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Explore the structure, replication, and repair processes of DNA. Learn about nucleotides, the double helix, DNA replication in prokaryotes and eukaryotes, helicases, synthesis, telomeres, telomerase, and DNA repair. Discover the fascinating world of DNA!
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DNA Part II: The "Stuff" of Life Its Structure, Replication, & Unending Repair
DNA Forming Chromosomes • Structure in eukaryotes. • the DNA is wrapped around proteins called histones forming nucleosomes. • This forms a fiber known as chromatin. • This forms a coil within a coil. 9
Experiment of Meselson and Stahl Matt Meselson (left) and Frank Stahl (right) in 1958 demonstrated that replication was semi-conservative using radioactive nucleotides with dividing bacteria supporting Watson’s and Crick’s hypothesis 12
Prokaryotic DNA Replication The prokaryotic chromosome attaches to the plasma membrane. The DNA is then replicated in both directions. 15
Eukaryotic DNA Replication DNA replication occurs simultaneously in many locations along the very long eukaryotic chromosomes. Three replication bubbles are visible along the DNA within this cultured Chinese hamster cell. The arrows indicate the directions of DNA replication at the two ends of the bubble. 16
Helicases Helicases are enzymes responsible for the unwinding of the DNA molecule. They unwind the DNA in both directions
Releasing Stress in the DNA Molecule Since DNA is a double helix, there will be tension in the DNA strand that causes it to tangle as it is unwound by the helicase. The enzymes topoisomerase I and II are responsible for relieving that stress by clipping one or two strands of the DNA. 18
Adding Nucleotides as Triphosphates Nucleotides are always added on as triphosphates. When the nucleotides are added then two phosphates are cleaved off making a pyrophosphate. 19
Synthesis ALWAYS occurs in the 5 to 3 direction! When a DNA molecule is being synthesized , the nucleotides are added as triphosphates, and two phosphates are removed. Nucleotides are always added to the 3 end!
In Need of a Primer! DNA polymerase must always attach the com-plementary nucleotide to a 3 end of the deoxyribose sugar molecule. So, in the very beginning a small RNA primer must be laid down in order to start the process of DNA replication. Primase is the enzyme responsible for this. 21
Putting Down a RNA Primer RNA nucleotides (red pentagons) are being laid down by primase before DNA polymerase begins DNA replication.
The Lagging Strand and Ligase This animation, shows the leading strand being synthesized followed by the lagging strand. The enzyme named ligase ties them together.
Function of Telomeres Once DNA has been replicated, there is one problem. The usual replication machinery provides no way to complete the 5 ends after the RNA primer is removed, so repeated rounds of replication produce shorter and shorter DNA molecules. To compensate for this repeated shortening process, repetitive sequences of DNA are added. These are noncoding sequences and called telomeres. 25
Maintaining Telomere Length in Gametes and Germ Cells In gametes, the shortening of telomeres would cause serious problems. If chromosomes of gametes became shorter each time during replication, then essential genes would eventually be missing. An enzyme complex called telomerase catalyzes the lengthening of telomeres in gametes. 27
DNA Repair Errors in DNA replication occur about 1 in every 10,000 base pairs. Not bad, but with 6 billion bases being replicated that amounts to 60,000 mistakes every time a cell divides. DNA repair systems repair about 99% of these mistakes. 29
