Source: Chapter 8 Review Sheet, Cell Biology (University of Florida)
Tags: eukaryotic gene regulation, transcription factors, RNA polymerase, enhancers, chromatin remodelling, HATs, HDACs, histone acetylation, glucocorticoid receptor, cell differentiation, stem cells, PCB 3023
Difficulty: Intermediate to Advanced Prerequisites: Part 1 of these notes (prokaryotic gene regulation, operons, repressors/activators). Solid understanding of chromatin and histone structure from earlier chapters.
Eukaryotic gene regulation is fundamentally more complex than prokaryotic regulation. Eukaryotes do not use operons. Instead, each gene has its own promoter and is regulated individually by a large cast of proteins, including general transcription factors that are needed for any gene and regulatory transcription factors that fine-tune expression of specific genes. On top of that, eukaryotic DNA is wrapped around histones and packaged into chromatin, which adds an entire physical layer of regulation: genes buried in tightly packed chromatin cannot be transcribed until that chromatin is opened up. This section also covers how all of this leads to cell differentiation, the process that produces the 200+ cell types in the human body from a single fertilised egg, and what stem cells are.
Eukaryotic transcription requires assembling a large initiation complex at the promoter and is typically inefficient without activators. Regulatory transcription factors bind enhancers (which can be far from the gene) and influence transcription by contacting the initiation complex through DNA looping or by recruiting enzymes that remodel chromatin. These layered controls, combined with positive feedback loops, allow cells to lock in stable patterns of gene expression during differentiation, and stem cells retain the ability to become multiple cell types.
General transcription factors (GTFs)
Proteins required for the transcription of virtually all genes by RNA Polymerase II. They assemble at the promoter to form the transcription initiation complex. Think of them as the standard crew needed to start any transcription job, regardless of which gene it is.
Regulatory transcription factors
Proteins that bind to specific regulatory DNA sequences (enhancers or silencers) and modulate the rate of transcription of particular genes. These are the fine-tuning dials, unlike GTFs which are the baseline machinery.
Transcription initiation complex
The assembly of RNA Polymerase II and the general transcription factors at the promoter, required before transcription can begin. In simple terms, this is the full team assembled at the starting line.
TFIID (including TBP)
A general transcription factor that recognises and binds the TATA box in the promoter, initiating the assembly of the transcription initiation complex. TBP (TATA-binding protein) is the subunit that makes direct contact with the DNA.
TFIIH
A general transcription factor with two enzymatic activities: it is a helicase (unwinds the DNA double helix at the promoter) and a kinase (phosphorylates the C-terminal domain of RNA Polymerase II, releasing it from the promoter to begin elongation).
Sigma factor (bacterial equivalent)
The bacterial protein that performs a role analogous to the general transcription factors in eukaryotes: it helps RNA polymerase recognise and bind the promoter. Eukaryotes use a whole set of GTFs instead of a single sigma factor.
Enhancer
A regulatory DNA sequence that increases transcription of a gene, often located thousands of base pairs away from the promoter, upstream, downstream, or even within introns. Think of enhancers as remote-control landing pads for activator proteins.
Silencer
A regulatory DNA sequence that decreases transcription when bound by a repressor protein. The repressive counterpart of an enhancer.
Action at a distance
The principle that enhancers (and silencers) can regulate transcription from positions very far from the promoter on the linear DNA. This works because DNA loops, bringing the enhancer-bound activator into physical contact with the transcription initiation complex at the promoter.
Upstream / downstream
Upstream means towards the 5' end of the DNA relative to the gene's transcription start site; downstream means towards the 3' end. Position +1 is the transcription start site. Negative numbers (e.g. -100, -1000) indicate positions upstream of +1.
Mediator complex
A large multi-protein complex that acts as a bridge between regulatory transcription factors bound at enhancers and the general transcription machinery at the promoter. It transmits the activating or repressing signal.
HAT (histone acetyltransferase)
An enzyme that adds acetyl groups to histone tails, loosening the interaction between histones and DNA. This opens chromatin and promotes transcription. Think of it as an enzyme that unpacks the DNA so it can be read.
