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Gene Expression – Protein Functions

by Kevin Ahern, PhD
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    00:01 The role of proteins in regulating gene expression is a very important function that they perform.

    00:05 It is especially important for multicellular organisms. Now in one of the other modules, I briefly showed how the RNA polymerase could be bound to a prokaryotic DNA using a protein called sigma, and sigma is a very simple protein that helps the RNA polymerase find its promoter. It's a much more complicated circumstance in multicellular organisms. In multicellular organisms, there are differentiated tissues, and the differentiated tissues each have their own specific need of proteins to be made. Further, multicellular organisms have many more genes than unicellular organisms, so picking which gene is to be synthesized at any given time and in any given place is an important function. The proteins play the role in doing that process. Now one of the ways in which proteins do that is by binding to special sequences within a DNA molecule. These sequences are called enhancer sequences, and you can see the enhancer sequences in this image at the top of the DNA strand. There you can see a couple of the orange balls that have bound at that that region to a sequence.

    01:13 The binding to the sequence is one of the first steps that happens in the process of initiating transcription. Now enhancer sequences are important because they are specific for tissues, that is an enhancer in a skin cell may be active when the same enhancer in a bone cell may not be active. How is that controlled? At the level of the enhancer, a very simple way of controlling that is whether or not the transcription factor is synthesized.

    01:40 A skin cell might make the enhancer that activates that and the bone cell might not. Other ways in which these things can be controlled are by modifying or binding other things to the protein to prevent it from binding to the DNA. Now this is very critical because this gives tissue specificity for the gene that's located nearby. In this case you can see that the enhancer sequence has been bent back to another region on the DNA, and the other region on the DNA at the bottom is where the gene is actually located. Now enhancers can be thousands of base pairs away and the DNA bind back and allow this to happen. Now this turns out to be very important because the binding of the enhancers to other proteins near the start site for transcription of the RNA is essential for the transcription to occur.

    02:30 If the enhancers are not there, the gene will not be transcribed.

    02:35 Now binding is a very, very important function and it happens as a result of proteins recognizing hydrogen bonds within the DNA duplex. I won't discuss that here because it's complicated, and you can see in the structure here, that in fact this is a complicated process. There are many proteins involved, but upon all of the proteins binding to the DNAs and interacting with the RNA polymerase, transcription can get initiated. Now here we see the transcription factor that has bound on to the DNA. There are many things that affect whether or not transcription occurs besides that. One is the bending of the DNA as I've already mentioned, the presence of other proteins in the area will also affect whether or not transcription occurs. The promoter complex is this complex of proteins that you see on the screen, and only after that promoter complex has formed, can RNA polymerase bind. It's a very different circumstance than we find in prokaryotic cells.


    About the Lecture

    The lecture Gene Expression – Protein Functions by Kevin Ahern, PhD is from the course Biochemistry: Basics.


    Included Quiz Questions

    1. Sigma factor
    2. TFIIA factor
    3. TFIIB factor
    4. TFIIF factor
    5. TFIIH factor
    1. Composed of a short region of DNA that binds to activator proteins to enhance the transcription of a particular gene
    2. A part of the promoter where a transcription factor binds to facilitate the smooth sliding of RNA polymerase over it
    3. A part of the operator where a repressor factor binds to stop the transcription
    4. A part of both promoter and operator region of a gene and inhibits the direct binding of RNA polymerase to the gene
    5. A part of the origin of replication and participates in the initiation of DNA replication process

    Author of lecture Gene Expression – Protein Functions

     Kevin Ahern, PhD

    Kevin Ahern, PhD


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