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Thread: Genetics Problem Set Help! Human Fibroblasts, DNA replication, cell cycle, labeled with 3H thymidine...

  1. #1 Genetics Problem Set Help! Human Fibroblasts, DNA replication, cell cycle, labeled with 3H thymidine... 
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    Aug 2012
    I just don't really know where to start on this problem. It's been a long summer...this is my 1st HW assignment back and I'm kinda struggling haha. Any help or clues would be much very much appreciated
    In typical human fibroblasts in culture, the G1 period of the cell cycle lasts about 10 hrs, S lasts about 9 hrs, G2 takes 4 hrs, and M takes 1 hr. Assume the cells in the culture are distributed throughout the cell cycle in direct proportion to the length of each phase. Suppose you added radioactive [3H]-thymidine to the growth medium, left it there 5 minutes, and then washed it out and replaced it with ordinary growth medium.

    a) What percentage of cells would you expect to become labeled by incorporating the [3H]-thymidine into their DNA?

    b) What is the shortest amount of time you would have to wait after removing the [3H]-thymidine before you would see labeled metaphase chromosomes?

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    Forum Professor Zwolver's Avatar
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    May 2006
    A) T's are not used in RNA, so usually only during cell reparations, or multiplications. Depending on the stability of the cells, and the number of mutations and also the number of reparations.

    Though this will only be visible during the reproductive phase of the cells. I have no idea if s is reproductive, or m or g2, and honestly i see no point in giving you the exact answer.

    total cycle is 10 hours, reproductive phase is 1 hour. so (1/(10+1))*2 cells will have an increased radioactive thymidine ratio in their DNA. which is 18%. But you will find deviation because of the prolonged time the thymidine will stay in the cells. But there should only be 18% of the cells be showing a highened rate. Taking in account that i am setting the rate of polymerisation in that hour equally.

    B) Totally unknown. As the change of the 18% as stated above would be totally unnoticable. I don't know. You might not ever see anything, possibly. Depending on the rest of the medium. You will have to test is and check your theory. It's biology, that's what biology is. TEST, TEST, TEST..

    Edit: There is a problem with the answer to A though. I projected the cells to actually multiply, and thus the *2. The extra cells that would form mean the +1. Still, if you would determin it fast enough, you would see no thymidine in the core. just in the cells and supernatant.

    Last edited by Zwolver; September 1st, 2012 at 07:38 AM.
    Growing up, i marveled at star-trek's science, and ignored the perfect society. Now, i try to ignore their science, and marvel at the society.

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