HDAC (histone deacetylase)
An enzyme that removes acetyl groups from histone tails, tightening chromatin and repressing transcription. The opposite of a HAT: it packs the DNA back up.
Histone acetylation
The addition of acetyl groups to lysine residues on histone tails. Acetylation neutralises the positive charge on histones, weakening their grip on the negatively charged DNA and opening chromatin structure.
Chromatin remodelling
The process of altering chromatin structure to make DNA more or less accessible for transcription. Includes histone modification (acetylation, methylation) and the physical repositioning of nucleosomes by remodelling complexes.
Glucocorticoid receptor
An intracellular receptor protein that, when bound to a glucocorticoid hormone (e.g. cortisol), enters the nucleus and acts as a transcription factor, binding to glucocorticoid response elements (GREs) in the DNA to activate or repress target genes.
Cell differentiation
The process by which a less specialised cell becomes a more specialised cell type, acquiring a distinct structure and function. This occurs primarily during embryonic development but also in adult tissue renewal.
Cell memory
The ability of a differentiated cell to maintain its identity through cell division, even after the original signal that triggered differentiation is gone. Typically maintained by positive feedback loops.
Positive feedback loop
A circuit in which a transcription factor activates its own gene, ensuring that once the factor is expressed, it continues to be produced in all daughter cells. This is the molecular basis of cell memory.
Stem cell
An undifferentiated cell that retains the ability to divide and produce both more stem cells (self-renewal) and specialised cell types (differentiation).
Totipotent
A cell capable of giving rise to all cell types in an organism, including extraembryonic tissues (e.g. placenta). Only the zygote and early embryonic cells are totipotent.
Pluripotent
A cell capable of giving rise to all cell types of the body but not extraembryonic tissues. Embryonic stem cells are pluripotent.
Multipotent
A cell capable of giving rise to a limited range of related cell types. Adult stem cells (e.g. haematopoietic stem cells) are typically multipotent.
Similarities:
Both use transcription factors (regulatory proteins) to control gene expression.
Both use activators and repressors.
Transcription is the most important control point in both.
Regulatory proteins recognise and bind specific DNA sequences.
Differences:
Prokaryotes organise related genes into operons (polycistronic mRNA); eukaryotes transcribe each gene individually (monocistronic mRNA).
Eukaryotic DNA is packaged into chromatin, adding a physical layer of regulation that prokaryotes lack.
Eukaryotes use three RNA polymerases; prokaryotes use one.
Eukaryotic transcription requires assembly of a multi-protein initiation complex; bacterial RNA polymerase with its sigma factor can initiate on its own.
Eukaryotic regulatory sequences (enhancers) can act at a distance, thousands of base pairs from the promoter.
Eukaryotic mRNA undergoes processing (5' capping, 3' polyadenylation, splicing) before translation; prokaryotic mRNA does not.
RNA Polymerase I: transcribes most ribosomal RNA (rRNA) genes (the large rRNA molecules: 28S, 18S, 5.8S).
RNA Polymerase II: transcribes protein-coding genes (produces mRNA), plus some small nuclear RNAs (snRNAs). This is the polymerase most relevant to gene regulation discussions.
RNA Polymerase III: transcribes transfer RNA (tRNA) genes, the 5S rRNA gene, and other small RNAs.
Eukaryotic RNA Polymerase II cannot bind the promoter or initiate transcription on its own. It requires the help of general transcription factors (GTFs).
The GTFs assemble at the promoter in a specific order to form the transcription initiation complex. This complex positions RNA Pol II correctly and unwinds the DNA to begin transcription.
TFIID (specifically its TBP subunit) binds the TATA box in the promoter first, serving as the platform for the rest of the complex.
TFIIH has two critical enzymatic functions:
Helicase activity: unwinds the DNA at the start site so the template strand is exposed.
Kinase activity: phosphorylates the C-terminal domain (CTD) of RNA Pol II, which releases the polymerase from the promoter and allows it to begin elongating the transcript.
The bacterial equivalent is the sigma factor, a single protein that helps bacterial RNA polymerase recognise the promoter. Eukaryotes replaced this one protein with an entire committee of GTFs.
General transcription factors:
Required at every RNA Pol II promoter.
