Question 1

The nucleus is bounded by a double membrane, the nuclear envelope, interrupted by:

Accepted Answer

Nuclear pores. Nuclear pores allow selective exchange (RNA, proteins) between nucleoplasm and cytoplasm.

Question 2

The loose, thread-like DNA–protein material seen in a non-dividing nucleus is:

Accepted Answer

Chromatin. Chromatin condenses into visible chromosomes during division; the nucleolus is the site of rRNA synthesis.

Question 3

A chromosome with the centromere in the middle (equal arms) is:

Accepted Answer

Metacentric. Metacentric = middle centromere; sub-metacentric = slightly off-centre; acrocentric = near one end; telocentric = terminal.

Question 4

The nucleolus, prominent in cells actively making proteins, is the site of:

Accepted Answer

Ribosomal RNA synthesis and ribosome assembly. The nucleolus is not membrane-bound and manufactures rRNA and ribosomal subunits.

Question 5

The secondary constriction on some chromosomes bears the nucleolus organiser and its distal segment is the:

Accepted Answer

Satellite (SAT chromosome). A satellite (distal to the secondary constriction) marks SAT chromosomes; the primary constriction is the centromere.

Question 6

The kinetochores at the centromere are essential during cell division because they:

Accepted Answer

Provide attachment sites for spindle fibres. Kinetochores are disc-shaped protein structures at the centromere to which spindle microtubules attach for chromosome movement.

Question 7

The nuclear envelope of a eukaryotic cell is perforated by nuclear pores, whose chief role is to:

Accepted Answer

Allow selective exchange of RNA and proteins between the nucleus and cytoplasm. Nuclear pores are passages through the double nuclear envelope that permit regulated movement of RNA and protein molecules in both directions between nucleoplasm and cytoplasm.

Question 8

The nucleoid differs from a true nucleus in that the nucleoid:

Accepted Answer

Is a region of DNA in prokaryotes not enclosed by a nuclear membrane. The nucleoid is the poorly defined region in a prokaryote where the naked circular DNA lies, without any surrounding nuclear membrane, nucleolus or pores.

Question 9

During the metaphase of cell division a chromosome is seen to consist of two sister chromatids joined at a constricted region. This primary constriction is the:

Accepted Answer

Centromere, on which the disc-shaped kinetochores are located. The primary constriction is the centromere, to which sister chromatids are attached and on which the kinetochores (spindle-fibre attachment sites) sit; the secondary constriction bears the NOR.

Question 10

A chromosome has its centromere very close to one end so that it has one extremely short arm and one very long arm. It is classified as:

Accepted Answer

Acrocentric. When the centromere lies near an end producing one very short and one very long arm, the chromosome is acrocentric; a strictly terminal centromere gives a telocentric chromosome.

Question 11

Assertion (A): Heterochromatin stains darkly and is transcriptionally less active than euchromatin. Reason (R): Heterochromatin is the more densely packed, condensed form of chromatin.

Accepted Answer

Both A and R are true and R is the correct explanation of A. Densely packed (condensed) heterochromatin stains darkly and is largely inactive, while the loosely packed, lighter euchromatin is transcriptionally active — the dense packing (R) explains its dark staining and low activity (A).

Question 12

Which statement about the nucleolus is INCORRECT?

Accepted Answer

It is a membrane-bound organelle that divides the nucleus into compartments. The nucleolus is a naked (non-membrane-bound) structure that is the site of rRNA synthesis and is more prominent in protein-synthesising cells; the claim that it is membrane-bound is false.

Question 13

The interphase chromatin of a eukaryotic nucleus is a complex of DNA with basic proteins. The repeating unit in which DNA is wrapped around a histone octamer is the:

Accepted Answer

Nucleosome. Chromatin is organised into nucleosomes — DNA wound around a core of eight histone proteins — giving the beads-on-a-string appearance; the nucleoid is the prokaryotic DNA region.

Question 14

The name 'chromatin' was given by

Accepted Answer

Fleming. Flemming introduced the term chromatin for the interphase nuclear material that takes up basic stains and consists of DNA and proteins.

Question 15

The outer membrane of the nucleus remains continuous with which cell organelle?

Accepted Answer

ER. The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum and often bears ribosomes on its surface.

Question 16

The perinuclear space is about

Accepted Answer

10–50 nm. The two parallel membranes of the nuclear envelope are separated by the perinuclear space, which is about 10 to 50 nm wide.

Question 17

The following cells are without a nucleus

Accepted Answer

All of these. Mature mammalian erythrocytes and mature sieve tube cells lack a nucleus, and bacterial (prokaryotic) cells lack a true membrane-bound nucleus, so all of these are without a nucleus.

Question 18

The site of active ribosomal RNA synthesis is

Accepted Answer

Nucleolus. The nucleolus is not membrane-bound and is the site of active ribosomal RNA (rRNA) synthesis.

Question 19

The site for rRNA synthesis is

Accepted Answer

Nucleolus. Ribosomal RNA is synthesised in the nucleolus, a dense spherical structure within the nucleus lacking a limiting membrane.

Question 20

A chromosome in which the centromere is situated at one end is

Accepted Answer

Telocentric. A telocentric chromosome has its centromere at the terminal end, giving a single arm.

Nucleus & Chromosomes — NEET Biology MCQs with Solutions

Questions with solutions