Chapter 15 Control of Gene Expression
I. A Cascade of Proteins and Cancer
A. Cyclin and cdc2 are proteins that initiate cell division.
B. In cancer cells these proteins are not destroyed after a division cycle; therefore, the cells keep dividing without controls.

II. The Nature of Gene Control
A. Because all cells in your body have the same genetic instructions, only a relatively small number of genes are active at any given time in any given tissue (example: only red blood cells activate hemoglobin genes).
B. Control Agents and Where They Operate
1. Transcriptional controls depend on two types of regulatory proteins:
a. Repressor proteins prevent RNA polymerases from binding to DNA—negative control.
b. Activator proteins enhance binding of RNA polymerases—positive control.
2. In vertebrates, hormones are agents of control.
3. Some control agents bind to promoter sequences at the start of a gene; others bind to the operator sequences between the promoter and the start of a gene.
C. Gene controls operate in response to chemical changes within the cell or its surroundings.

III. Gene Control in Prokaryotes
A. Negative Control of Lactose Metabolism
1. E. coli bacteria (common in the human digestive tract) can metabolize lactose because of a series of genes that code for lactose-digesting enzymes.
a. The three genes are preceded by a promoter and an operator—all together called an operon.
b. 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.
B. Positive Control of Nitrogen Metabolism
1. Activators bind with the promoter which in turn enhances binding of RNA polymerase.
2. In the nitrogen-related operon, a cascade of phosphorylated activators turn on the genes for glutamine synthetase.

IV. Gene Control in Eukaryotes
A. Selective Gene Expression
1. Much less is known about gene controls in multicelled eukaryotes because patterns of gene expression vary within and between body tissues.
2. All body cells have the same genes, but the cells of different tissues are differentiated (specialized) because of selective gene expression.
a. Some hormones such as somatotropin have widespread effects because most of the body’s cells have receptors for it.
b. Prolactin affects only the mammary glands because only they have the receptors.
B. Levels of Control in Eukaryotes
1. Transcriptional controls influence time and extent of gene transcription by activators and nucleosome arrangements.
2. Transcript processing controls govern modification of the initial mRNA transcripts in the nucleus.
3. Transport controls dictate which mature transcripts will be shipped to the cytoplasm for translation.
4. Translational controls govern rates at which mRNA transcripts will be translated.
5. Post-translational controls govern the modifications to polypeptides—such as attachment of sugars or phosphate groups.
C. Evidence of Control Mechanisms
1. Chromosome Loops and Puffs
a. In amphibians and insects, the chromosomes decondense during meiosis I into thousands of looped domains (so-called “lampbrush” chromosomes) for easier transcriptional access.
b. Polytene chromosomes in the salivary glands of insects are so active that they puff out during transcription.
2. X Chromosome Inactivation
a. In mammalian females, the gene products of only one X chromosome are needed; the other is condensed and inactive—called a Barr body.
b. 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.
c. 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.
3. Transcript-Processing Controls
a. mRNA transcripts may be edited and spliced to yield varying sequences of nucleotides.
b. There are several versions of troponin-T, but all function in contraction—in different ways.
D. When the Controls Break Down
1. While not well understood, the rate of cell division for different tissues under different conditions varies.
2. When controls are lost, cancer can occur.