Necessary but not sufficient for efficient transcription.
They set up the machinery but do not determine whether a specific gene is highly expressed or barely expressed.
Regulatory transcription factors:
Gene-specific. Different combinations of regulatory TFs control different genes.
Bind to enhancers or silencers, not the core promoter.
Determine the rate of transcription: they are the reason a liver cell expresses albumin at high levels while a skin cell does not.
A regulatory TF is "on" or "active" when it is capable of binding its target DNA and influencing transcription. It is "off" or "inactive" when it cannot.
Ways a regulatory TF can be turned on or off:
Ligand binding (e.g. a hormone binds the receptor, activating it).
Phosphorylation or dephosphorylation.
Binding of an inhibitory protein that masks the DNA-binding or activation domain.
Regulated entry into the nucleus (some TFs are held in the cytoplasm until a signal triggers their nuclear import).
Enhancers are clusters of short DNA sequences that serve as binding sites for regulatory transcription factors (activators).
They can be located:
Upstream of the promoter (sometimes thousands of base pairs away).
Downstream of the gene.
Within introns of the gene itself.
Action at a distance refers to the ability of enhancers to influence transcription from these remote positions. The mechanism: the intervening DNA loops out, bringing the enhancer-bound activators into direct physical contact with the transcription initiation complex at the promoter. The Mediator complex often serves as the bridge.
Gene structure terminology:
+1: the transcription start site (the first nucleotide transcribed).
Upstream: positions with negative numbers relative to +1 (e.g. -100 is 100 base pairs before the start site).
Downstream: positions with positive numbers relative to +1.
Typical positions: the TATA box is around -25 to -30; enhancers can be at -1000, -10,000, or further.
Eukaryotic transcription initiation is inherently inefficient. Without activators, even with GTFs assembled, transcription occurs at a very low rate. Activators boost transcription through two main mechanisms:
Direct association with the transcription machinery:
Activators bound at enhancers contact the Mediator complex or components of the transcription initiation complex directly (via DNA looping).
This stabilises the initiation complex and increases the rate at which RNA Pol II begins transcription.
Chromatin remodelling (relaxing chromatin):
Activators recruit histone acetyltransferases (HATs) to the gene.
HATs add acetyl groups to histone tails, which neutralises positive charges on the histones and loosens the histone-DNA interaction.
This decondenses the chromatin, making the DNA physically accessible to the transcription machinery.
Activators may also recruit chromatin-remodelling complexes that physically slide or eject nucleosomes from the promoter region.
Repressors work by the reverse of these same two mechanisms:
Direct interference with the transcription machinery:
Some repressors compete with activators for the same DNA binding sites.
Others bind the Mediator or initiation complex and block its assembly or function.
Chromatin condensation:
Repressors recruit histone deacetylases (HDACs), which remove acetyl groups from histones.
This restores the positive charge on histones, tightening the histone-DNA interaction and condensing the chromatin.
Condensed chromatin physically blocks the transcription machinery from accessing the gene.
HATs (histone acetyltransferases):
Add acetyl groups to lysine residues on histone tails.
Effect: open chromatin (euchromatin), promote transcription.
Recruited by activators.
HDACs (histone deacetylases):
Remove acetyl groups from histone tails.
Effect: condensed chromatin (heterochromatin), repress transcription.
Recruited by repressors.
The balance between HAT and HDAC activity at a given gene determines its chromatin state and, consequently, its transcriptional activity.
The glucocorticoid receptor is an intracellular receptor that doubles as a transcription factor.
In the absence of hormone, the receptor is inactive, typically held in the cytoplasm bound to chaperone proteins (heat shock proteins).
When a glucocorticoid hormone (e.g. cortisol) enters the cell and binds the receptor:
The receptor changes conformation and releases from the chaperones.
It dimerises (pairs with another receptor).
The dimer translocates into the nucleus.
It binds to specific DNA sequences called glucocorticoid response elements (GREs) in the enhancers of target genes.
It then activates (or in some cases represses) transcription of those genes.
This is a clear example of a regulatory transcription factor being turned "on" by ligand binding.
Cell differentiation is the process by which a cell becomes specialised in structure and function.
