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Cell Biology Ch. 8
Control of Gene Expression
Question | Answer |
---|---|
Gene expression | process by which a gene makes a product that is useful to the cell or organism by directing the synthesis of a protein or an RNA molecule with a characteristic activity |
differentiation | process by which a cell undergoes a progressive, coordinated change to a more specialized cell type |
differentiation is brought about by long-term changes in what? | gene expression |
different cell types of a multicellular organism contain the same what? | DNA |
if different cell types of a multicellular organism contain the same DNA, how is it that they differ? | they differ not because they contain different genes, but because they express them differently |
a cell can change the expression of its genes in response to _____________ ____________ | external signals |
Give an example of a cell changing the expression of its genes in response to external signals | when exposed to cortisol, a liver cell dramatically increases the production of several proteins |
Gene expression can be ______________ at various steps from DNA to RNA to protein | regulated |
gene expression can be controlled at any of the steps between a gene and its ultimate functional product. For the majority of genes, however, what is the most important point of control? | the initiation of transcription |
the promoter region of a gene binds to the enzyme, ____ ____________, and correctly orients the enzyme to begin its task of making an RNA copy of the gene | RNA polymerase |
the promoters of both bacterial and eukaryotic genes include a ________________ ______________ _______, where RNA synthesis begins | transcription initiation site |
nearly all genes have ___________________ DNA sequences that are used to switch the gene on or off | regulatory DNA sequences |
regulatory DNA sequence | DNA sequence to which a transcription regulator binds to determine when, where, and in what quantities a gene is to be transcribed into RNA |
transcription of individual genes is switched on and off in cells by transcription regulator _____________, which bind to short stretches of regulatory DNA sequences | proteins |
transcription regulators | protein that binds specifically to a regulatory DNA sequence and is involved in controlling whether a gene is switched on or off |
many transcription regulators bind to the DNA helix as dimers. What does this dimerization do? | doubles the area of contact with the DNA, thereby greatly increasing the strength and specificity of the protein-DNA interaction |
transcriptional repressor | binds to a regulatory sequence on the DNA and PREVENTS TRANSCRIPTION (switches gene off) |
transcriptional activator | binds to regulatory DNA sequence to PERMIT TRANSCRIPTION (switch gene on) |
activator proteins often have to do what to be able to bind DNA? | have to interact with a second molecule (like protein CAP having to bind cAMP before binding DNA) |
In many cases, the activity of a single promoter is controlled by two different transcription regulators. What is a good example? | the activator and repressor control of the Lac operon in E. coli |
What does the Lac operon encode? | proteins required to import and digest disaccharide lactose |
What are the two transcription regulators that control the lac operon? | the Lac repressor and the CAP |
Control of the Lac operon: what happens when lactose is absent? | Lac repressor binds to the Lac operator and shuts off expression of the operon |
Control of the Lac operon: What results from the addition of lactose? | allolactose increases, which binds to Lac repressor, causing conformational changes that unbinds the protein from DNA |
Control of the Lac operon: What happens when glucose is absent? | cAMP is produced, which activates CAP to bind to DNA. LacZ encodes enzyme beta-galactosidase which breaks down lactose to galactos and glucose |
In bacteria, transcription regulators usually bind to regulatory DNA sequences close to where RNA polymerase binds. This binding can either activate or repress transcription of the gene. How do eukaryotes differ? | in eukaryotes, regulatory DNA sequences are often separated from the promoter by many thousands of nucleotide pairs |
in eukaryotes, how can gene ACTIVATION occur at a distance? | an activator protein bound to a distant enhancer attract RNA polymerase and general transcription factors to the promoter. |
in eukaryotes, activation can occur at a distance when an activator protein bound to a distant enhancer attracts RNA polymerase and general transcription factors to the promoter. What does the looping of the intervening DNA permit? | contact between the activator and the transcription initiation complex bound to the promoter |
Often times, additional proteins serve to link the distantly bound transcription regulators to these proteins at the promoter. What is the most important of these regulators? | a large complex known as mediator |
eukaryotic repressor proteins do the opposite: | they decrease transcription by preventing the assembly of the same protein complex |
Eukaryotic transcription regulators have an additional mechanism of action: | they can help initiate transcription by recruiting chromatin-modifying proteins |
in eukaryotic cells, activator and repressor proteins exploit chromatin structure to do what? | help turn genes on and off |
chromatin structure can be altered by ___________-____________ complexes and by enzymes that covalently modify histone proteins that form the core of the nucleosome | chromatin-remodeling complexes |
many gene activators recruit chromatin-modifying proteins to promoters. What can this modification do? | alters chromatin structure, allowing greater accessibility to the underlying DNA to help initiate gene transcription |
gene repressor proteins can modify chromatin in ways that do what? | reduce the efficiency of transcription initiation |
eukaryotic genes are controlled by combinations of ________________ ____________________ | transcription regulators |
