Chapter 16 The Molecular Basis of Inheritance PowerPoint Lectures for Biology Eighth Edition Neil Campbell and Jane Reece Additional Evidence That DNA Is the Genetic Material 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 5 end O 5 O P O CH2 O 1 O 4 H H H H 2 3 H O O P O CH2 O O H H H H H O O P O CH2 O O H H H H H Figure 16 5 O 5 O P O CH2 O 1 O 4 H H PhosphateH H 3 2 OH H Sugar deoxyribose 3 end Nitrogenous bases CH3 O H N N H O Thymine T H N H N N N N H H Adenine A H H H N H N N O Cytosine C H N N N O N H N H H Guanine G DNA nucleotide 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 Figure 16 6 a b a Rosalind Franklin b Franklin s X ray diffraction Photograph of DNA 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 Watson and Crick deduced that DNA was a double helix Through observations of the X ray crystallographic images of DNA G C A T T A 1 nm C G C A T G C T A T A A T T A G A Figure 16 7a c 3 4 nm G C 0 34 nm T a Key features of DNA structure c Space filling model Each base pair forms a different number of hydrogen bonds Adenine and thymine form two bonds cytosine and guanine form three bonds 5 end O OH P O O Hydrogen bond O O O P O O H2 C A T O CH2 O O O P O O H2 C O O O P O O H2 C 3 end OH G O O P O O C O CH2 C O O A N Sugar T O OH 3 end b Partial chemical structure Figure 16 7b CH2 O O P O O CH2 O O P O O N N Adenine A O O P O O G N H N H O N N Sugar N H O CH3 H N N O Sugar Thymine T H H N N N N N H O Sugar H Guanine G Cytosine C 5 end Figure 16 8 DNA replication The parent molecule unwinds and two new daughter strands are built based on basepairing rules T A T A T A T G C G C G C G T A T A T A T A T A T T A T A T C G C C G C G C A T A T A C G C G C T A T A A T A G C G A G 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 Figure 16 9 a d 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 Semi conservative replication Parent cell a Conservative model The two parental strands reassociate after acting as templates for new strands thus restoring the parental double helix b Semiconservative model The two strands of the parental molecule separate and each functions as a template for synthesis of a new complementary strand c Dispersive model Each strand of both daughter molecules contains a mixture of old and newly synthesized DNA First replication Second replication A eukaryotic chromosome May have hundreds or even thousands of replication origins Origin of replication 1 Replication begins at specific sites where the two parental strands separate and form replication bubbles Bubble Parental template strand Daughter new strand 0 25 m Replication fork 2 The bubbles expand laterally as DNA replication proceeds in both directions 3 Eventually the replication bubbles fuse and synthesis of the daughter strands is complete Two daughter DNA molecules a In eukaryotes DNA replication begins at many sites along the giant DNA molecule of each chromosome Figure 16 12 a b b In this micrograph three replication bubbles are visible along the DNA of a cultured Chinese hamster cell TEM Elongating a New DNA Strand Elongation of new DNA at a replication fork Is catalyzed by enzymes called DNA polymerases which add nucleotides to the 3 end of a growing strand New strand 5 end Sugar A Base Phosphate P P A T C G C G G C G C A T A T P 3 end 5 end T OH Figure 16 13 Template strand 3 end Nucleoside triphosphate OH Pyrophosphate 3 end P C P C 2 P 5 end 5 end Synthesis of leading and lagging strands during DNA replication 1 DNA pol Ill elongates DNA strands only in the 5 3 direction 3 5 Parental DNA 5 3 Okazaki fragments 2 1 3 5 DNA pol III 2 One new strand the leading strand can elongate continuously 5 3 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 Template strand 3 Leading strand Lagging strand 2 Template strand Figure 16 14 1 DNA ligase Overall direction of replication 4 DNA ligase joins Okazaki fragments by forming a bond between their free ends This results in a continuous strand 1 Primase joins RNA nucleotides into a primer 3 5 5 3 Template strand 2 DNA pol III adds DNA nucleotides to the primer forming an Okazaki fragment RNA primer 3 5 3 After reaching the next RNA primer not shown DNA pol III falls off 3 Okazaki fragment 3 5 1 3 5 1 5 4 After the second fragment is primed DNA pol III adds DNA nucleotides until it reaches the first primer and falls off 5 3 5 3 2 5 1 DNA pol 1 replaces the RNA with DNA adding to the 3 end of fragment 2 3 5 2 1 6 DNA ligase forms a bond 7 The lagging strand between the newest DNA and the adjacent DNA of fragment 1 5 3 Figure 16 15 2 3 5 in this region is now complete 3 1 Overall direction of replication 5 Other Proteins That Assist DNA Replication Helicase topoisomerase single strand binding protein Are all proteins that assist DNA replication …
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