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Structure of DNA and RNA – DNA Structure and Replication

by Georgina Cornwall, PhD
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    00:01 So now you know all about chromosomes and that they replicate during S phase of the cell cycle.

    00:08 Let us take a deeper look into what actually happens during S phase in replication of DNA. By the end of this lecture, you will be able to diagram the manner in which nucleotides assemble themselves in DNA as well as explain the implications of semiconservative replication.

    00:28 You will be able to describe the process of DNA replication and the role of each of the enzymes in that process as well as explain modifications that are necessary for eukaryotic chromosomes. We will begin with a quick review of DNA structure.

    00:45 You will recall that there are a phosphate and sugar backbone. Much like a spiral staircase, there are steps made of nitrogenous bases and these steps and nitrogenous bases with the phosphate, sugar backbone form a whole molecular DNA, which has an antiparallel orientations.

    01:05 That is each strand is in opposition to the other strand, so they run in an antiparallel fashion, which lends itself to some particular situations during DNA replication that need a little bit of adaptation. Looking at that antiparallel orientation, we know that there is a three prime OH group that is going to be really important to us during DNA replication and on the other strand that same end, you have a five prime phosphate hanging off.

    01:38 The reason this is important is DNA polymerase can only read strands from the three prime to five prime directions. It can only go if we were counting 1, 2, 3, 4, 5. It can only read the three prime to five prime. Keep that in mind. You also will recall that the nitrogenous base steps are held together with hydrogen bonds. There are two bonds between adenine and thymine and there are three bonds between guanine and cytosine. The main point here though is that it's hydrogen bonds that hold the two strands that are antiparallel together.

    02:18 And the backbone is held together by what kind of bonds? Phosphodiester bonds that are covalent bonds. And are they stronger or weaker than the bonds between the strands, the hydrogen bonds? Right, they are stronger. When we pull on the DNA molecule and try to rip it apart, it conveniently unzips right down the center, so we have access to the nitrogenous bases, DNA polymerase can do its job. What happens during DNA replication? In synthesis of the cell cycle S phase, you will recal that cells spent most of their time in gap 1 doing their daily routine and they may not be dividing again or if they are dividing, they may spend a fair amount of time in gap 1. But once they are committed to divide, it is time to get the machinery together for synthesis phase and we are going to learn all about the machinery in this lecture. Synthesis phase is the highlight. Everything in this lecture is about synthesis phase. First of all, we need to understand that DNA replication is semiconservative. That is, the DNA will separate and then be replicated.

    03:39 Each new molecule of DNA is composed of one old strand and one new strand. The parent chromosome separates and new strands are replicated for each of the daughter chromosomes. Then


    About the Lecture

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


    Included Quiz Questions

    1. DNA polymerase
    2. RNA polymerase
    3. Helicase
    4. Primase
    5. Ligase
    1. …hydrogen bond zipper.
    2. …covalent bond zipper.
    3. …phosphodiester bonds.
    4. …peptide bonds.
    5. …ionic bond zipper.

    Author of lecture Structure of DNA and RNA – DNA Structure and Replication

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD


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