DNA Replica,on. An,parallel Elonga,on. DNA polymerases. Add nucleo1des only to the free 3 end of a growing strand

Transcription

1 DNA Replica,on nucleoside triphosphate Nucleodes that are added to a growing DNA Deoxyribonucleoside triphosphate (dntp) supply nucleodes to growing DNA As monomer dntp joins DNA loses two phosphate groups as a molecule of pyrophosphate Fig. 6 4 New end end end end Sugar Phosphate A Base T A T G G G G end T A DNA polymerase T A Pyrophosphate end Nucleoside triphosphate end end An,parallel Elonga,on DNA polymerases Add nucleodes only to the free end of a growing Therefore DNA elongates only in to direcon DNA polymerase synthesizes leading connuously moving toward the replicaon fork

2 Fig. Fig. 6 5 Overview Origin of replication Primer Parental DNA Overall directions of replication Origin of replication RNA primer Sliding clamp DNA poll III lagging Strand constructed away from replicaon fork Okazaki fragments series of segments An,parallel Elonga,on joined together by DNA ligase

3 Fig. Fig. 6 6a Overview Origin of replication Overall directions of replication Fig. 6 6b Fig. 6 6b RNA primer

4 Fig. Fig. 6 6b3 RNA primer Okazaki fragment Fig. 6 6b4 RNA primer Okazaki fragment Fig. 6 6b5 RNA primer Okazaki fragment

5 Fig. Fig. 6 6b6 RNA primer Okazaki fragment Overall direction of replication Fig. 6 7 Overview Origin of replication Single- binding protein Overall directions of replication Helicase Parental DNA DNA pol III Primer Primase DNA pol III 4 DNA pol I 3 DNA ligase DNA replicaon complex The DNA Replica,on omplex proteins that parcipate in replicaon DNA replicaon machine Recent studies probably staonary during the replicaon process support a model in which DNA polymerase molecules reel in parental DNA and extrude newly made daughter DNA molecules DNA Replication Review

6 Proofreading Proofreading and Repairing DNA Eukaryoc DNA ploymerases replace any incorrect nucleodes Mismatch repair repair enzymes correct errors in base pairing DNA can be damaged by chemicals, radioacve emissions, X rays, UV light, and certain molecules (in cigareoe smoke for example) Nucleo,de excision repair nuclease cuts out and replaces damaged stretches of DNA Fig. 6 8 Thymine dimer Nuclease DNA polymerase DNA ligase Replica,ng the Ends of DNA Molecules Limitaons of DNA polymerase create problems for the linear DNA of eukaryoc chromosomes Standard replicaon machinery provides no way to complete the ends repeated rounds of replicaon produce shorter DNA molecules

7 Fig. Fig. 6 9 Ends of parental DNA s Last fragment Previous fragment Parental RNA primer Removal of primers and replacement with DNA where a end is available Second round of replication New leading New lagging Further rounds of replication Shorter and shorter daughter molecules Telomeres Replica,ng the Ends of DNA Molecules Nucleode sequences At ends of eukaryoc chromosomal DNA molecules Do not prevent the shortening of DNA molecules Postpone the erosion of genes near the ends of DNA molecules It has been proposed that the shortening of telomeres is connected to aging Fig. 6 0 µm

8 Then what about gametes? If germ cell chromosomes became shorter in every cell cycle telomerase Replica,ng the Ends of DNA Molecules genes eventually damaged and gametes they produce catalyzes the lengthening of telomeres in germ cells fountain of youth? hop:// html Replica,ng the Ends of DNA Molecules The shortening of telomeres might protect cells from cancerous growth by liming the number of cell divisions evidence of telomerase acvity in cancer cells may allow cancer cells to persist DNA packaging Bacterial chromosome ircular, small amount of protein supercoiled, in nucleoid region Eukaryoc chromosome Linear, large amount of protein

9 DNA packaging hroma,n complex of DNA and protein Histones proteins that are responsible for the first level of DNA packing in chroman Fig. 6 a DNA double helix ( nm in diameter) Histones Nucleosome (0 nm in diameter) Histone tail DNA, the double helix Histones Nucleosomes, or beads on a string (0-nm fiber) H Fig. 6 b hromatid (700 nm) 30-nm fiber Loops Scaffold 300-nm fiber Replicated chromosome (,400 nm) 30-nm fiber Looped domains (300-nm fiber) Metaphase chromosome

10 DNA packaging hroman is organized into fibers 0 nm fiber DNA winds around histones to form nucleosome beads Nucleosomes are strung together like beads on a string by linker DNA 30 nm fiber Interacons between nucleosomes cause the thin fiber to coil or fold into this thicker fiber 300 nm fiber DNA packaging The 30 nm fiber forms looped domains that aoach to proteins Metaphase chromosome Looped domains coil further Width of a chromad is 700 nm 30-nm fiber 30-nm fiber LoopsScaffold Looped domains (300-nm fiber) 300-nm fiber hromatid (700 nm) Replicated chromosome (,400 nm) Metaphase chromosome Most chroman DNA packaging loosely packed during interphase Euchroma,n condenses prior to mitosis Loosely packed chroman Heterochroma,n Highly condensed chroman centromeres and telomeres During interphase Dense packing of the heterochroman makes it difficult for the cell to express genec informaon coded in these regions

11 You should now be able to:. Describe the contribuons of the following people: Griffith; Avery, Mcary, and MacLeod; Hershey and hase; hargaff; Watson and rick; Franklin; Meselson and Stahl. Describe the structure of DNA 3. Describe the process of DNA replicaon; include the following terms: anparallel structure, DNA polymerase, leading, lagging, Okazaki fragments, DNA ligase, primer, primase, helicase, topoisomerase, single binding proteins 4. Describe the funcon of telomeres 5. ompare a bacterial chromosome and a eukaryoc chromosome