Chapter 11.3: Chromosomal Packaging and Gene Expression

Question 1

In eukaryotes, why is less DNA transcribed the more it is compacted

Answer 1

o Gene promoters are hidden from RNA polymerase and transcription factors when the promoter DNA is wrapped around the histone core of a nucleosome

Question 2

How does the promoter region change from a DNase resistant site to a DNase hypersensitive site?

Answer 2

 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

Question 3

What is the function of remodeling complexes?

Answer 3

o One type of chromatin modulator consists of multisubunit remodeling complexes that use the energy of ATP hydrolysis to alter nucleosome positioning

Question 4

Describe heterochromatin and euchromatin when viewed under a light micrscope

Answer 4

o When stained and viewed under a light microscope, the darker regions are heterochromatin and the lighter regions are euchromatin

Question 5

Describe heterochromatin and euchromatin when viewed under a electron micrscope

Answer 5

o Under an electron microscope, the heterochromatin appears much more condensed than the euchromatin

Question 6

Define constitutive heterochromatin

Answer 6

o Chromosome regions that remain condensed in heterochromatin at most times in all cells

Question 7

What are o Chromosome regions that remain condensed in heterochromatin at most times in all cells ?

Answer 7

constitutive heterochromatin

Question 8

What does autoradiography reveal about cells actively expressing genes and what does this indicate?

Answer 8

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

Question 9

What are transposable elements?

Answer 9

segments of DNA that move around the genome

Question 10

What produces position-effect variegation (PEV)?

Answer 10

o Rearrangement that silence gene expression in some cells and not others, produces position-effect variegation (PEV)

Question 11

What does position-effect variegation (PEV) reveal?

Answer 11

o The phenomenon of position-effect variegation thus reflects the existence of facultative heterochromatin

Question 12

Define facultative heterochromatin

Answer 12

regions of chromosomes (or even whole chromosomes) that are heterochromatic in some cells and euchromatic in other cells of the same organism

Question 13

What are regions of chromosomes (or even whole chromosomes) that are heterochromatic in some cells and euchromatic in other cells of the same organism?

Answer 13

facultative heterochromatin

Question 14

What is the function of barrier insulators?

Answer 14

 DNA elements called barrier insulators block the spread of heterochromatin

Question 15

What are histone tails?

Answer 15

N-terminal regions of the four core enzymes

Question 16

What is the purpose of histone tail modification?

Answer 16

 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

Question 17

What is the purpose of lysine acetylation?

Answer 17

 Lysine acetylation accomplished by a family of enzymes called histone acetyl transferases (HATs), “opens” chromatin by preventing the close packing of nucleosomes

Question 18

What does histone acetylation favor?

Answer 18

• Histone acetylation favors the expression of genes in euchromatic regions, as their promoters are now accessible to RNA polymerase and its associated proteins

Question 19

How are Barr bodies examples of facultative heterochromatin?

Answer 19

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

Question 20

What is the most important gene in the XIC?

Answer 20

o The most important of several genes in the XIC is the Xist gene (X inactive specific transcript) responsible for X inactivation

Question 21

How does the Xist gene make a Barr body?

Answer 21

 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