Nucleus and Chromatin Practice Questions

Correct answer: Nuclear envelope

The nucleus is surrounded by a double-layered nuclear envelope that separates nuclear contents from the cytoplasm. This envelope regulates exchange of materials via nuclear pores.

Correct answer: Rough endoplasmic reticulum

The outer membrane of the nuclear envelope is continuous with the rough endoplasmic reticulum. Ribosomes are often attached to this outer membrane.

Correct answer: Selective bidirectional transport between nucleus and cytoplasm

Nuclear pore complexes regulate the bidirectional exchange of macromolecules between the nucleus and cytoplasm. RNA molecules (mRNA, rRNA, tRNA) are exported outward while proteins, such as transcription factors and histones, are imported inward. This selective transport is energy-dependent and mediated by importins and exportins.

Correct answer: Chromatin

Chromatin consists of DNA associated with proteins and contains the genetic information of the cell. It is distributed throughout the nucleoplasm.

Correct answer: Lightly stained and transcriptionally active

Euchromatin is loosely packed, lightly stained, and actively involved in transcription. It contains genes that are frequently expressed.

Correct answer: Highly condensed and transcriptionally less active

Heterochromatin is densely packed, stains darkly with basic dyes, and is largely transcriptionally silent. It often contains repetitive sequences and gene-poor regions. This contrasts with euchromatin, which is loosely packed and transcriptionally active.

Correct answer: Constitutive heterochromatin

Constitutive heterochromatin remains condensed throughout the cell cycle. It is usually found at centromeres and telomeres.

Correct answer: Ribosomal RNA (rRNA) for ribosome biogenesis

The nucleolus is the primary site of ribosomal RNA (rRNA) transcription by RNA polymerase I. Both large and small ribosomal subunits are partially assembled in the nucleolus before being exported to the cytoplasm, making the nucleolus central to ribosome biogenesis and thus to all cytoplasmic protein synthesis.

Correct answer: Nuclear pore complexes

The nucleolus consists of three morphologically distinct regions: the fibrillar center (where rDNA is transcribed), the dense fibrillar component (where early rRNA processing occurs), and the granular component (where ribosome subunits are assembled). Nuclear pore complexes are embedded in the nuclear envelope and are entirely separate from the nucleolus.

Correct answer: Disappears temporarily

The nucleolus disassembles during prophase of mitosis and reappears during telophase. This reflects the temporary halt in rRNA synthesis.

Correct answer: Nuclear lamina

The nuclear lamina is a fibrous network underlying the inner nuclear membrane. It maintains nuclear shape and organizes chromatin.

Correct answer: Chromatin

Histones bind DNA to form nucleosomes, the fundamental units of chromatin. This association helps package DNA efficiently within the nucleus.

Correct answer: Nucleosome

A nucleosome consists of DNA wrapped around a core of histone proteins. It is the fundamental repeating unit of chromatin structure.

Correct answer: H1, which binds linker DNA between nucleosomes

Histone H1 (the linker histone) binds to the DNA that enters and exits the nucleosome core (linker DNA). It compacts the nucleosome array into a 30 nm chromatin fiber, stabilizing higher-order chromatin folding. The four core histones (H2A, H2B, H3, H4) form the octamer around which the DNA wraps.

Correct answer: The aqueous fluid matrix that fills the nucleus

The nucleoplasm is the semi-fluid matrix that fills the interior of the nucleus, analogous to the cytoplasm of the cell. It suspends chromatin, the nucleolus, and other nuclear structures, and contains the enzymes, nucleotides, and regulatory molecules required for DNA replication and transcription.

Correct answer: The nucleolus

The nucleolus is the site of ribosomal subunit assembly, combining rRNA with ribosomal proteins. The completed subunits are then exported through nuclear pores to the cytoplasm.

Correct answer: Chromatin condenses during cell division

During cell division, chromatin becomes highly condensed to form visible chromosomes. This condensation ensures accurate segregation of genetic material to daughter cells.

Correct answer: Nucleoporins

Nuclear pore complexes are made up of proteins called nucleoporins. They form selective channels for molecular transport across the nuclear envelope.

