Glossary
Cell Differentiation
The process by which a less specialized cell becomes a more specialized cell type, driven by differential gene expression.
Example:
A stem cell can undergo cell differentiation to become a specialized neuron, expressing genes unique to nerve cell function.
Coordinated Regulation
The control of multiple genes together, ensuring they are expressed simultaneously and in appropriate amounts for a specific cellular process.
Example:
In bacteria, genes for breaking down a specific sugar are often under coordinated regulation, ensuring all necessary enzymes are produced at once.
DNA Methylation
An epigenetic modification involving the addition of a methyl group to cytosine bases in DNA, typically leading to gene repression.
Example:
In some cancer cells, tumor suppressor genes are silenced due to excessive DNA methylation, preventing them from producing protective proteins.
Enhancers
Regulatory sequences that increase the rate of transcription of a gene, often located far from the promoter.
Example:
When a cell needs to rapidly produce a stress-response protein, an enhancer acts like a turbo boost, significantly speeding up the gene's transcription.
Epigenetic Changes
Heritable modifications to gene function that occur without altering the underlying DNA sequence, often influenced by environmental factors.
Example:
Identical twins can develop different traits over time due to varying lifestyles leading to distinct epigenetic changes on their shared DNA.
Histone Modification
Epigenetic changes to histone proteins, around which DNA is wrapped, altering chromatin structure and affecting DNA accessibility for transcription.
Example:
When histones are acetylated, the DNA loosens, making genes more accessible for transcription; this is a type of histone modification.
Homeobox Genes
A group of master regulatory genes that control the development of body structures in animals, specifying the identity of body segments.
Example:
Mutations in Homeobox genes in fruit flies can lead to legs growing where antennae should be, demonstrating their critical role in body plan formation.
Inducible System
A type of gene regulation where gene expression is normally off but can be turned on in the presence of a specific molecule called an inducer.
Example:
The lac operon is an inducible system because its genes are only expressed when lactose, the inducer, is present.
Operons
Units of genetic function in prokaryotes consisting of a promoter, an operator, and a group of structural genes that are transcribed together.
Example:
The lac operon in E. coli is a classic example of how bacteria efficiently regulate genes involved in lactose metabolism.
Phenotype
The observable characteristics or traits of an organism, resulting from the interaction of its genotype with the environment.
Example:
A plant's height, flower color, and disease resistance are all aspects of its phenotype, determined by its genes and growing conditions.
Promoters
Specific DNA sequences located near the start of a gene where RNA polymerase and transcription initiation factors bind to begin transcription.
Example:
The promoter is like the 'start' button on a recording device; RNA polymerase must bind there to begin transcribing the genetic message.
Regulatory Proteins (Transcription Factors)
Proteins that bind to specific regulatory DNA sequences to either activate or repress gene transcription.
Example:
A cell's response to a hormone might involve a specific regulatory protein binding to an enhancer, thereby turning on a set of genes.
Regulatory Sequences
Segments of DNA that do not code for proteins but control the timing and amount of gene expression by interacting with regulatory proteins.
Example:
Imagine a light switch for a gene; regulatory sequences determine if the light (gene) is on or off, and how bright it shines.
Repressible Operons
Operons whose transcription is normally on but can be turned off when a specific molecule (corepressor) binds to the repressor protein, activating it.
Example:
The trp operon is a repressible operon; it produces tryptophan-synthesizing enzymes until enough tryptophan is present, at which point it is shut off.
Repressor Protein
A regulatory protein that binds to an operator region of an operon, blocking RNA polymerase and preventing gene transcription.
Example:
In the absence of lactose, a repressor protein binds to the lac operon, preventing the bacteria from wasting energy producing lactose-digesting enzymes.
Sequential Gene Expression
The activation or repression of different sets of genes at specific stages during an organism's development or in response to changing conditions.
Example:
During human embryonic development, different genes are turned on and off in a precise sequential gene expression pattern to form organs at the correct times.
Silencers
Regulatory sequences that decrease or repress the rate of transcription of a gene.
Example:
To prevent uncontrolled cell growth, a silencer might bind to a gene involved in cell division, effectively putting the brakes on its expression.
Terminators
DNA sequences that signal the end of transcription, causing RNA polymerase to detach from the DNA template.
Example:
After transcribing a gene, RNA polymerase encounters a terminator sequence, which acts as a 'stop sign,' signaling the completion of the mRNA molecule.
lac Operon
An inducible operon in *E. coli* that controls the genes necessary for the metabolism of lactose, activated in the presence of lactose.
Example:
When E. coli encounters lactose, the lac operon is switched on, allowing the bacterium to utilize this sugar as an energy source.
trp Operon
A repressible operon in *E. coli* that controls the genes for tryptophan synthesis, turned off when tryptophan levels are high.
Example:
When an E. coli cell has sufficient tryptophan, the trp operon is repressed, stopping the production of enzymes that make more tryptophan.