Prokaryotic Replication – DNA Structure and Replication

by Georgina Cornwall, PhD

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    00:00 chromosome separates and new strands are replicated for each of the daughter chromosomes. Then we need to understand how replication happens. What is the general pattern and then we will step into the details. First we are going to begin by looking at prokaryotic replication. In prokaryotic replication, you recall the bacteria have a single circular chromosome. It may not look like a circle, it could be all crumbled up in the nucleus, but it is single and it is circular and so each of them will have a specific origin of replication and a specific sequence that says termination of replication.

    00:36 In this bacterial chromosome, we will see a replicon opened up or a space opened up in which we can have our enzymes or enzymes machinery. We call that machinery a replisome and those replisomes will proceed bidirectionally around the circular chromosomes until they both run into the terminus at which case the two new chromosomes will separate from each other and go into their respective cells. Pretty simple process right? Well not quite.

    01:12 It is the machinery that is in the two replicons in the replisome that is involved in the whole replication process that we have to get to know in this lecture. Key again here I might have repeat it because it comes up over and over is that DNA polymerase are primary enzyme that is laying down at nucleotides can only read in the three prime to five prime directions.

    01:40 The other key piece of information is that DNA polymerase also needs that three prime OH group. Those three prime OH group that I keep emphasizing is really important to know. It needs a three prime OH group to grab on to the previous nucleotide and lay down the next nucleotide. Let us say we are looking at a template strand and we are looking at the newly synthesized strand. We will say that the three prime is on this template strand and we are proceeding in this direction. DNA polymerase has to read the template strand in this direction, but it is laying down a complimentary strand in this direction.

    02:22 So we have one nucleotide with the three prime OH hanging out on that end, and another nucleotide with three prime OH hanging out on that end. The newly synthesized strand is now going to be oriented in the five prime to three prime directions. Three prime OH is critical reading, always three prime to five prime, laying down new nucleotides five prime to three prime. Critical to understand in order to see why DNA replication can get quite confusing.

    03:00 Let us look at more details. Here is our nucleotide. We will emphasize the piece with the sugar. We have our three prime OH and our five prime phosphates. Super important parts of a molecule to understand how they are all put together on the newly synthesized DNA strand. These parent strands are going to be antiparallel. We have a replication fork. When that replication fork opens up, we can see that the template strands are sort of in opposite orientation. We have a leading strand that is oriented from three prime, the green piece, three prime to five prime no problem whatsoever. DNA polymerase can run along that strand and lay down nucleotides continuously. However, the other strand, the old complimentary strand has a little bit of an issue because now we can only read at that replication fork, there is a five prime end sticking out on top and the green strand goes to a three prime end in which case DNA polymerase has an issue because he cannot read five to three. It has to read the green strand from three prime to five prime and lay down nucleotides in an antiparallel fashion. So that requires a little bit of jumping back.

    04:33 We will look at the details of how that works shortly, but as we jump back, we need to create different fragments. We call these fragments Okazaki fragments. I will show you how we will deal with them shortly. But Okazaki fragment is a word that you need to know. Keep in mind that replication is bidirectional. There is a replication fork on both ends. One side we have talked about. Here we have a lagging strand up on top, a leading strand down below.

    05:03 On the other end, the lagging strand is going to be on the bottom and the leading strand is going to be on the top, but it is on this bottom strand now that we are reading the template from five to three that doesn't work out for DNA polymerase. So we are going to have Okazaki fragments on this lower strands also, bi-directional replication.

    About the Lecture

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

    Included Quiz Questions

    1. semiconservative replication.
    2. conservative replication.
    3. dispersive replication.
    4. semidiscontinuous replication
    1. 3'
    2. 1'
    3. 2'
    4. 4'
    5. 5'
    1. …on the lagging strand.
    2. …on the leading strand.
    3. …in the replication fork.
    4. …in the replicon.
    5. …in the terminus.

    Author of lecture Prokaryotic Replication – DNA Structure and Replication

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD

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