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Biology LibreTexts

6.7: Regulation of Gene Expression

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    22479
  • Express Yourself

    This sketch in Figure \(\PageIndex{1}\) illustrates some of the variability in human cells. The shape and other traits that make each type of cell unique depend mainly on the particular proteins that the cell type makes. For example, an ovum is a cell with a large cytoplasm because it nourishes the embryo after fertilization. Sperm is a small cell with essentially just genetic material. The job of the sperm is to transfer genetic material to the ovum. The sketch shows various other cell types. Proteins are encoded in genes. All the cells in an organism have the same genes, so they all have the genetic instructions for the same proteins. Obviously, different types of cells must use, or express, different genes to make different proteins.

    variety of animal cell types
    Figure \(\PageIndex{1}\): Different cells have different shapes to perform their particular jobs.

    What Is Gene Expression?

    Using a gene to make a protein is called gene expression. It includes the synthesis of the protein by the processes of transcription of DNA and translation of mRNA. It may also include further processing of the protein after synthesis.

    Gene expression is regulated to ensure that the correct proteins are made when and where they are needed. Regulation may occur at any point in the expression of a gene, from the start of the transcription phase of protein synthesis to the processing of a protein after synthesis occurs. The regulation of transcription is one of the most complicated parts of gene regulation in eukaryotic cells and is the focus of this concept.

    Regulation of Transcription

    As shown in Figure \(\PageIndex{2}\), transcription is controlled by regulatory proteins or transcription factors. These proteins bind to regions of DNA, called regulatory elements which are located near promoters. The promoter is the region of a gene where RNA polymerase binds to initiate transcription of the DNA to mRNA. After regulatory proteins bind to regulatory elements, the proteins can interact with RNA polymerase. Regulatory proteins are typically either activators or repressors. Activators are regulatory proteins that promote transcription by enhancing the interaction of RNA polymerase with the promoter. Repressors are regulatory proteins that prevent transcription by impeding the progress of RNA polymerase along the DNA strand so the DNA cannot be transcribed to mRNA.

    Transcription enhancers
    Figure \(\PageIndex{2}\): An enhancer is a DNA sequence that promotes transcription. Each enhancer is made up of short DNA sequences called distal control elements. Activators bound to the distal control elements interact with mediator proteins and transcription factors. Two different genes may have the same promoter but different distal control elements, enabling differential gene expression.

    Enhancers

    Although regulatory proteins and elements are typically the key players in the regulation of transcription, other factors may also be involved. For example, regulation of transcription may also involve enhancers. Enhancers are distant regions of DNA that can loop back to interact with a gene's promoter and enhance transcription.

    Regulation During Development

    The regulation of gene expression is extremely important during the early development of an organism. Regulatory proteins must turn on certain genes in particular cells at just the right time so the individual develops normal organs and organ systems. Homeobox genes are a large group of genes that regulate development during the embryonic stage. In humans, there are an estimated 235 functional homeobox genes. They are present on every chromosome and generally grouped in clusters. Homeobox genes contain instructions for making chains of 60 amino acids called homeodomains. Proteins containing homeodomains are transcription factors that bind to and control the activities of other genes. The homeodomain is the part of the protein that binds to the target gene and controls its expression.

    Review

    1. Define gene expression.
    2. Why must gene expression be regulated?
    3. Explain how regulatory proteins may activate or repress transcription.
    4. Describe homeobox genes and their role in the development of an organism.
    5. Discuss the role of regulatory gene mutations in cancer.
    6. Explain the relationship between proto-oncogenes and oncogenes.
    7. If a newly fertilized egg contained a mutation in a homeobox gene, what effect do you think this might have on the developing embryo? Explain your answer.
    8. Which of the following are proteins?
      1. Repressors
      2. Promoters
      3. Regulatory elements
      4. All of the above
    9. Which of the following is a region of DNA?
      1. Homeodomain
      2. Activator
      3. TATA box
      4. Both A and C
    10. Compare and contrast enhancers and activators.
    11. True or False. Mutations in genes that normally either promote or suppress cell division can both cause cancer.
    12. True or False. Gene expression is only regulated at the transcriptional stage.
    13. True or False. If RNA polymerase cannot bind to the promoter of a gene, it cannot transcribe that gene into mRNA.

    Explore More

    Mutations in the regulation of gene expression can lead to uncontrolled cell division, also known as cancer. Learn more here:

    Attributions

    1. Animal cell variety by Sunshineconnelly, licensed CC BY 3.0 via Wikimedia Commons
    2. Eukaryotic Transcription Gene by OpenStax via Lumen Learning, CC BY 4.0
    3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0