DNA and Replication copyright cmassengale 1 DNA Structure
- Slides: 33
DNA and Replication copyright cmassengale 1
DNA Structure Continued copyright cmassengale 2
copyright cmassengale 3
Antiparallel Strands • One strand of DNA goes from 5’ to 3’ • The other strand is opposite in direction going 3’ to 5’ copyright cmassengale 4
P 5 DNA O 3 3 P 5 O O C G 1 P 5 3 2 4 4 P 5 P 2 3 1 O T A 3 O 3 5 O copyright cmassengale 5 P P 5
Question: • If there is 30% Adenine, Adenine how much Cytosine is present? copyright cmassengale 6
Answer: • There would be 20% Cytosine • Adenine (30%) = Thymine (30%) • Guanine (20%) = Cytosine (20%) • Therefore, 60% A-T and 40% C -G copyright cmassengale 7
History of DNA copyright cmassengale 8
History of DNA • Early scientists thought protein was the cell’s genetic material because it was more complex than DNA copyright cmassengale 9
Griffith Experiment copyright cmassengale 10
Transformation • Fred Griffith worked with virulent S and nonvirulent R strain Pneumoccocus bacteria • He found that R strain could become virulent when it took in DNA from heat-killed S strain • Study suggested that DNA was probably the genetic material copyright cmassengale 11
DNA Structure • Rosalind Franklin took diffraction x-ray photographs of DNA crystals • In the 1950’s, Watson & Crick built the first model of DNA using Franklin’s x-rays copyright cmassengale 12
Rosalind Franklin copyright cmassengale 13
DNA Replication copyright cmassengale 14
DNA replication overview copyright cmassengale 15
Replication Facts • DNA has to be copied before a cell divides • DNA is copied during the S phase of interphase S • New cells will need phase • identical DNA G 1 G 2 • strands Mitosis copyright cmassengale 16
DNA Replication • Begins at Origins of Replication • Two strands open forming Replication Forks (Y-shaped region) • New strands grow at the forks 5’ Parental DNA Molecule 3’ copyright cmassengale 3’ Replication Fork 17 5’
DNA Replication • DNA replications is Semiconservative. • Each new double-stranded DNA contains one old strand (template) and one newly-synthesized complementary strand copyright cmassengale 18
DNA Replication • As the 2 DNA strands open at the origin, Replication Bubbles form • Chromosomes have MANY bubbles Bubbles copyright cmassengale 19
DNA Replication Enzymes • Helicase-unwinds and separates the 2 DNA strands by breaking the weak hydrogen bonds • Single-Strand Binding Proteins- attach and keep the 2 DNA strands separated and untwisted copyright cmassengale 20
DNA Replication Enzymes • Topoisomerase attaches to DNA ahead of the bubble to relieve stress on the DNA molecule as it separates. Video Enzyme DNA copyright cmassengale 21
DNA Replication • Before new DNA strands can form, there must be RNA primers present to start the addition of new nucleotides • Primase is the enzyme that synthesizes the RNA Primer • DNA polymerase III can then add the new nucleotides copyright cmassengale 22
DNA Replication • DNA polymerase can only add nucleotides to the 3’ end of the DNA • This causes the NEW strand to be built in a 5’ to 3’ direction 5’ 3’ Nucleotide DNA Polymerase Direction of Replication copyright cmassengale RNA Primer 23 5’
Synthesis of the New DNA Strands • The Leading Strand is synthesized as a single strand from the point of origin toward the opening replication fork 5’ 3’ Nucleotides DNA Polymerase copyright cmassengale 5’ RNA Primer 24
Synthesis of the New DNA Strands • The Lagging Strand is synthesized discontinuously against overall direction of replication • This strand is made in MANY short segments It is replicated from the replication fork toward the origin Leading Strand 5’ 3’ DNA Polymerase 5’ 3’ Lagging Strand RNA Primer copyright cmassengale 3’ 5’ 25
Lagging Strand Segments • Okazaki Fragments - series of short segments on the lagging strand • Must be joined together by an enzyme DNA Okazaki Fragment RNA Primer 5’ 3’ Polymerase 3’ 5’ Lagging Strand copyright cmassengale 26
Joining of Okazaki Fragments • The enzyme Ligase joins the Okazaki fragments together to make one strand DNA ligase 5’ 3’ Okazaki Fragment 1 Okazaki Fragment 2 3’ 5’ Lagging Strand copyright cmassengale 27
Replication of Strands Replication Fork Point of Origin copyright cmassengale 28
Proofreading New DNA • DNA polymerase initially makes about 1 in 10, 000 base pairing errors • Enzymes proofread and correct these mistakes • The new error rate for DNA that has been proofread is 1 in 1 billion base pairing errors copyright cmassengale 29
DNA Damage & Repair • Chemicals & ultraviolet radiation damage the DNA in our body cells • Cells must continuously repair DAMAGED DNA • Excision repair occurs when any of over 50 repair enzymes remove damaged parts of DNA • DNA polymerase and DNA ligase replace and bond the new nucleotides together copyright cmassengale 30
Question: • What would be the complementary DNA strand for the following DNA sequence? DNA 5’-CGTATG-3’ copyright cmassengale 31
Answer: DNA 5’-CGTATG-3’ 3’-GCATAC-5’ copyright cmassengale 32
copyright cmassengale 33
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