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Dna replication
DNA replication involves copying the DNA of a cell before it divides. It occurs during the S phase of interphase and results in two identical DNA molecules, each with one original and one new strand. The double helix unwinds and an enzyme adds complementary nucleotides to each strand to replicate the DNA. On the lagging strand, DNA is synthesized discontinuously in short segments called Okazaki fragments which are later joined together. DNA replication ensures that daughter cells have identical copies of the parental DNA.
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Dna replication
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- 2. 2 Replication Facts • DNAhas to be copied before a cell divides • DNA is copied during the S (or synthesis) phase of interphase • New cells will need identical DNA strands
- 3. 3 Synthesis Phase (Sphase) • S phase during interphase of the cell cycle • Nucleus of eukaryotes Mitosis -prophase -metaphase -anaphase -telophase G1 G2 S phase interphase DNA replication takes place in the S phase.
- 4. 4 DNA Replication • Beginsat Origins of Replication • Two strands open forming Replication Forks (Y-shaped region) • New strands grow at the forks Replication Fork Parental DNA Molecule 3’ 5’ 3’ 5’
- 5. 5 DNA Replication • Asthe 2 DNA strands open at the origin, Replication Bubbles form • Prokaryotes (bacteria) have a single bubble • Eukaryotic chromosomes have MANY bubbles Bubbles Bubbles
- 6. 6 DNA Replication • EnzymeHelicase 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
- 7. 7 DNA Replication • EnzymeTopoisomerase attaches to the 2 forks of the bubble to relieve stress on the DNA molecule as it separates Enzyme DNA Enzyme
- 8. 8 DNA Replication • Beforenew 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 can then add the new nucleotides
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- 10. 10 DNA Replication • DNApolymerase 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 RNA PrimerDNA Polymerase Nucleotide 5’ 5’ 3’ Direction of Replication
- 11. 11 Remember HOW the CarbonsAre Numbered! O O=P-O O Phosphate Group N Nitrogenous base (A, G, C, or T) CH2 O C1 C4 C3 C2 5 Sugar (deoxyribose)
- 12. 12 Remember the Strandsare Antiparallel P P P O O O 1 2 3 4 5 5 3 3 5 P P P O O O 1 2 3 4 5 5 3 5 3 G C T A
- 13. 13 Synthesis of theNew DNA Strands • The Leading Strand is synthesized as a single strand from the point of origin toward the opening replication fork RNA PrimerDNA PolymeraseNucleotides 3’5’ 5’
- 14. 14 Synthesis of theNew 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 RNA Primer Leading Strand DNA Polymerase 5 ’ 5’ 3’ 3’ Lagging Strand 5’ 5’ 3’ 3’
- 15. 15 Lagging Strand Segments •Okazaki Fragments - series of short segments on the lagging strand • Must be joined together by an enzyme Lagging Strand RNA Primer DNA Polymerase 3’ 3’ 5’ 5’ Okazaki Fragment
- 16. 16 Joining of OkazakiFragments • The enzyme Ligase joins the Okazaki fragments together to make one strand Lagging Strand Okazaki Fragment 2 DNA ligase Okazaki Fragment 1 5’ 5’ 3’ 3’
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- 18. 18 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
- 19. 19 Semiconservative Model of Replication •Idea presented by Watson & Crick • The two strands of the parental molecule separate, and each acts as a template for a new complementary strand • New DNA consists of 1 PARENTAL (original) and 1 NEW strand of DNA Parental DNA DNA Template New DNA
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- 21. 21 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
- 22. 22 Question: • What wouldbe the complementary DNA strand for the following DNA sequence? DNA 5’-CGTATG-3’
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- 24. Post-Replicative Modification ofDNA Methylation; one of the major post- replicative reactions. Site of methylation of eukaryotic DNA is always on cytosine residues in CG dinucleotide. DNA methylation plays an important role for epigenetic gene regulation in development and disease.
- 25. Epigenetics • Epigenetics refersto the study of changes in the regulation of gene activity and expression that are not dependent on gene DNA sequence. • While epigenetics often refers to the study of single genes or sets of genes, epigenomics refers to more global analyses of epigenetic changes across the entire genome.
- 26. Epigenetics Mechanisms Gene Expression RNA Interference HistoneModifications DNA Methylation
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- 28. Natural Roles ofDNA Methylation in Mammalian System Imprinting X chromosome inactivation Heterochromatin maintenance Developmental controls Tissue specific expression controls
- 29. DNA Methylation and OtherHuman Diseases -- Imprinting Disorder: • Beckwith-Wiedemann syndrom (BWS) • Prader-Willi syndrome (PWS) • Transient neonatal diabetes mellitus (TNDM) -- Repeat-instability diseases • Fragile X syndrome (FRAXA) • Facioscapulohumeral muscular dystroph -- Defects of the methylation machinery • Systemic lupus erythemtosus (SLE) • Immunodeficiency, centromeric instability and facial anomalies (ICF) syndrome