Chapter 11.3: Chromosomal Packaging and Gene Expression Flashcards
In eukaryotes, why is less DNA transcribed the more it is compacted
o Gene promoters are hidden from RNA polymerase and transcription factors when the promoter DNA is wrapped around the histone core of a nucleosome
How does the promoter region change from a DNase resistant site to a DNase hypersensitive site?
Transcription regulatory proteins (transcription factors) bind DNA at nearby enhancer and recruit proteins that remove the promoter-blocking nucleosomes or reposition them in relation to the gene
What is the function of remodeling complexes?
o One type of chromatin modulator consists of multisubunit remodeling complexes that use the energy of ATP hydrolysis to alter nucleosome positioning
Describe heterochromatin and euchromatin when viewed under a light micrscope
o When stained and viewed under a light microscope, the darker regions are heterochromatin and the lighter regions are euchromatin
Describe heterochromatin and euchromatin when viewed under a electron micrscope
o Under an electron microscope, the heterochromatin appears much more condensed than the euchromatin
Define constitutive heterochromatin
o Chromosome regions that remain condensed in heterochromatin at most times in all cells
What are o Chromosome regions that remain condensed in heterochromatin at most times in all cells ?
constitutive heterochromatin
What does autoradiography reveal about cells actively expressing genes and what does this indicate?
o Autoradiography reveals that cells actively expressing genes incorporate radioactive RNA precursors into RNA exclusively in regions of euchromatin
Indicates that euchromatin contains most of the sites of transcription and thus almost all of the genes
What are transposable elements?
segments of DNA that move around the genome
What produces position-effect variegation (PEV)?
o Rearrangement that silence gene expression in some cells and not others, produces position-effect variegation (PEV)
What does position-effect variegation (PEV) reveal?
o The phenomenon of position-effect variegation thus reflects the existence of facultative heterochromatin
Define facultative heterochromatin
regions of chromosomes (or even whole chromosomes) that are heterochromatic in some cells and euchromatic in other cells of the same organism
What are regions of chromosomes (or even whole chromosomes) that are heterochromatic in some cells and euchromatic in other cells of the same organism?
facultative heterochromatin
What is the function of barrier insulators?
DNA elements called barrier insulators block the spread of heterochromatin
What are histone tails?
N-terminal regions of the four core enzymes
What is the purpose of histone tail modification?
Such modifications of the histone tails (N-terminal regions of the four core enzymes) can influence the packaging of nucleosomes, and the modified tails can also serve as platforms to which chromatin modifier proteins can bind
What is the purpose of lysine acetylation?
Lysine acetylation accomplished by a family of enzymes called histone acetyl transferases (HATs), “opens” chromatin by preventing the close packing of nucleosomes
What does histone acetylation favor?
• Histone acetylation favors the expression of genes in euchromatic regions, as their promoters are now accessible to RNA polymerase and its associated proteins
How are Barr bodies examples of facultative heterochromatin?
o The inactive X chromosomes, or Barr bodies, are examples of facultative heterochromatin: An entire X chromosome becomes nearly completely heterochromatic in some cells, while other copies of this same X chromosome remain euchromatic in other cells
What is the most important gene in the XIC?
o The most important of several genes in the XIC is the Xist gene (X inactive specific transcript) responsible for X inactivation
How does the Xist gene make a Barr body?
Its noncoding RNA (ncRNA) coats the X chromosome that produces it and recruits histone modifying enzymes to inactivate the chromosome, which becomes a Barr body