It occurs primarily during embryonic development, as cells in the developing embryo commit to specific fates (skin, muscle, neuron, etc.).
A skin cell and a muscle cell have the same DNA. They differ because they express different sets of genes, controlled by different combinations of transcription factors.
How differentiation is maintained (cell memory):
Once a cell commits to a particular fate, it must maintain that identity through subsequent cell divisions.
This is accomplished by positive feedback loops: a master regulatory transcription factor activates its own gene, ensuring that the protein continues to be produced in all daughter cells.
Example: a transcription factor that specifies muscle cell identity activates the genes needed for muscle function and also activates its own gene. Even when the original external signal is gone, the cell keeps making this transcription factor, locking in the muscle identity.
A stem cell is an undifferentiated (or partially differentiated) cell that has two defining properties:
Self-renewal: it can divide to produce more stem cells.
Differentiation potential: it can give rise to one or more specialised cell types.
Types by potency:
Totipotent: can become any cell type, including placenta (zygote and very early embryo).
Pluripotent: can become any cell type of the body, but not extraembryonic tissues (embryonic stem cells).
Multipotent: can become a limited range of cell types (adult stem cells, e.g. haematopoietic stem cells produce all blood cell types but not neurons).
Why stem cells matter for research and therapy:
Basic research: stem cells allow scientists to study the mechanisms of differentiation, gene regulation, and development in real time.
Therapeutic potential: stem cells could be used to replace damaged or diseased tissues (e.g. replacing insulin-producing cells in diabetes, regenerating nerve tissue after spinal cord injury, producing new blood cells for leukaemia patients).
Drug testing: stem cell-derived tissues can be used to test drug safety and efficacy without animal models.
The glucocorticoid receptor pathway is the basis of corticosteroid medications (e.g. prednisone, dexamethasone) used to treat inflammation, autoimmune diseases, and allergic reactions. These drugs work by activating the glucocorticoid receptor, which then modulates the transcription of genes involved in immune responses. Understanding how this receptor regulates gene expression explains both the therapeutic effects and the side effects of these widely prescribed drugs.
Stem cell research has led to bone marrow transplants (a form of haematopoietic stem cell therapy) that have been used for decades to treat blood cancers. More recently, induced pluripotent stem cells (iPSCs), created by reprogramming adult cells back to a pluripotent state, have opened new possibilities for personalised regenerative medicine.
"General transcription factors are enough for efficient transcription." They are not. On their own, GTFs and RNA Pol II produce only very low levels of transcription. Activators and regulatory TFs are needed for any meaningful gene expression in eukaryotes.
"Enhancers must be right next to the gene they regulate." Enhancers can be tens of thousands of base pairs away, upstream, downstream, or within introns. They act at a distance through DNA looping.
"HATs turn genes on and HDACs turn genes off." More precisely, HATs open chromatin (making transcription possible) and HDACs close it (making transcription difficult). Whether a gene is actually transcribed also depends on the presence of the right transcription factors.
"All stem cells can become any cell type." Only totipotent and pluripotent stem cells have that broad a range. Most adult stem cells are multipotent, meaning they can only produce a limited set of related cell types.
⚠️ Know the specific functions of TFIID (binds the TATA box, nucleates the complex) and TFIIH (helicase + kinase). These are commonly tested as discrete facts.
⚠️ Be able to distinguish general transcription factors (needed for all genes, assemble at the promoter) from regulatory transcription factors (gene-specific, bind enhancers/silencers, fine-tune expression).
⚠️ Understand the two mechanisms by which activators work: direct contact with the initiation complex, and chromatin remodelling via HAT recruitment. Repressors use the mirror-image mechanisms.
⚠️ The glucocorticoid receptor is a classic example of a ligand-activated transcription factor. Know the steps: hormone binds receptor in cytoplasm, receptor releases from chaperones, dimerises, enters nucleus, binds GRE, activates transcription.
⚠️ Positive feedback loops are the molecular explanation for cell memory. Be able to explain how a transcription factor that activates its own gene maintains a differentiated state.
⚠️ Know the differences between totipotent, pluripotent, and multipotent stem cells, with examples.
