Following statements are regarding to the mechanism of action of cholera toxin.

A. cholera toxin is a homodimeric protein, target G proteins, interfering with normal signaling in host cells.

B. cholera toxin associates with a small G protein, known as ARF6.

C. association with ARF6 activates cholera toxin, which catalyzes the transfer of ADP-ribose from NAD+ to the critical Arg residue in the P loop of the alpha subunit of Gs.

D. ADP-ribosylation blocks the GEF activity of Gs and thereby renders Gs permanently inactive.

E. This results in continuous activation of the adenylyl cyclase of intestinal epithelial cells, chronically high cAMP, and active PKA dephosphorylates the CFTR Cl- channel and a sodium-proton exchanger in the intestinal epithelial cells.

Which of the following combination is correct?

#Question id: 2885

#Unit 2. Cellular Organization

Which of the following function of DNA methylation in mammalian cell is incorrect?

#Question id: 2886

#Unit 2. Cellular Organization

Match the following sites of modifications on histones given in Column I with their effect on transcription given in Column II.

COLUMN I                                               COLUMN II

1) Methylation of H3K36 in transcribed region               a) Activation

2) Ubiquitinylation of H2BK120 in mammals                b) Repression

3) Phosphorylation of H2AT120                                    c) Elongation

4) Hypoacetylated Lysine

5) Acetylation of H4K8

#Question id: 2887

#Unit 2. Cellular Organization

Which of the following reasons has led to the evolutionary selection of an extra methyl group in DNA?

#Question id: 2888

#Unit 2. Cellular Organization

Repressive complexes have been discovered that are composed of multiple repressing proteins bound to RNAs many kilobases in length that do not contain long open reading frames and are consequently called long noncoding RNAs or lncRNAs. Which of the following statements is correct regarding the regulation of these lncRNAs?

#Question id: 2889

#Unit 2. Cellular Organization

The edges of each base pair are exposed in the major and minor grooves, creating a pattern of hydrogen-bond donors and acceptors and of hydrophobic groups (allowing for Vander Waals interactions) that identifies the base pair. The edge of an C : G base pair displays the following chemical groups in the following order in the major groove:

#Question id: 2890

#Unit 2. Cellular Organization

Match individual variant histones i) to iv) to one or more of the possible functions listed in a) to g).

Variant histones              Functions 

i) CENP-A                         a) DNA replication

ii) H2A.X                           b) As a barrier to stop heterochromatin spreading

iii) H2A.Z                          c) DNA repair

iv) H3.3                             d) Transcriptional activation

                                          e) Kinetochore assembly 

                                           f) Recombination

                                          g) Genome maintenance