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Topic 7.1 7.1.U4 DNA replication is continuous on the leading strand and discontinuous on the lagging strand • Distinguish between the lead strand and the lagging strand. A: On the leading strand synthesis occurs continuously while on the lagging strand it occurs in fragments. • What are Okazaki fragments and which strand do they form on? A: Fragments formed on the lagging strand. 7.1.U5 DNA replication is carried out by a complex system of enzymes. • Summarise the roles of the enzymes of DNA Replication: a. DNA gyrase- releases the strain ahead of DNA helicase. b. DNA helicase- unwinds the DNA at the replication fork. c. DNA primase- creates a RNA primer on the leading strand and many on the lagging. 1. RNA primer- initiates the activity of DNA polymerase on the lagging strand. 7.1.U5 DNA replication is carried out by a complex system of enzymes. • Enzymes cont: d. DNA polymerase- responsible for linking the DNA nucleotides to the 3’ end of the strand. 1. Also can proofread and remove RNA primers. e. DNA ligase- connects the gaps between fragments. 7.1.U5 DNA replication is carried out by a complex system of enzymes. Summary of the enzymes involved in DNA Replication DNA Gyrase (aka topoisomerase) moves in advance of helicase and relieves strain and prevents supercoiling on the separated strands DNA Helicase unwinds and separates the double stranded DNA by breaking the hydrogen bonds between base pairs http://commons.wikimedia.org/wiki/File:DNA_replication_en.svg 7.1.U5 DNA replication is carried out by a complex system of enzymes. Summary of the enzymes involved in DNA Replication DNA Ligase joins the Okazaki fragments together to create a continuous strand RNA Primase synthesises a short RNA primer on each template strand to provide an attachment and initiation point for DNA polymerase III DNA Polymerase III adds deoxynucleoside triphosphates (dNTPs) to the 3' end of the polynucleotide chain, synthesising in a 5' 3' direction DNA Polymerase I removes the RNA primers and replaces them with DNA http://www.ib.bioninja.com.au/_Media/dna_replication_med.jpeg 7.1.U3 DNA polymerases can only add nucleotides to the 3’ end of a primer. • In which direction does DNA polymerase move along the template strand? What implication does this have for the addition of bases on the growing strand? A: DNA polymerase always moves in 5’ to 3’ direction. • Contrast replication in prokaryotes and eukaryotes. A: In prokaryotes there is one site of replication. In eukaryotes there are many. 7.1.U6 Some regions of DNA do not code for proteins but have other important functions. • Distinguish between coding and non-coding regions of DNA. A: Coding genes code for the production of polypeptides non coding do not. Some have functions in regulating of gene expression. 7.1.U6 Some regions of DNA do not code for proteins but have other important functions. • Most of the eukaryotic genome is non-coding. There are two types of repetitive sequences: moderately repetitive sequences and highly repetitive sequences otherwise known as satellite DNA. Give an example of a region of DNA that contains highly repetitive sequences and outline the function of that region. A: The ends of the chromosomes called telomeres. They protect the ends of the chromosomes.