Chapter 2 - Nucleus

Question 1

The nuclear pore complex:
(A) permits free communication between the nucleus and the cytoplasm
(B) is bridged by a unit membrane
(C) is located only at specific nuclear pore sites
(D) permits passage of proteins via receptor-
mediated transport
(E) has a luminal ring that faces the cytoplasm.

Answer 1

D. The NPC contains a central aqueous channel that permits passage of small water-soluble molecules. However, movement of proteins in and out of the nucleus is selectively controlled by the NPC via receptor-mediated transport (see Chapter 2 II E).

Question 2

Which one of the following nucleotides is
present only in RNA?
(A) Thymine
(B) Adenine
(C) Uracil
(D) Cytosine
(E) Guanine

Answer 2

C. DNA contains the purines, adenine and guanine, and the pyrimidines, cytosine and thymine. In RNA, uracil, a pyrimidine, replaces thymine (see Chapter 2 VII A, VIII).

Question 3

Anticodons are located in:
(A) mRNA
(B) rRNA
(C) tRNA
(D) snRNP
(E) hnRNP

Answer 3

C. Each tRNA possesses a triplet of nucleotides, called an anticodon, which recognizes the complementary codon in mRNA (see Chapter 2 VIII B).

Question 4

DNA is duplicated in the cell cycle during the:
(A) G2 phase.
(B) S phase.
(C) M phase.
(D) G1 phase.
(E) G0 phase.

Answer 4

B. The S (synthesis) phase of the cell cycle is the period during which DNA replication and histone synthesis occur, resulting in duplication of the chromosomes. At the end of the S phase, each chromosome consists of two identical chromatids attached to one another at the centromere (see Chapter 2 IX B).

Question 5

A male child at puberty is determined to have Klinefelter syndrome. Although the par­ents have been informed of the clinical signifi­cance, they have asked for an explanation of what happened. Identify the item that should be discussed with the parents.
(A) Trisomy of chromosome 21
(B) Loss ofan autosome during mitosis
(C) Loss of the Y chromosome during meiosis
(D) Nondisjunction of the X chromosome
(E) Loss ofthe X chromosome

Answer 5

D. Klinefelter syndrome occurs only in men. This condition results from nondisjunction of the X chromosome during meiosis, resulting in an extra X chromosome in somatic cells. These cells therefore have a normal complement of autosomal chromosomes (22 pairs), and instead of one pair of sex chromosomes (XY), there is an extra X chromosome. These individuals have an XXY genotype, resulting in 47 total chromosomes rather than the normal complement of 46. This syndrome is an example of trisomy of the sex chromosomes. Down syndrome is an example of an autosomal trisomy, specifically trisomy of chromosome number 21. Both syndromes have profound complications (see Chapter 2 XI B Clinical Considerations).

Question 6

Which one of the following is an inclusion not bounded by a membrane that is observable only during interphase?
(A) Nuclear pore complex
(B) Nucleolus
(C) Heterochromatin
(D) Outer nuclear membrane
(E) Euchromatin

Answer 6

B. The nucleolus is an inclusion, not bounded by a membrane, within the nucleus. It is observable during interphase but disappears during mitosis (see Chapter 2 III A).

Question 7

A structure that is continuous with RER is the:
(A) Nuclear pore complex
(B) Nucleolus
(C) Heterochromatin
(D) Outer nuclear membrane
(E) Euchromatin

Answer 7

D. The outer nuclear membrane is continuous with the RER (see Chapter 2 II A).

Question 8

Identify the structure that controls movement of proteins in and out of the nucleus.
(A) Nuclear pore complex
(B) Nucleolus
(C) Heterochromatin
(D) Outer nuclear membrane
(E) Euchromatin

Answer 8

A. The NPC selectively controls movements of water-soluble molecules and proteins in and out of the nucleus (see Chapter 2 II E) .

Question 9

The site of transcriptional activity is the:
(A) Nuclear pore complex
(B) Nucleolus
(C) Heterochromatin
(D) Outer nuclear membrane
(E) Euchromatin

Answer 9

E. The pale-staining euchromatin is the transcriptionally active chromatin in the nucleus (see Chapter 2 V A).

Question 10

Clumps of nucleoprotein concentrated near the periphery of the nucleus are called:
(A) Nuclear pore complex
(B) Nucleolus
(C) Heterochromatin
(D) Outer nuclear membrane
(E) Euchromatin

Answer 10

C. Heterochromatin is the dark-staining nucleoprotein near the periphery of the nucleus. It is transcriptionally inactive, but may be responsible for proper chromosome segregation during meiosis (see Chapter 2 V A).