The principal pathway for transport of lysosomal hydrolases from the trans-Golgi network (pH 6.6) to the late endosomes (pH 6.0) and the recycling of M6P (mannose 6 phosphate) receptors back to the Golgi depends on thee pH difference between those compartments. From what you know about M6P receptor binding and recycling and the pathways for delivery of material to lysosomes, predict what would happen if the pH in late endosomes was raised to 6.6?

• M6P will bind to hydrolases but will not release the hydrolases in the late endosomes.

• M6P will bind to hydrolases and will release the hydrolases in the late endosomes.

• At higher endosomal pH, the receptor would not release the hydrolase and could not be recycled back to the trans-Golgi network.

• M6P will be degraded at higher pH

The following small peptide substrates are used for determining elastase activity and the following data have been recorder

The arrow indicates the cleavage site. From the above observations, it appears that:

(A) P A P A F is digested most rapidly.

(B) P A P A G is digested most rapidly.

(C) A hydrophobic residue at the C-terminus seems to be favored.

(D) A smaller residue at the C-terminus seems to be favored.

(E) Elastase always required a smaller residue at the N-terminus of the cleavage site.

Which of the following is true?

• (A), (C) and (E)

• (B), (D) and (E)

• (E) only

• (D), (E) only

The diploid genome of a species comprises 6.4 × 10⁹ bp and fits into a nucleus that is 6μm in diameter. If base pairs occur at intervals of 0.34 nm along the DNA helix, what is the total length of DNA in a resting cell?

• 3.0  m

• 3.5 m

• 2.2 m

• 4.0 m

Cells that grow and divide in a medium containing radioactive thymidine covalently incorporate the thymidine into their DNA during the S phase. Consider a simple experiment in which cells are labeled by a brief (30 minutes) exposure to radioactive thymidine. The medium is then replaced with one containing unlabeled thymidine, and the cells grow and divide for some additional time. At different time points after replacement of the medium, cells are examined under a microscope. Cells in mitosis are easy to recognize by their condensed chromosomes and the fraction of mitotic cells that have radioactive DNA can be estimated by autoradiography and plotted as a function of time after the thymidine labeling as in the figure below:

The rise and fall of the curve is because:

• The initial rise of the curve corresponds to cells that were just finishing DNA replication when radioactive thymidine was added (S phase).

• The peak of the curve corresponds to cells in M phase

• The rise in curve after 20 min corresponds to cells in the apoptotic phase.

• The fall in curve after 10 min indicates the cells exiting the M phase.

Phosphorylation of serines as well as methylation and acetylation of lysines in histone tails affect the stability of chromatin structure above the nucleosome level and have important consequences for gene expression. The resulting changes in charge are expected to affect the ability of the tails to interact with DNA because.

• DNA is negatively charged.

• DNA-histone interaction is independent of net charge.

• Phosphorylation of serine increases DNA-histone interaction.

• Methylation and acetylation of lysine increases DNA-histone interaction

You are following the intracellular sorting of an integral plasma membrane protein in a living cell, in culture. You have decided to probe this protein by metabolic labeling technique with ³⁵S-methionine (pulse-chase-technique). After one cycle of division, the cells were treated with a potent inhibitor of protein biosynthesis and processed for subcellular fractionation. In which of the following fractions will you expect the presence of this protein upon immunoprecipitation with a specific antibody?

• only cytoplasm

• Only plasma membrane

• Both endoplasmic reticulum and plasma membrane

• Only secretory vesicles and endoplasmic reticulum.

The amino acid alanine has a high propensity to occur in a helical conformation. The circular dichroism spectrum of an equimolar mixture of two 20-residue peptides, one composed of only L-alanine and the other only D-alanine is recorder in the region of 185-250 nm. Which one of the following will be observed?

• No signal: as the chiroptical properties of the two peptides will cancel out

• bands with only negative ellipticity: as helix formed by the D-Ala peptide will be unstable

• Bands with only positive ellipticity: as both the peptides will form right-handed helices

• Bands with identical negative and positive ellipticity

In order to determine the primary structure of an octapeptide, amino acid composition was determined by acid hydrolysis (A). The intact oligopeptide was treated with carboxypeptidase (B), chymotrypsin (C), trypsin, and CNBr (E). The peptides were separated in each case and acid hydrolysis was carried out for B - E. The Following results were obtained (the brackets represent mixtures of amino acids in each fragment):

(A) (2Ala, Arg, Lys, Met, Phe, 2Ser)

(B) (Ala, Arg, Lys, Met, Phe, 2Ser) and Ala

(C) (Ala, Arg, Phe, Ser), (Ala, Lys, Met, Ser)

(D) (Ala, Arg), (Lys, Phe, Ser), (Ala, Met, Ser)

(E) (Ala, Arg, Lys, Met, Phe, Ser), (Ala, Ser)

Which one is the correct sequence of the oligopeptide?

• Arg-Ala-Ser-Lys-Met-Phe-Ser-Ala

• Arg-Ala-Ser-Lys-Phe-Met-Ser-Al

• Ala-Arg-Ser-Phe-Lys-Met-Ser-Ala

• Ala-Arg-Phe-Ser-Lys-Met-Ser-Ala

The apparent pH of a fluid is 7.45, where bicarbonate buffer is involved for maintaining its pH. Values of pKa of carbonic acid are 6.15 and 10.45. The molar ratio of [conjugate base] : [acid] is

• 1 : 20

• 20 : 1

• 1 : 1000

• 1000 : 1

A segment of B-DNA encodes an enzyme of molecular mass 50 kD. The estimated length of this segment in μm would be

• 0.1547

• 0.1547 × 10^-3

• 0.4641

• 0.4641 × 10^-3