Controls Over Genes

 

  1. Overview of Gene Controls
    1. Because all cells in your body have a complete genome (well almost) only about 90 % are used at a time.
      1. Which genes are expressed depends on the type of cell, its responses to chemical signals, and built-in control systems.
      2. Regulatory proteins interact with DNA, RNA, or actual gene products.
    2. Two kinds of control systems are used by cells:
      1. In negative control systems, a regulatory protein binds to the DNA to block transcription; it can be removed by an inducer.
      2. In positive control systems, a regulatory proteins binds to the DNA and promotes initiation of transcription.
  2. Controls in Bacterial Cells
    1. Negative Control of Transcription
      1. Escherichia coli bacteria (common in the human digestive tract) can metabolize lactose because of a series of genes that code for lactose-digesting enzymes.
        1. A promoter and operator precede the three genes and –together they are called an operon.
        2. A regulator gene nearby codes for a repressor protein that binds to the operator when lactose concentrations are low and effectively blocks RNA polymerase’s access to the promoter.
      2. When milk is consumed, the lactose binds to the repressor changing its shape and effectively removing its blockage of the promoter; thus RNA polymerase can now initiate transcription of the genes.
    2. Positive Control of Transcription
      1. The lactose operon also is subject to positive control by an activator protein called CAP.
        1. RNA polymerase will bind to the promoter if CAP is there.
        2. And in turn, CAP must first be activated by cAMP.
      2. When glucose is scarce, the CAP-cAMP complex forms and turns on the lactose-metabolism genes.
  3. Controls in Eukaryotic Cells
    1. Much less is known about gene controls in multicelled eukaryotes because patterns of gene expression vary within and between body tissues.
    2. A Case of Cell Differentiation
      1. All body cells have the same genes, but the cells of different tissues are differentiated (specialized) because of selective gene expression.
      2. For example: hemoglobin genes are activated only in red blood cells.
    3. Selective Gene Expression at Many Levels
      1. Controls related to transcription include:
        1. gene amplification (more replicates of DNA);
        2. DNA rearrangements (cutting and splicing of DNA segments);
        3. chemical modifications (histone interactions)
      2. Post-transcriptional controls include:
        1. transcript processing (introns and exons);
        2. transport controls (dictate which mature transcripts will be shipped to the cytoplasm for translation);
        3. post-translational controls (govern the mod. to polypeptides).
  4. Evidence of Gene Control
    1. Transcription in Lampbrush Chromosomes
      1. In amphibians and insects, the chromosomes decondense during meiosis I into thousands of looped domains (so-called "lampbrush" chromosomes) for easier transcriptional access
      2. The proteins of the nucleosomes are responsible for this action.
    2. X Chromosome Inactivation
      1. In mammalian females, the gene products of only one X chromosome are needed; the other is condensed and inactive–called a Barr body.
      2. Because in some cells the paternal X chromosome is inactivated, while in other cells the maternal X chromosome is inactivated, each adult female is a mosaic of X-linked traits, called Lyonization.
      3. This mosaic effect is seen in human females affected by anhidrotic ectodermal dysplasia in which a mutant gene on one X chromosome results in patches of skin with no sweat glands.
  5. Examples of Signaling Mechanisms
    1. Hormone Signals
      1. Hormones are major signaling molecules that can stimulate or inhibit gene activity in target cells.
        1. Some hormones bind to membrane receptors on cell surfaces.
        2. Others enter cells to bind with regulatory proteins to initiate transcription, often with the aid of enhancer sequences.
      2. In the salivary glands of insect larvae, the polytene chromosomes respond to the hormone ecdysone by puffing out during transcription.
      3. In vertebrates, some hormones such as somatotropin have widespread effects because most of the body’s cells have receptors for it; whereas, prolactin affects only the mammary glands because only they have the receptors.
    2. Sunlight as a Signal
      1. Plant seedlings will respond to a single burst of light by making chlorophyll.
      2. Phytochrome is a blue-green pigment that helps plants adapt to the changing light conditions of day/night and seasons by signaling genes responsible for germination, stem elongation, branching, leaf expansion, and formation of flowers, fruits, and seeds.