11.3 Chromosomal packaging and gene expression

Question 1

Compare heterochromatin and Euchromatin

Answer 1

Heterochromatin: It is highly condensed, usually inactive transcriptionally. It is the darkly stained regions of chromosomes. It is constitutive and facultative.
Euchromatin: it is relaxed, and usually active transcriptionally. It is the lightly stained regions of chromosomes.

Question 2

Define constitutive and Facultative

Answer 2

constitutive- condensed in all cells (eg most of the Y chromosome and all pericentromeric regions)
Facultative- condensed in only some cells and relaxed in other cells ( e.g position effect variegation, X chromosome in female mammals)

Question 3

How does transcription change chromatin structure ?

Answer 3

Promoters of inactive genes are hidden in nucleosomes. Transcription factors bind to enhancers to activate a gene and recruit chromatin remodelling proteins.
Promoters are exposed by removing or repositioning nucleosomes.

Question 4

Explain position-effect variegation (PEV) in Drosophila

Answer 4

white^+ (w^+) gene is normally located in euchromatin. w^+ gene is expressed in all cells causing red pigment (red eyes)

Chromosomal inversion can result in the w^+ gene being located adjacent to heterochromatin. w^+ gene is silenced in some cells causing (no pigment) but is expressed in other cells (red pigment) (red in some parts of the eye but no pigment in other parts, Variegated eyes).
Gene silencing can be caused by the spreading of heterochromatin into nearby genes. Spreading can occur over >1000 kb of chromatin. Heterochromatin spreads further in some cells than in others (slide 23 and slide 12 tutor slides)

Question 5

identification of molecules involved in heterochromatin formation:

Answer 5

Genetic screens in Drosophila were used to identify genes involved in chromatin modification.
- Looked for changes in the amount of position-effect variegation.
- Mutations that enhanced heterochromatin formation made eyes more white (inactive w^+)
- Mutations that suppressed heterochromatin made eyes less white.

(some genes encode proteins that localize to heterochromatin)

Question 6

How are the 4 core histones protein tails modified

Answer 6

The tails extend outward from the nucleosome. Enzymes can add chemical groups ( mehtyl groups, phosphate groups, ubiquitin, etc) Modified tails can alter nucleosomes and bind chromatin modifier proteins.

Question 6

What groups can modify the four core histone tails?

Answer 6

chemical groups

Question 7

Histone tail modifications alter __________

Answer 7

chromatin structure

Question 7

Explain how histone tail modifications alter chromatin structure (Acetylation)

Answer 7

1. Histones acetyltransferases add acetyl groups to histone tails.
2. This prevents the close packing of nucleosomes
3. Thus favoring expression of genes in euchromatin.
4. The process is reversed by histone deacetylases

Question 8

X-chromosome inactivation in female mammals occurs through __________

Answer 8

Heterochromatin formation

Question 9

Explain an example of facultative heterochromatin

Answer 9

Dosage compensation in mammals so that X-linked genes in XX and XY individuals are expressed at the same level. Random inactivation of all except one X chromosome in each cell (females). The inactivated X chromosome is a Barr body.

Question 10

What is a Barr body

Answer 10

Darkly stained heterochromatin masses observed in somatic cells in interphase.

Question 11

What is facultative heterochromatin

Answer 11

regions of chromosomes that are heterochromatic in some cells and euchromatic in other cells of the same organism

Question 12

After how many weeks of fertilization does one X chromosome get inactivated in female humans

Answer 12

2 weeks. Some cells have maternal X inactivation, other cells have paternal X inactivation. All cell descendants have the same inactivate X.

Question 13

Adult females are _________ at X-linked genes

Answer 13

mosaic

Question 14

Explain what X chromosome mosaicism is

Answer 14

It occurs when an individual has cells with different X chromosome patterns

Question 15

What initiates X chromosome inactivation?

Answer 15

the X inactivation centre (XIC) of the Xist gene expression

Question 16

What is Xist?

Answer 16

it is the X-inactivation specific transcript that is expressed on the inactive X and not the active X.

Question 17

What is Xist RNA and what does it do?

Answer 17

It is a large, non-coding, cis-acting regulatory RNA. It binds to the X chromosome that it was expressed from. It initiates histone modifications (methylation, deacetylation) that result in heterochromatin formation.