Controls Over Genes
- Overview of Gene Controls
- Because all cells in your body have a complete genome (well almost) only about 90 % are
used at a time.
- Which genes are expressed depends on the type of cell, its responses to chemical
signals, and built-in control systems.
- Regulatory proteins interact with DNA, RNA, or actual gene products.
- Two kinds of control systems are used by cells:
- In negative control systems, a regulatory protein
binds to the DNA to block transcription; it can be removed by an inducer.
- In positive control systems, a regulatory proteins
binds to the DNA and promotes initiation of transcription.
- Controls in Bacterial Cells
- Negative Control of Transcription
- Escherichia coli bacteria (common in the human digestive tract) can
metabolize lactose because of a series of genes that code for lactose-digesting enzymes.
- A promoter and operator precede the three genes and together they are called an operon.
- A regulator gene nearby codes for a repressor protein that binds to the operator
when lactose concentrations are low and effectively blocks RNA polymerases access to
- 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.
- Positive Control of Transcription
- The lactose operon also is subject to positive control by an activator protein called CAP.
- RNA polymerase will bind to the promoter if CAP is there.
- And in turn, CAP must first be activated by cAMP.
- When glucose is scarce, the CAP-cAMP complex forms and turns on the lactose-metabolism
- Controls in Eukaryotic Cells
- Much less is known about gene controls in multicelled eukaryotes because patterns of
gene expression vary within and between body tissues.
- A Case of Cell Differentiation
- All body cells have the same genes, but the cells of different tissues are differentiated (specialized) because of selective gene
- For example: hemoglobin genes are activated only in red blood cells.
- Selective Gene Expression at Many Levels
- Controls related to transcription include:
- gene amplification (more replicates of DNA);
- DNA rearrangements (cutting and splicing of DNA segments);
- chemical modifications (histone interactions)
- Post-transcriptional controls include:
- transcript processing (introns and exons);
- transport controls (dictate which mature transcripts will be shipped
to the cytoplasm for translation);
- post-translational controls (govern the mod. to polypeptides).
- Evidence of Gene Control
- Transcription in Lampbrush Chromosomes
- In amphibians and insects, the chromosomes decondense during meiosis I into thousands of
looped domains (so-called "lampbrush" chromosomes) for easier transcriptional
- The proteins of the nucleosomes are responsible for
- X Chromosome Inactivation
- In mammalian females, the gene products of only one X chromosome are needed; the other
is condensed and inactivecalled a Barr body.
- 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,
- 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.
- Examples of Signaling Mechanisms
- Hormone Signals
- Hormones are major signaling molecules that can stimulate or inhibit gene
activity in target cells.
- Some hormones bind to membrane receptors on cell surfaces.
- Others enter cells to bind with regulatory proteins to initiate transcription, often
with the aid of enhancer sequences.
- In the salivary glands of insect larvae, the polytene chromosomes respond to the hormone
ecdysone by puffing out during transcription.
- In vertebrates, some hormones such as somatotropin
have widespread effects because most of the bodys cells have receptors for it;
whereas, prolactin affects only the mammary glands because only they have the receptors.
- Sunlight as a Signal
- Plant seedlings will respond to a single burst of light by making chlorophyll.
- 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.