Eukaryotic Replication – DNA Structure and Replication

by Georgina Cornwall, PhD

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    00:02 Now we can think about how things are different in prokaryotes and eukaryotes. Like it isn't complicated enough in the prokaryotic cell, eukaryotic cells have things going on that are little bit more complex. Good news is we don't understand them very well, as far as we understand, it is very similar to the bacterial replisome. The only issue is prokaryotic chromosomes are circular, eukaryotic chromosomes are linear so that leads us into another little piece of a problem with the telomeres. What are telomeres? Telomeres are the ends of linear chromosomes and they have long repeats. At the end of those chromosomes right, so long protective pieces. Now why do we have to have protective pieces? Let us take a look.

    00:58 When we look at the lagging strand, we have a sequence of Okazaki fragments that need to get glued together. Each of them had a primer that need to be replaced, but we have a leftover primer at the end and what did we say. We need a three primer OH handle in order to replace the primer and there is nothing to grab onto there and so even when we removed the primer, we cannot replace the primer. So by the second round of replication, we actually end up with a shorter template because those overhanging pieces of DNA template strands cannot be duplicated and so enzymes come along and gobble them up. And so each cell division, telomeres in a cell will get shorter unless they have around a special enzyme.

    01:52 The special enzyme that is involved in called telomerase. It constructs telomeres. Telomerase has an RNA template inside of it and it will launch onto the end of the chromosome and use its RNA template as a template for DNA nucleotides thus extending the telomeres making them longer so that it keeps going in round making them longer so that it is protected when we have this overhang leftover that is going to get clipped on. Telomerase extends the telomeres and that is really great when cells are undergoing a lot of cell division.

    02:38 In general, we will see a high level of telomeric activity or telomerase activity in cancer cells because cancer cells need to divide over and over and over and cells sort of have a finite amount of this telomere sequence that shortens and shortens and shortens with each cell division. In early development, all the cells have a high expression of telomerase meaning lots of telomerase is being made because they are going to have a lot of divisions, but by adulthood the expression of telomerase drops way down. It is the shortening of telomeres that's said to cause cellular damage and may have an impact on aging, so naturally there is lots of research going on there to see how we can get telomerase production up and avoid aging because we all want to avoid that. Also, we see cancer cells having a much higher expression rate. What is going on in cancer cells to promote the transcription and translation of telomerase and thus lengthening of the chromosomes in cancer cells? They uncontrollably divide, they have a finite number of divisions hardly fair is it? Anyway, interesting stuff there with telomerase.

    03:56 Now we have described how DNA replication occurs and addressed some of the things that are different in eukaryotic cells. You should be able to diagram how DNA is assembled thinking about the three prime and five prime ends and directionality it really helps to spend some time with the pen and paper and diagram how these leading and lagging strands get synthesized. You should also be able to explain the implications of semiconservative replication as well as describe the process of DNA replication and the roles of each of the enzymes in the process. Put those in your diagram, very good thing to have and then explain modifications that are necessary in eukaryotic chromosomes in order for successful replication. Thank you so much for listening. I will look forward to seeing you in the next lecture.

    About the Lecture

    The lecture Eukaryotic Replication – DNA Structure and Replication by Georgina Cornwall, PhD is from the course Understanding Genetics.

    Included Quiz Questions

    1. Long repeating sequences at the ends of chromosomes used to protect the coding portions from damage
    2. Short RNA sequences that provide an attachment point for Okazaki fragments
    3. A short region of DNA where transcription begins
    4. A portion of the gene that is spliced out before transcription
    5. The terminal end of tRNA where amino acids attach
    1. To decrease the rate at which telomeres shorten throughout a eukaryotes life
    2. To repair breaks in the coding region of DNA
    3. To bond Okazaki fragments together
    4. To prevent upstream supercoiling of DNA during replication
    5. To unwind DNA at the replication fork
    1. Telomeric shortening
    2. A decreasing number of replication forks during replication
    3. An increasing number of Okazaki fragments
    4. Lengthening of centromeres
    5. The disappearance of centromeres

    Author of lecture Eukaryotic Replication – DNA Structure and Replication

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD

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