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7.1 DNA structure and replication Understanding: -DNA structure suggested a mechanism for DNA replication -Nucleosomes help to supercoil the DNA -DNA replication

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  • 7.1 DNA structure and replication Understanding: -DNA structure suggested a mechanism for DNA replication -Nucleosomes help to supercoil the DNA -DNA replication is continuous on the leading strand and discontinuous on the lagging strand -DNA replication is carried out by a complex system of enzymes -DNA polymerase can only add nucleotides to the 3 end of the primer -Some regions of DNA do not code for proteins but have some other important function Applications: -Rosalind Franklins and Maurice Wilkins investigation of DNA structure by X-ray diffraction -Tandem repeats are used in DNA profiling -Use of nucleotides containing dideoxyribonucleic acid to stop DNA replication Skills: -Analysis of results of the Hershey and Chase experiment providing evidence that DNA is the genetic material -Utilisation of molecular visualisation software to anaylyse the association between proteins and DNA within a nucleosome Nature of science: -Making careful observations: Rosalind Franklins X-ray diffraction provided crucial evidence that DNA is a double helix
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  • Hershey-Chase experiments Is DNA the genetic material? Alfred Hershey and Martha Chase used radioisotopes to help prove this.
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  • Radioisotopes Forms of elements that decay over time at a predictable rate. Particles given off in this decay allow detection of the specific isotope used
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  • Hershey-Chase experiments Grew bacteriophage viruses in two different types of cultures. Radioactive phosphorous-32 - Detectable phosphorous in DNA Radioactive sulphur-32 -Detectable sulphur found in outer coat of virus Allowed to infect bacteria (E.coli)
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  • Hershey-Chase experiments As DNA contains phosphorous and not sulphur, they concluded that DNA was the genetic material not protein.
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  • Hershey-Chase experiments Describe the Hershey-Chase experiment:
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  • DNA structure 3 components: Pentose sugar (deoxyribose in DNA) Phosphate Organic base (always contains nitrogen) Phosphate sugar base Stay the same Changes Contains nitrogen & carbon Pentose sugar (5 Carbon atoms)
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  • Single strand of DNA Backbone is made of alternating phosphate and deoxyribose sugar They are held together by a phosphodiester bond (just ester at SL!) Condensation reaction producing water This produces a chain of DNA (as this links the single nucleotides together)
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  • Phosphodiester bond Where does the phosphodiester bonds occur? Phosphate group reacts on the 5 carbon Hydroxyl group reacts on the 3 carbon
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  • Double strand of DNA The strands of DNA run in opposite directions to each other There is a 5 carbon free to bond on this end and a 3 carbon free to bond on this end There is a 3 carbon free to bond on this end and a 5 carbon free to bond on this end
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  • Double strand of DNA Each end is either the 5-prime or 3-prime end 5 3 5
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  • Label the phosphodiester bond, hydrogen bonds, 3 end and 5 end for both strands, bases, ribose sugar, phosphate group, 3 carbon and 5 carbon in ribose.
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  • DNA packaging DNA molecules are paired with a protein called histone Histones help to package DNA Essential as DNA can be 4cm long, it must fit into a microscopic nucleus
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  • DNA packaging Unfolded DNA looks like beads on a string These beads are nucleosomes 8 histones (proteins) make up a nucleosome core DNA wraps round these histones twice (slight negative charge on DNA attracts to positive charge of histones) Between the nucleosomes are strings of DNA
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  • DNA sequences From the Multinational Genome Project we have learnt that less than 2% of human DNA codes for proteins. What does the other 98% do? -Regulate gene expression -Telomeres (protects DNA) -Introns (interruptions in coding region)
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  • Types of DNA sequences Find out what the following are: -Protein-coding genes -Highly repetitive sequences -Structural DNA -Short tandem repeats
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  • Protein-coding genes Highly repetitive sequences Structural DNA Short-tandem repeats
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  • Protein coding genes Genes that have coding functions Provide base sequence to produce proteins Genes will have interruptions of non-coding regions in between them then must be spliced out before proteins are made. Coding regions = Exons Non-coding regions = Introns
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  • Highly repetitive sequences 3-500 base pairs long in a repetitive sequence It could go up to 100,000 repeats of a single unit If it is clustered together satellite DNA So far not discovered any coding function (not genes) Transposable can move from one location to another AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT T
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  • Structural DNA Highly coiled DNA with no coding function. Near centromere and telomeres Could have lost their function due to mutations involving a base sequence change
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  • Short Tandem Repeats Your DNA is almost identical to the person next to you Specific regions are varied polymorphisms Analyse these using DNA profiling Usually look at short tandem repeats Short repeating sequences of DNA 2-5 base pairs T T T C C C T C A T C A T C A T C A T C C C G A A A G G G A G T A G T A G T A G T A G G G C
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  • DNA replication Replication starts as a bubble Helicase breaks the hydrogen bonds between nucleotides Two strands separate At each end of the bubble there is a replication fork where DNA strands open. Bubbles enlarge in both directions bidirectional Bubble eventually fuse with one another to produce two identical daughter DNA molecules
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  • DNA replication 1.Primer (short piece of RNA) is produced at the replication fork by primase 2.Primers match exposed DNA bases, marks the start of replication 3.DNA polymerase III allows addition of nucleotides in 5 to 3 direction 4.DNA polymerase I removes the primers from the 5 end
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  • Energy to create the bonds Each nucleotide molecule is a deoxynucleoside triphosphate (dNTP) molecule Contains -Deoxyribose -A base -Three phosphate groups As the molecules are added together to form nucleotides, two phosphates are lost This provides energy needed for the nucleotides to bind
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  • DNA replication When DNA is replicated it assembles 5 to 3 due to the action of polymerase III. 5 3 5
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  • DNA replication This means that there is a difference in the assembly of DNA from the templates Leading strand continuous and fast (5 to 3) Lagging strand slowly (3 to 5)
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  • DNA replication 1.The leading strand assembled continuously in 5 to 3 direction 2.Lagging strand is assembled by fragments produced moving away from the replication fork in the 5 to 3 direction 3.Fragments of the lagging strand called Okazaki fragments 4.Primer, primase and DNA polymerase III are needed to begin both the leading and lagging strand 5.Once Okazaki fragments assembled, DNA ligase enzyme attaches them together to make a single strand
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  • Formation of the lagging strand 1. 2. 3. 4. 5. 6.
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  • Proteins in replication What do they do? ProteinRole Helicase Primase DNA polymerase III DNA polymerase I DNA ligase
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  • Proteins in replication What do they do? ProteinRole HelicaseUniwnds double helix Breaks H bonds PrimaseSynthesises RNA primer DNA polymerase IIISynthesises new strand by adding nucleotides onto the primer (5 to 3 direction) DNA polymerase IRemoves primer DNA ligaseJoins Okazaki fragments
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  • Complete for your homework DNA sequencing -What is DNA sequencing? -What is the process? -What is it used for? DNA profiling -What is DNA profiling? -What is the process? -What is it used for? -How is it different to DNA sequencing?