Correct answer: Constitutive heterochromatin

Constitutive heterochromatin contains repetitive DNA sequences and remains condensed throughout the cell cycle. It plays a structural role at centromeres and telomeres.

Correct answer: Number of rRNA gene clusters

Nucleoli form around nucleolar organizer regions, which contain clusters of rRNA genes. Cells with more rRNA gene clusters generally have multiple nucleoli to meet their ribosome production demands.

Correct answer: Perinuclear space

The perinuclear space is the gap between the inner and outer nuclear membranes, typically measuring 10–50 nm in width. It is continuous with the lumen of the endoplasmic reticulum.

Correct answer: Nuclear lamina

The nuclear lamina is composed of intermediate filament proteins called lamins. It maintains the shape of the nucleus and serves as an attachment site for chromatin.

Correct answer: Nucleolus

NORs contain the genes for ribosomal RNA (rRNA). The nucleolus forms around these specific chromosomal regions during the interphase of the cell cycle.

Correct answer: Assembly of ribosomal subunits

The nucleolus has three distinct zones; the granular component is the outermost region where maturing ribosomal subunits are assembled before being exported to the cytoplasm.

Correct answer: Heterochromatin

Heterochromatin remains tightly packed throughout the cell cycle. Because it is so condensed, the transcriptional machinery cannot access the DNA, rendering these regions mostly silent.

Correct answer: H1

The nucleosome core octamer consists of two molecules each of H2A, H2B, H3, and H4. Histone H1 is the 'linker histone' that sits outside the core particle to help stabilize the 30nm fiber.

Correct answer: Nuclear pore complexes

Nuclear Pore Complexes are massive protein gates that span the double membrane. They allow small molecules to diffuse but require specific transport receptors for large macromolecules like mRNA and proteins.

Correct answer: Facultative heterochromatin

Facultative heterochromatin is not permanently silenced. It can be de-condensed into euchromatin when specific genes in that region need to be expressed during development.

Correct answer: Nucleoplasm

Nucleoplasm (or nuclear sap) is a highly gelatinous substance that supports the nuclear contents and provides a medium for the movement of molecules involved in signaling and metabolism.

Correct answer: About 1.67 turns

Roughly 147 base pairs of DNA wrap around the histone core in approximately 1.67 left-handed superhelical turns. This is the first level of DNA compaction.

Correct answer: Fibrillar center

The fibrillar center (FC) contains the DNA templates (NORs) and RNA polymerase I required for the transcription of ribosomal RNA precursors.

Correct answer: Nucleoplasmic side

The nuclear pore complex is asymmetrical. It has eight filaments extending into the cytoplasm and a cage-like 'basket' structure extending into the nucleoplasm.

Correct answer: Lysine and Arginine

Histones are rich in Lysine and Arginine. Their positive charge allows them to bind tightly to the negatively charged phosphate backbone of the DNA molecule.

Correct answer: Near the telomeres and centromeres

Constitutive heterochromatin is permanently condensed in all cells of an organism. It usually consists of highly repetitive DNA sequences located at the centromeres and telomeres.

Correct answer: It fully fragments and disappears

In 'open mitosis' (typical of animal cells), the nuclear envelope breaks down into small vesicles. This allows the spindle fibers to access the chromosomes for segregation.

Correct answer: The nucleolus

The nucleolus is a 'subnuclear organelle' but lacks a surrounding membrane. It is formed by the physical aggregation of rRNA, proteins, and chromatin.

Correct answer: Transporting proteins into nucleus

Importins are karyopherins that recognize Nuclear Localization Signals (NLS) on cargo proteins and facilitate their transport through the nuclear pore complex into the nucleus.

Correct answer: Chemical modification of rRNA

snoRNAs are found in the nucleolus and guide the enzymes responsible for modifying (such as methylation) ribosomal RNA during its maturation process.

Correct answer: Chromatin

Chromatin consists of DNA, histones, and non-histone proteins. Its primary functions are to package DNA into a smaller volume and to control gene expression and DNA replication.

Correct answer: Active transport through nuclear pores

Assembled 40S and 60S ribosomal subunits are too large to diffuse. They are actively transported out of the nucleus through the nuclear pore complexes using exportin proteins.