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings To elongate the other new strand of DNA, the lagging strand (the DNA strand that is discontinuously synthesized against the overall direction of replication) is synthesized. DNA polymerase III must work in the direction away from the replication fork. The lagging strand – Is synthesized as a series of segments called Okazaki fragments, which are then joined together by DNA ligase

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Parental DNA DNA pol Ill elongates DNA strands only in the 5 3 direction. 1 Okazaki fragments DNA pol III Template strand Lagging strand 3 2 Template strand DNA ligase Overall direction of replication One new strand, the leading strand, can elongate continuously 5 3 as the replication fork progresses. 2 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). 3 DNA ligase joins Okazaki fragments by forming a bond between their free ends. This results in a continuous strand. 4 Figure Leading strand 1 Synthesis of leading and lagging strands during DNA replication

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Priming DNA Synthesis DNA polymerases cannot initiate the synthesis of a polynucleotide, they can only add nucleotides to the 3 end of already existing chain. Before new DNA strand can be form, there must be a small pre- existing primers to start the addition of new nucleotides. Primer is a short RNA segments that are complementary to a DNA segments and that is necessary to begin DNA replication. Primers are polymerized by by an enzyme called: primase. Only one primer is necessary for replication of the leading strand, but many are required to replicate the lagging strand. An RNA primer must initiate the synthesis of each Okazaki fragments. DNA polymerase removes the RNA primer and replaces it with DNA. DNA ligase joins these fragments forming a continuous DNA strand

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In summary; there are three types of proteins that function in DNA synthesis; 1.DNA polymerase 2.DNA ligase 3.DNA primase

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Other proteins assist DNA replication There are three types of proteins also involved in DNA replication: Helicase: enzyme which unwinds parental double helix at the replication fork (Strand separation) causing a tight twist for the DNA strand. Topoisomerase; helps relief this strain (the tight twist). Single-strand binding proteins: proteins which keep the separated strands apart and stabilize the unwound DNA until new complementary strand is synthesized.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overall direction of replication Template strand RNA primer Okazaki fragment Figure Primase joins RNA nucleotides into a primer. 1 DNA pol III adds DNA nucleotides to the primer, forming an Okazaki fragment. 2 After reaching the next RNA primer (not shown), DNA pol III falls off. 3 After the second fragment is primed. DNA pol III adds DNA nucleotides until it reaches the first primer and falls off. 4 DNA pol 1 replaces the RNA with DNA, adding to the 3 end of fragment 2. 5 DNA ligase forms a bond between the newest DNA and the adjacent DNA of fragment 1. 6 The lagging strand in this region is now complete. 7

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Other Proteins That Assist DNA Replication Helicase, topoisomerase, single-strand binding protein – Are all proteins that assist DNA replication Table 16.1

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure Overall direction of replication Leading strand Lagging strand Lagging strand Leading strand OVERVIEW Leading strand Replication fork DNA pol III Primase Primer DNA pol III Lagging strand DNA pol I Parental DNA Origin of replication DNA ligase Helicase unwinds the parental double helix. 1 Molecules of single- strand binding protein stabilize the unwound template strands. 2 The leading strand is synthesized continuously in the 5  3 direction by DNA pol III. 3 Primase begins synthesis of RNA primer for fifth Okazaki fragment. 4 DNA pol III is completing synthesis of the fourth fragment, when it reaches the RNA primer on the third fragment, it will dissociate, move to the replication fork, and add DNA nucleotides to the 3 end of the fifth fragment primer. 5 DNA pol I removes the primer from the 5 end of the second fragment, replacing it with DNA nucleotides that it adds one by one to the 3 end of the third fragment. The replacement of the last RNA nucleotide with DNA leaves the sugar- phosphate backbone with a free 3 end. 6 DNA ligase bonds the 3 end of the second fragment to the 5 end of the first fragment. 7 A summary of DNA replication

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The DNA Replication Machine as a Stationary Complex The various proteins that participate in DNA replication – Form a single large complex, a DNA replication “machine”. The DNA replication machine – Is probably stationary during the replication process.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Proofreading and Repairing DNA DNA replication is very accurate. Paring errors occurs at a rate of one to 10 billions in a complete DNA molecule. DNA polymerases proofread newly made DNA by replacing any incorrect nucleotides In Mismatch repair of DNA: Repair enzymes correct errors in base pairing (DNA synthesis) In bacteria: DNA polymerase proofread each newly added nucleotide against its template. If polymerase founds an incorrectly paired nucleotide, the enzyme removes it and replaces it before continuing with synthesis. In eukaryotes: additional proteins as well as polymerase act in the mismatch repair.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure Nuclease DNA polymerase DNA ligase A thymine dimer distorts the DNA molecule. 1 A nuclease enzyme cuts the damaged DNA strand at two points and the damaged section is removed. 2 Repair synthesis by a DNA polymerase fills in the missing nucleotides. 3 3 DNA ligase seals the Free end of the new DNA To the old DNA, making the strand complete. 4 Nucleotide excision repair : Enzymes cut out and replace damaged stretches of DNA. The damaged segment is excised (cut) by an enzyme (Nuclease), and the remaining gap is filled in by base-pairing nucleotides. DNA polymerase and Ligase are enzymes that catalyze the filling-in process.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Replicating the Ends of DNA Molecules The ends of eukaryotic chromosomal DNA (Get shorter with each round of replication). Because DNA polymerase can only add nucleotides to the 3’ end of pre-existing polynucleotide, repeated replication of linear DNA would result in successively shorter molecules, potentially deleting genes. Prokaryotes do not have this problem because they have circular DNA.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Replicating the Ends of DNA Molecules Figure End of parental DNA strands Leading strand Lagging strand Last fragmentPrevious fragment RNA primer Lagging strand Removal of primers and replacement with DNA where a 3 end is available Primer removed but cannot be replaced with DNA because no 3 end available for DNA polymerase Second round of replication New leading strand New lagging strand 5 Further rounds of replication Shorter and shorter daughter molecules

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Eukaryotic chromosomal DNA molecules – Have at their ends nucleotide sequences, called telomeres (short non-coding nucleotide sequence), that postpone the erosion of genes near the ends of DNA molecules Figure µm Mouse chromosomes with telomeres at their tips

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings If the chromosomes of germ cells became shorter in every cell cycle – Essential genes would eventually be missing from the gametes they produce An enzyme called telomerase – Catalyzes the lengthening of telomeres in germ cells

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings End of Chapter 16