True or False: RNA Polymerase II can bind the eukaryotic promoter and initiate transcription without any general transcription factors.
Fill in the blank: TFIIH acts as both a __________ (to unwind DNA) and a __________ (to phosphorylate RNA Pol II's CTD).
True or False: Enhancers must be located upstream of the gene they regulate.
Fill in the blank: HATs __________ chromatin by adding acetyl groups to histones, while HDACs __________ chromatin by removing them.
True or False: A pluripotent stem cell can give rise to placental tissue.
Answers: 1. False. 2. Helicase, kinase. 3. False (they can be upstream, downstream, or within introns). 4. Open (or relax/decondense), close (or condense/compact). 5. False (that requires totipotency; pluripotent cells produce body cell types but not extraembryonic tissues).
Q: What are the three eukaryotic RNA polymerases and what does each transcribe?
A: RNA Pol I transcribes most rRNA genes (28S, 18S, 5.8S). RNA Pol II transcribes protein-coding genes (mRNA) and some snRNAs. RNA Pol III transcribes tRNA genes, the 5S rRNA gene, and other small RNAs.
Q: Why is a transcription initiation complex necessary in eukaryotes?
A: Unlike bacterial RNA polymerase (which uses a sigma factor to recognise promoters directly), eukaryotic RNA Pol II cannot bind the promoter or initiate transcription on its own. It requires GTFs to recognise the promoter, position the polymerase, unwind the DNA, and release the polymerase for elongation.
Q: Compare general transcription factors and regulatory transcription factors.
A: GTFs are required at every RNA Pol II promoter and are necessary for basal transcription of all genes. Regulatory TFs are gene-specific, bind to enhancers or silencers rather than the core promoter, and modulate transcription rates for particular genes. GTFs set up the machinery; regulatory TFs determine how much (or whether) it runs.
Q: How do activators stimulate transcription through chromatin remodelling?
A: Activators recruit HATs to the gene region. HATs acetylate histone tails, neutralising their positive charge and weakening the histone-DNA interaction. This decondenses the chromatin, making the DNA accessible to the transcription machinery. Activators may also recruit nucleosome-remodelling complexes that reposition or evict nucleosomes.
Q: How do repressors inhibit transcription?
A: Repressors can directly interfere with the transcription initiation complex (competing with activators for binding sites or blocking assembly). They can also recruit HDACs, which remove acetyl groups from histones, leading to chromatin condensation and physical blockage of transcription.
Q: Describe the steps by which the glucocorticoid receptor regulates gene expression.
A: In the absence of hormone, the receptor is inactive in the cytoplasm, bound to chaperone proteins. When cortisol (a glucocorticoid) binds the receptor, it changes shape, releases from chaperones, dimerises, enters the nucleus, and binds glucocorticoid response elements in the enhancers of target genes. It then activates (or represses) transcription of those genes.
Q: What is cell differentiation, and how is it maintained through cell division?
A: Cell differentiation is the process by which a cell becomes specialised. It is maintained by positive feedback loops: a master regulatory transcription factor activates its own gene, so the protein keeps being produced in all daughter cells. This self-sustaining loop is the basis of cell memory, allowing a muscle cell to remain a muscle cell through countless divisions even after the original differentiation signal is gone.
Q: What are stem cells, and why are they important for research and medicine?
A: Stem cells are undifferentiated cells capable of self-renewal and differentiation into specialised types. They are important for basic research because they allow study of differentiation mechanisms. Therapeutically, they offer the potential to replace damaged or diseased tissues, and they can be used for drug testing on human cell types without animal models.
Chromatin remodelling (HATs, HDACs, histone modification) connects to epigenetics, a growing field that studies heritable changes in gene expression that do not involve changes to the DNA sequence itself.
The glucocorticoid receptor is one example of a nuclear receptor superfamily member, which also includes oestrogen, thyroid, and retinoic acid receptors, each regulating distinct sets of genes via the same general mechanism. This connects to endocrinology and signal transduction topics.
Stem cell biology connects to developmental biology and regenerative medicine, and the concept of induced pluripotent stem cells (iPSCs) links back to the nuclear transfer experiments discussed in Part 1, reinforcing that differentiation is reversible.
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