combinatorial control | describes the way in which groups of transcription regulators work together to regulate the expression of a single gene |
the expression of different genes can be coordinated by a single ____________ | protein |
In addition to day-to-day regulation of cell function, what can combinatorial control generate? | different cell types |
How can combinatorial control generate different cell types? for example, the development of muscle cells | A set of regulators activate the transcription of genes that code for muscle-specific proteins by binding to specific DNA sequences present in their regulatory regions |
Some transcription regulators can even convert one specialized cell type to another. how? | by artificially expressing an appropriate set of transcription regulators |
transcription regulators can also coax various differentiated cells to de-differentiate into pluripotent stem cells. What are pluripotent stem cells? | cell capable of giving rise to any of the specialized cell types in the body |
induced pluripotent stem cells (iPS cells) | somatic cells that have been reprogrammed to resemble and behave like a pluripotent embryonic stem cell through the artificial introduction of a set of genes encoding particular transcription regulators |
What can human iPS cells be used for? | they can be directed to generate a population of differentiated cells for use in the study or treatment of disease |
the formation of an entire organ can be trigger by a single transcription factor. how? (generally) | the action of a single transcription factor can produce a cascade of regulators that work in combination, leading to the formation of an organized group of many different types of cells |
Once a cell has become differentiated into a particular cell type, it will generally remain differentiated, and all its progeny will remain that same cell type. For a proliferating cell to maintain its identity it must have cell memory, which is what? | the ability of differentiated cells and their descendants to maintain their identity |
cells in multicellular organisms have mechanisms that enable their progeny to "remember" what type of cell they should be. A prominent mechanism for propagating cell memory relies on what? | transcription regulators that perpetuate transcription of their own gene- a form of positive feedback |
positive feedback loop | form of regulation in which the end product of a reaction or pathway stimulated continued activity (controls gene expression) |
DNA methylation is used to reinforce cell identity.. how? | the pattern of DNA methylation can be transmitted from one cell generation to the next, producing a form of epigenetic inheritance that helps a cell remember the state of gene expression in its parent cell |
epigenetic inheritance | the transmission of a heritable pattern of gene expression from one cell to its progeny that does not involve altering the nucleotide sequence of the DNA |
cells can regulate gene expression by controlling events that occur after transcription has begun. Many of these post-transcriptional mechanisms rely on RNA molecules that can do what? | influence their own stability or translation |
each mRNA controls its own __________________ and _______________ | degradation and translation |
Whether bacterial or eukaryotic, an mRNA's lifetime is dictated by specific ______________ _____________ within the untranslated regions that lie upstream and downstream of the protein-coding sequence. | nucleotide sequences |
These nucleotide sequences often harbor binding sites for proteins that are involved in what? | RNA degradation |
each mRNA possesses a sequence that help control how often or how efficiently it will be translated into protein. These sequences control ______________ _______________ | translation initiation |
how do bacterial mRNAs control translation initiation? | by blocking or exposing the ribosome-binding sequence, the bacterium can inhibit or promote the translation of mRNA |
how do eukaryotic mRNAs control translation initiation? | they possesses a 5' cap that guide the ribosome to the start codon. repressors can inhibit translation initiation by binding to 5' untranslated region on mRNA, preventing the ribosome from finding the start codon |
______________ RNAs control the expression of thousands of genes | regulatory RNAs |
Regulatory RNA | RNA molecule that plays a role in controlling gene expression (noncoding RNA) |
What are the three major types of regulatory RNAs? | microRNAs, small interfering RNAs, and long noncoding RNAs |
microRNA (miRNA) are tiny RNA molecules that control gene expression how? | by base-pairing with specific mRNAs and inhibiting their stability and translation |
two features of miRNAs make them useful regulators of gene expression. (1) a single miRNA is capable of what? | inhibiting the transcription of a whole set of mRNAs (as long as all the mRNAs carry a common sequence) |
two features of miRNAs make them useful regulators of gene expression. (2) a gene that encodes an miRNA occupies relatively little _______ in the genome compared with one that encodes a transcription regulator. | space |
cells have a defense mechanism for destroying "foreign" double-stranded RNAs, many of which are produced by viruses. It makes use of what? | small interfering RNAs (siRNA) |
What are siRNAs produced from? | the foreign RNAs in a process called RNA interference (RNAi) |
RNA interference (RNAi) | cellular mechanism activated by double-stranded RNA molecules that results in the destruction of RNAs containing a similiar mucleotide sequence |
What is RNA interference widely exploited for? | an experimental tool for preventing the expression of selected genes (gene silencing) |
RNAi (1) the double-stranded, foreign RNAs are what? | cut into short fragments by a protein dicer to make siRNAs |
RNAi (2) the siRNAs are then taken up by RISCs, which discard one strand of foreign siRNA duplex and use the other strand to what? | to locate and destroy foreign RNAs with a complementary sequence |
the recent discovery of thousands of long noncoding RNAs in mammals has what? | opened a new window to the roles of RNAs in gene regulation |