control og gene expression: regulation, transcription, and cell differentiation in eukaryotes and bacteria

What is the main difference between a neuron and a liver cell?

A neuron has long, branching extensions for communication, while a liver cell is smaller and specialized for metabolism.

What does the frog experiment demonstrate about differentiated cells?

It shows that differentiated cells can revert to an embryonic state and develop into a full organism.

How can a single carrot cell regenerate into a whole plant?

Many plant cells can de-differentiate and proliferate into a mass of dividing cells.

What is the significance of the cloning of Dolly the sheep?

It demonstrated that a nucleus from a differentiated cell can develop into a genetically identical organism.

What are the key steps in regulating eukaryotic gene expression?

Regulation occurs at transcription, RNA processing, mRNA transport, mRNA degradation, translation, protein degradation, and protein activity.

What is transcriptional control?

It regulates whether and how much RNA is transcribed from DNA, serving as the main control point for most genes.

What role does RNA processing play in gene expression?

It modifies the RNA transcript through splicing, capping, and polyadenylation before it becomes mRNA.

How does mRNA localization affect gene expression?

It determines whether mRNA is exported from the nucleus and where it is localized in the cytoplasm.

What is the purpose of mRNA degradation control?

It determines mRNA stability and how quickly it is broken down.

What does translation control regulate?

It regulates how efficiently mRNA is translated into protein.

How does protein degradation control affect gene expression?

It controls protein stability and degradation rate, affecting the amount of active protein in the cell.

What is the role of transcription regulators?

They bind to regulatory DNA sequences to control the transcription of genes.

How do transcription regulator proteins interact with DNA?

They use α-helices to fit into the major groove of DNA and make specific contacts with base pairs.

What is the Trp operon in E. coli?

It is a cluster of genes required for tryptophan biosynthesis, transcribed together from a single promoter.

How is the Trp operon regulated when tryptophan levels are low?

The Trp repressor is inactive, allowing RNA polymerase to bind and transcribe the operon.

What happens to the Trp operon when tryptophan levels are high?

Tryptophan activates the repressor, which binds to the operator and blocks transcription.

What is an example of negative feedback regulation in gene expression?

The Trp operon shuts down its own synthesis pathway when tryptophan is abundant.

How do eukaryotic activator proteins function in gene expression?

They bind to enhancer sequences and can regulate genes from a distance by looping DNA.

What is the role of the Mediator in eukaryotic transcription?

It bridges the interaction between activators and RNA polymerase, promoting transcription initiation.

How do eukaryotic activators regulate transcription?

They regulate transcription at a distance, allowing complex and flexible control of gene expression.

What are general transcription factors?

They are the same for all genes and assemble at the promoter with RNA polymerase.

What role do transcription regulators play in gene expression?

They bind to specific regulatory DNA sequences, often far from the promoter, and interact with the transcription machinery through the Mediator complex.

What are chromatin-modifying proteins?

They include chromatin-remodeling complexes and histone-modifying enzymes that influence DNA accessibility.

What is the function of spacer DNA in eukaryotic transcription?

Spacer DNA separates regulatory sequences but is not bound by regulators.

How do the combined effects of transcription regulators influence gene expression?

They determine the rate of transcription initiation for each gene, allowing precise, gene-specific control.

What is the process of reprogramming differentiated cells into induced pluripotent stem (iPS) cells?

By artificially expressing three transcription regulator genes (Oct4, Sox2, Klf4), a fibroblast can be reprogrammed into an iPS cell.

What properties do iPS cells share with embryonic stem (ES) cells?

iPS cells can divide indefinitely in culture and differentiate into various cell types, such as smooth muscle cells, neurons, or fat cells.

What is DNA methylation?

It is a chemical modification of cytosine that affects gene expression by influencing chromatin structure and silencing certain genes.

Where does DNA methylation occur in vertebrates?

It occurs specifically at cytosines followed by guanine in the sequence 5′-CG-3′ (called CpG sites).

How does DNA methylation affect gene expression?

It does not alter base-pairing but influences chromatin structure, affecting gene expression.

What is epigenetic inheritance?

It is the transmission of gene expression patterns without altering the DNA sequence itself.

How are histone modifications inherited during cell division?

Parental nucleosomes with modified histones are distributed randomly, and enzymes restore the modification pattern on new histones.

What is the role of RNA-binding repressor proteins in bacteria?

They can block the ribosome-binding site on an mRNA, preventing translation initiation.

What is a thermosensor in bacterial mRNAs?

It is a special RNA structure that hides the ribosome-binding site, which is exposed at higher temperatures, allowing translation.

What are microRNAs (miRNAs)?

They regulate gene expression by targeting mRNAs for destruction or inhibiting their translation.

What is the RISC complex?

It is the RNA-induced silencing complex that miRNAs assemble with to search for complementary target mRNAs.

What happens to mRNAs when there is extensive matching with miRNAs?

The mRNA is rapidly degraded by nucleases within RISC.

What is the function of small interfering RNAs (siRNAs)?

They protect cells from infections by degrading foreign double-stranded RNAs.

How are siRNAs processed in the cell?

The enzyme Dicer cleaves foreign RNA into small double-stranded fragments, which are then loaded into RISC.

What is the outcome when RISC binds to complementary foreign RNAs?

RISC degrades the matching RNA, silencing it and protecting the cell from harmful genetic elements.