1 DNA and Replication. 2 History of DNA Early scientists thought protein was the hereditary material –Protein is more complex than DNA –Proteins are composed.

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Presentation transcript:

1 DNA and Replication

2 History of DNA Early scientists thought protein was the hereditary material –Protein is more complex than DNA –Proteins are composed of 20 different amino acids in long polypeptide chains

3 Transformation Fred Griffith worked with virulent “S” and nonvirulent “R” strains of Pneumoccocus bacteria He found –R strain becomes virulent when it absorbs DNA from killed S strain bacteria –Transformation- bacteria gets DNA from a dead bacteria and aquires new traits Study suggested that DNA was probably the genetic material

4 Griffith Experiment

5 History of DNA Viruses are made of DNA and protein Hershey & Chase used radioactively tagged bacteriophage viruses to prove DNA is the genetic material –Tag DNA with 32 P –Tag protein with 35 S –32P shows up in the bacteria, 35 S doesn’t

6 DNA Replication

7 Replication Facts DNA must be copied before a cell dividesDNA must be copied before a cell divides DNA is copied during S (synthesis) phase of interphaseDNA is copied during S (synthesis) phase of interphase New cells need identical DNA strandsNew cells need identical DNA strands

8 Synthesis Phase (S phase) S phase during interphase of the cell cycle Nucleus of eukaryotes Mitosis -prophase -metaphase -anaphase -telophase G1G1 G2G2 S phase interphase DNA replication takes place in the S phase.

9 DNA Replication Enzyme Helicase unwinds& separates the 2 DNA strands by breaking weak hydrogen bondsEnzyme Helicase unwinds& separates the 2 DNA strands by breaking weak hydrogen bonds Begins at Origins of ReplicationBegins at Origins of Replication Two strands open forming Replication Forks (Y-shaped region)Two strands open forming Replication Forks (Y-shaped region) ReplicationFork Parental DNA Molecule 3’ 5’ 3’ 5’

10 DNA Replication DNA polymerase adds new nucleotides Replication Bubbles form as the DNA strands open at the originReplication Bubbles form as the DNA strands open at the origin Eukaryotic chromosomes have Multiple bubbles Prokaryotes (bacteria) have one Single bubble Bubbles

11 Proofreading New DNA DNA polymerase makes about 1 in 10,000 base pairing errorsDNA polymerase makes about 1 in 10,000 base pairing errors Enzymes proofread and correct mistakesEnzymes proofread and correct mistakes Error rate for proofread DNA is 1 in 1 billion base pairing errorsError rate for proofread DNA is 1 in 1 billion base pairing errors

12 DNA Damage & Repair Chemicals & ultraviolet radiation damage DNA in our cells Cells continuously repair DAMAGED DNA Excision repair- one of over 50 different repair enzymes removes damaged DNA DNA polymerase and DNA ligase replace and tie in (bond) new, correct nucleotides

13 Replication is Semiconservative Watson & Crick suggested: TheThe parental DNA acts a template for a new complementary strand Final, new DNA molecules consist of 1 Parental (original) strand and 1 New strand of DNA Parental DNA DNA Template New DNA

14 Question: What would be the complementary DNA strand for the following DNA sequence? DNA 5’-CGTATG-3’

15 Answer: DNA 5’-GCGTATG-3’ DNA 3’-CGCATAC-5’