DNA Replication. Packet #17 Chapter #16

Transcription

1 DNA Replication Packet #17 Chapter #16 1

2 HISTORICAL FACTS ABOUT DNA 2

3 Historical DNA Discoveries 1928 Frederick Griffith finds a substance in heat-killed bacteria that transforms living bacteria 1944 Oswald Avery, Cloin MacLeod and Maclyn McCarty chemically identify Griffith s transforming principle as DNA 1949 Erwin Chargaff reports relationships among DNA bases that provide a clue to the structure of DNA 1953 Alfred Hersey and Martha Chase demonstrate that DNA, not protein, is involved in viral reproduction Rosalind Franklin produces an x-ray diffraction image of DNA 3

4 Historical DNA Discoveries II 1953 James Watson and Francis Crick propose a model of the structure of DNA Matthew Meselson and Franklin Stahl demonstrate that DNA replication is semi conservative replication 1962 James Watson, Francis Crick and Maurice Wilkins are awarded the Nobel Prize in Medicine for discoveries about the molecular structure of nucleic acids Alfred Hershey is awarded the Nobel Prize in Medicine for discovering the replication mechanism and genetic structure of viruses 4

5 Griffith Experiment The Griffith experiment, conducted in 1928, was one of the first experiments suggesting that bacteria are capable of transferring genetic information through a process known as transformation. 5

6 Hershey Chase Experiment Hershey and Chase conduced an experiment using viral DNA to show that the DNA was the genetic material being inserted into the bacteria and used to replicate more viruses. 6

7 STRUCTURE OF DNA 7

8 Introduction I DNA is an organic macromolecule known as a nucleic acid. Nucleic Acids are composed of building blocks known as nucleotides. Nucleotides have three parts: - Phosphate Sugar Nitrogenous bases 8

9 DNA Nucleotides Multiple DNA nucleotide subunits link together to form a single DNA strand. DNA nucleotides are composed of: - Phosphate Sugar Deoxyribose Nitrogenous Bases Purines (Two Rings) Adenine Guanine Pyrimidines (One Ring) Thymine Cytosine 9

10 DNA Nucleotides II Nucleotides are linked together by covalent phosphodiester bonds Each phosphate attaches to the 5 end (carbon #5) of one deoxyribose and to the 3 end (carbon #3) of the neighboring deoxyribose Makes up the sugarphosphate backbone 10

11 DNA Strands Each DNA strand, that is composed of multiple nucleotides, has a head and a tail. Head = 5 end Phosphate group Tail = 3 end Hydroxyl group 11

12 DNA Molecule Each DNA molecule consists of two DNA strands (polynucleotide chains) that associate as a double helix The two strands/chains run antiparallel 12

13 Base-Pairing Rules for DNA Chargaff Rules The two DNA strands are joined together at the nitrogenous bases. Holding the bases together, and allowing the formation of the double helix, are hydrogen bonds. 13

14 Base-Pairing Rules for DNA Chargaff Rules II Adenine forms two hydrogen bonds with thymine Guanine forms three hydrogen bonds with cytosine These pairings are known as Chargaff s rules A always pairs with T G always pairs with C Complementary base pairing 14

15 Chargaff Rules III 15

16 MODELS OF DNA REPLICATION 16

17 Models of DNA Replication There were three models proposed about how DNA replicates. However, the one that stood the test was semi-conservative replication. 17

18 Models of DNA Replication II In semi-conservative replication, each old strand of DNA is used to create a new complementary strand. 18

19 The Players INTRODUCTION TO DNA REPLICATION 19

20 Introduction to the Strands Template Strands {The Parental Strands} These are the strands being copied The original DNA strands During DNA replication, both strands are copied This means that there are TWO template strands 20

21 Introduction to the Strands II Complementary Strands {The Daughter Strands} The NEW DNA strands produced from the Template Strands During DNA replication, there are TWO complementary strands Always remember that the process started with TWO template strands 21

22 Origin of Replication & Bi-directionality. DNA replication is bidirectional and starts at the origin of replication The process proceeds in both directions from that point. A eukaryotic chromosome may have multiple origins of replication Allows the process to occur faster and more efficient 22

23 Introduction to the Making of the Complementary Strand DNA replication/synthesis, of the complementary strands, proceed in a 5 to 3 direction. Nucleotides can ONLY be added to the 3 end. 23

24 Introduction to the Making of the Complementary Strand Since DNA nucleotides can only be added to the 3 end, it causes one of the complementary strands to be produced continuously and the other discontinuous The continuous strand is called the leading strand The discontinuous strand is called the lagging strand Is first synthesized as short Okazaki fragments before becoming one strand 24

25 ENZYMES OF DNA REPLICATION & THE STEPS OF DNA REPLICATION 25

26 Enzymes of DNA Replication Helicase Unzips DNA double-helix Topoisomerases Prevents tangling and knotting of DNA as the while the strands are unzipped. RNA primase Initiates the formation of daughter strands Forms a segment known as the RNA primer The RNA primer contains the nitrogenous base Uracil 26

27 Enzymes of DNA Replication II DNA Polymerase III Enzyme that catalyzes the polymerization (making) of nucleotides Adds Deoxyribonucleotides (nucleotides only found in DNA, as opposed to RNA) to the 3 end of a growing nucleotide chain Acts at the replication fork DNA Polymerase I A type of DNA polymerase will change the RNA primers into DNA Changing the base Uracil into Thymine 27

28 Enzymes of DNA Replication III DNA Ligase Enzyme responsible for joining Okazaki fragments forming the Lagging Strand Gyrase Returns the DNA strands into a Double Helix Zips the DNA back together 28

29 DNA Replication The Big Picture 29

30 DNA Replication Lagging Strand 30

31 POST DNA REPLICATION 31

32 Check Points Recall, from the cell cycle, that there are check points during the process. One of those check points is to check for incorrect base-pair matching.

33 DNA Excision Repair DNA Polymerase II On some occasions, errors in nucleotides may occur while making the new DNA strand. Errors such as mismatches & dimers may occur. To correct these errors, the enzymes nuclease, DNA polymerase II and DNA ligase are used during the process known as excision repair. 33

34 Telomeres, Telomerase & DNA Shortening At the end of eukaryotic chromosomes are known as telomeres Short, repetitive DNA sequences that do not contain genes. Typically 100 to 1000 nucleotides TTAGGG (Humans) Telomeres help protect the organism's genes from being eroded through successive rounds of DNA replication. 34

35 Telomeres, Telomerase & DNA Shortening Telomeres shorten each cell cycle (DNA replication sequence) but can be extended using the enzyme telomerase Absence of telomerase in certain cells may be the cause of cell aging Cells having a limited number of cell divisions Most cancer cells have telomerase to maintain the telomeres and possibly resist apoptosis. 35

36 REVIEW