3.A.1 DNA and RNA: Structure and Replication

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1 3.A.1 DNA and RNA: Structure and Replication

2 Each DNA polymer is made of Nucleotides (monomer) which are made of: a) Phosphate group: Negatively charged and polar b) Sugar: deoxyribose- a 5 carbon sugar c) Nitrogen base A, C, T, G

3 Nitrogen Bases Four bases divided into two groups: Purines Pyrimidines Double ring Single ring Adenine (A) Thymine (T) Guanine (G) Cytosine (C)

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5 A DNA strand is directional; it has a 3 end and a 5 end. It is antiparallel to the opposite strand

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7 antiparallel = run parallel to each other but with opposite alignments. Ex. the two complementary strands of DNA, which run in opposite directions alongside each other.

8 DNA replication ensures continuity of hereditary information.

9 Replication occurs during S phase of the cell cycle.

10 Replication is a semi conservative process; that is, one strand serves as the template for a new, complementary strand.

11 Replication requires many essential cellular enzymes.

12 Essential Replication Enzymes DNA is unwound by HELICASE PRIMASE- produces the RNA primer to initiate replication DNA POLYMERASE III adds new complementary nucleotides to the growing strand in the 5 à 3 direction DNA POLYMERASE I proofreads and corrects mistakes LIGASE- acts like molecular tape joining new DNA bases together.

13 The copying of DNA is remarkable in its speed and accuracy.

14 In DNA replication, the parent molecule unwinds and two new daughter strands are built following base-pairing rules.

15 The replication of a DNA molecule begins at special sites called origins of replication (ORI), where the two strands are separated. Replication is bidirectional.

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17 A eukaryotic chromosome may have hundreds or even thousands of replication origins.

18 A replication fork is formed by the action of two enzymes, topoisomerase and DNA helicase. Topoisomerase helps relieve the strain created by unwinding the parent strand, while DNA helicase breaks the hydrogen bonds between base pairs.

19 DNA polymerase III adds nucleotides to the growing strand in the 5 à 3 direction.

20 DNA is antiparallel, so the two new strands have to be made differently.

21 First, an RNA primer is formed by the enzyme DNA primase. A primer is necessary because DNA polymerase cannot initiate the synthesis of new strand; it can only add nucleotides to the 3 DNA template strand

22 Along the leading strand, DNA polymerase can synthesize a complementary strand continuously, moving toward the replication fork in the 5 à3 direction.

23 To elongate the other new strand of DNA, the lagging strand, DNA polymerase must work in the direction away from the replication fork. The lagging strand is discontinuous, resulting in fragments: each with its own RNA primer.

24 The lagging strand is synthesized as a series of segments called Okazaki fragments, which are then joined together by DNA ligase. The primers are then removed and replaced with DNA.

25 DNA polymerase I proofreads newly made DNA, replacing any incorrect nucleotides. In mismatch repair of DNA, repair enzymes correct errors in base pairing.

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27 The ends of eukaryotic chromosomal DNA get shorter with each round of replication. The primers are removed and replaced with DNA. Telomeres (ends of DNA) delay the shortening.

28 The ends of eukaryotic DNA molecules have nucleotide sequences called telomeres that postpone the erosion of genes near the ends of DNA molecules. An enzyme called telomerase catalyzes the lengthening of telomeres in germ cells so that the ends do not get shorter each time a germ cell is produced.

29 DNA Replication Process: Summary The weak hydrogen bonds between the base pairs are broken and the two strands at the origin of replication. The partial unwinding of the double helix occurs at the replication fork by which enzyme?

30 Primase does what? What is the difference between leading and lagging strands? Which enzyme adds free nucleotides to a growing strand? Which one proofreads? What are Okazaki fragments? Which enzyme joins the DNA bases together? What is the difference between prokaryotic and eukaryotic replication? What is a telomere and what does telomerase catalyze?

31 Learning Objectives: LO 3.3 The student is able to describe representations and models that illustrate how genetic information is copied for transmission between generations. [See SP 1.2]