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1.30.07 - 1.30.2007 Eukaryotes have linear DNA the new...

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1.30.2007 Eukaryotes have linear DNA, the new strand is always shorter than the old strand because there is no way a primer could be made and then replaced in a way for the lagging strand to be that long o Telomerase is what puts DNA on the ends, it is like DNA polymerase and can only add to the 3’ end, so it cannot extend the 5’ end of the new growing strand o It instead adds nucleotides to the longer, old, template strand’s 3’ end, this is the only place that enzymatically and chemically this can work o Where is the template that it uses? Telomerase has its own portable template that it reuses over and over, it is part of the enzyme itself and is made out of RNA There is a gene that is transcribed in order to make the RNA template It aligns by base pairing a little of that template with the overhang of the old template strand at its 3’ end The protein part of the telomerase enzyme picks up triphosphate DNA nucleotides and base pairs it to the RNA template, the RNA template dictates what it added to this longer 3’, template strand the template is relatively short and can only add 6 nucleotides to the end telomerase lets go, repositions itself six nucleotides further down after where it just added them (to the new 3’ end) and it does this again, GGGTGG, GGGTGG, same sequences over and over, whatever is dictated by the template that it carries this sequence encodes nothing, this is the point, it is junk, it will never be transcribed, this is nonsensical and useless but its useful in its uselessness, this sequence could be lost off of the end of the chromosomes and no information is lost to the cell through several replications this sequence can be lost and nothing bad happens, it can be in a state of flux, its sacrificial o We still haven’t accomplished our goal, but now we have a lot more room along the template strand that can now be used for the other strand to make an Okazaki fragment, before there was no space for a fragment o DNA pol alpha can now make a primer in the direction opposite the direction that telomerase was adding, so that it can add 3’ 5’ and then DNA pol delta continues the strand and ligase seals the gap in the lagging strand o This happens regularly when the telomeres (GGGTGG is the telomeric sequence that is repeated over and over) shorten through several replications and then telomerase can put it back, maintaining continuity of chromosome size over time, but not all eukaryotic cells have telomerase activity If you try to grow cells in culture, if they are young cells, they divide quickly for a while as long as they have food and space, you can keep giving them food and space but after a while they still don’t divide quickly, and then not at all, they lose capacity to divide, one of the reasons cells do this is because a lot of eukaryotic cells do not have telomerase in them so they start out with long telomeres and with each replication they lose some
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