Javad Jamshidi Fasa University of Medical Sciences Chromatin Organization And DNA Replication 2 The DNA-protein complex which exist in eukaryotic cells, is highly conserved in all eukaryotes. Both histones and non-histones are involved in physical structure of the chromosome Histones are abundant, small proteins. The five main types are H1, H2A, H2B, H3 and H4, highly conserved The DNA is organized inside the nucleus from linear double strand to chromosomes 3 4 Consists of a protein core with DNA wound around its surface The core is an octamer containing two copies each of histones H2A, H2B, H3, and H4 Nucleosomes from all eukaryotes contain 147 base pairs of DNA wrapped one and two-thirds turns around the protein core The length of the linker DNA is variable among species, and even between different cells of one organism, ranging from about 10 to 90 base pairs 5 6 7 8 9 10 All organisms must duplicate their DNA before each cell division Duplicating DNA at rates as high as 1000 nucleotides per second Extraordinary accuracy is Needed, < 1 in 10 9 nucleotides per generation The complementary nature of DNA make replication possible 11 12 13 14 Unwind the strands (Helicases) Keep it Unwound ( Single Stranded DNA Binding Proteins, or SSBs) A free 3 OH end (Primase) Add dNTPs to 3 end (DNA Polymerases) 15 16 17 18 19 20 21 22 23 24 25 26 27 28 During replication, DNA becomes overwound ahead of a replication fork Topoisomerases are enzymes that regulate the overwinding or underwinding of DNA There are 2 types of topoisomerases 29 30 Accuracy of just one mistake for every 10 9 nucleotides, is much higher than just base-pairing mechanism. Proofreading mechanisms act to correct any initial mispairings The 3 5 proofreading exonuclease activity of DNA pol. Depend on its requirement for a perfectly base-paired primer terminus, not in RNA 31 32 33 34 Eukaryotic chromosomal DNA molecules contain multiple replication origins separated by tens to hundreds of kilobases A six-subunit protein called ORC -Origin Recognition Complex- binds to each origin and load other proteins Replication of cellular DNA and other events leading to proliferation of cells are tightly regulated