DNA Structure. DNA: The Genetic Material. Chapter 14

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1 DNA: The Genetic Material Chapter 14 DNA Structure DNA is a nucleic acid. The building blocks of DNA are nucleotides, each composed of: a 5-carbon sugar called deoxyribose a phosphate group (PO 4 ) a nitrogenous base adenine, thymine, cytosine, guanine 2 3 1

2 DNA Structure The nucleotide structure consists of the nitrogenous base attached to the 1 carbon of deoxyribose the phosphate group attached to the 5 carbon of deoxyribose a free hydroxyl group (-OH) at the 3 carbon of deoxyribose 4 5 DNA Structure Nucleotides are connected to each other to form a long chain phosphodiester bond: bond between adjacent nucleotides formed between the phosphate group of one nucleotide and the 3 OH of the next nucleotide The chain of nucleotides has a 5 to 3 orientation. 6 2

3 7 DNA Structure Determining the 3-dimmensional structure of DNA involved the work of a few scientists: Erwin Chargaff determined that amount of adenine = amount of thymine amount of cytosine = amount of guanine This is known as Chargaff s Rules 8 DNA Structure Rosalind Franklin and Maurice Wilkins Franklin performed X-ray diffraction studies to identify the 3-D structure discovered that DNA is helical discovered that the molecule has a diameter of 2nm and makes a complete turn of the helix every 3.4 nm 9 3

4 DNA Structure James Watson and Francis Crick, 1953 deduced the structure of DNA using evidence from Chargaff, Franklin, and others proposed a double helix structure 10 DNA Structure The double helix consists of: 2 sugar-phosphate backbones nitrogenous bases toward the interior of the molecule bases form hydrogen bonds with complementary bases on the opposite sugar-phosphate backbone

5 DNA Structure The two strands of nucleotides are antiparallel to each other one is oriented 5 to 3, the other 3 to 5 The two strands wrap around each other to create the helical shape of the molecule DNA Replication DNA replication includes: initiation replication begins at an origin of replication elongation new strands of DNA are synthesized by DNA polymerase termination replication is terminated differently in prokaryotes and eukaryotes 15 5

6 Prokaryotic DNA Replication The chromosome of a prokaryote is a circular molecule of DNA. Replication begins at one origin of replication and proceeds in both directions around the chromosome Prokaryotic DNA Replication The double helix is unwound by the enzyme helicase DNA polymerase III (pol III) is the main polymerase responsible for the majority of DNA synthesis DNA polymerase III adds nucleotides to the 3 end of the daughter strand of DNA 18 6

7 19 Prokaryotic DNA Replication DNA replication is semidiscontinuous. pol III can only add nucleotides to the 3 end of the newly synthesized strand DNA strands are antiparallel to each other leading strand is synthesized continuously (in the same direction as the replication fork) lagging strand is synthesized discontinuously creating Okazaki fragments

8 Prokaryotic DNA Replication The enzymes for DNA replication are contained within the replisome. The replisome consists of the primosome - composed of primase and helicase 2 DNA polymerase III molecules The replication fork moves in 1 direction, synthesizing both strands simultaneously Eukaryotic DNA Replication The larger size and complex packaging of eukaryotic chromosomes means they must be replicated from multiple origins of replication. Simple DNA replication More detailed animation The enzymes of eukaryotic DNA replication are more complex than those of prokaryotic cells. 24 8

9 Eukaryotic DNA Replication Synthesizing the ends of the chromosomes is difficult because of the lack of a primer. With each round of DNA replication, the linear eukaryotic chromosome becomes shorter Eukaryotic DNA Replication telomeres repeated DNA sequence on the ends of eukaryotic chromosomes produced by telomerase telomerase contains an RNA region that is used as a template so a DNA primer can be produced 27 9

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