Test: Nucleus & Chromosomes - NEET MCQ

Histone H₁ is one of the five main histone protein families which are components of chromatin in eukaryotic cells. Though highly conserved, it is nevertheless the most variable histone in sequence across species. Unlike the other histones, H1 does not make up the nucleosome "bead". Instead, it sits on top of the structure, keeping in place the DNA that has wrapped around the nucleosome.

• Some cells such as the red blood cells or the erythrocytes of humans and the sieve tube cells of vascular plants lack a nucleus in their mature cells.
• Leukocytes and thrombocytes possess a nucleus.

• The nuclear matrix or the nucleoplasm contains the nucleolus and chromatin.
• The nucleolus is the site for active ribosomal RNA or rRNA synthesis.
• The nucleolus is not bound by a membrane.

• The nucleus contains the nucleolus which is not a membrane-bound organelle.
• The mitochondria, the endoplasmic reticulum, and lysosomes are present in the cytosol.
• They are membrane-bound organelles of the cell.

Lampbrush Chromosomes (LBCs) are present in the oocytes of birds, lower vertebrata and invertebrates during the prolonged prophase of the first meiotic division.

• Every visible chromosome has two sister chromatids which are constricted together at a point known as the centromere.
• During mitosis, the spindle fibres attach to the chromosomes via the centromere.

Based on the position of the centromere, the chromosomes can be classified into four types. The metacentric chromosome has its centromere in the middle, thus forming two equal arms of the chromosome.

Nucleus controls the activity of cytoplasm by sending RNA. The nucleus pores allows the movement of large molecules such as proteins and RNA through the pores is required for both gene expression and the maintenance of chromosomes. The interior of the nucleus does not contain any membrane-bound sub compartments, its contents are not uniform, and a number of sub-nuclear bodies exist, made up of unique proteins, RNA molecules, and particular parts of the chromosomes. The best-known of these is the nucleolus, which is mainly involved in the assembly of ribosomes. After being produced in the nucleolus, ribosomes are exported to the cytoplasm where they translate mRNA.

Diplotene chromosomes are the lampbrush chromosomes. Lampbrush chromosomes are a special form of chromosome found in the growing oocytes (immature eggs) of most animals, except mammals. Chromosomes transform into the lampbrush form during the diplotene stage of meiotic prophase I due to an active transcription of many genes. They are highly extended meiotic half-bivalents, each consisting of 2 sister chromatids.

Based on the position of the centromere, the chromosomes can be classified into four types. In telocentric chromosomes, the centromere is located at the proximal end or the tip of the chromosome.

• Some chromosomes have secondary constrictions which are non – staining, apart from the centromere.
• They give the appearance of a small fragment.
• This small fragment is known as a satellite.

A nucleoside triphosphate (NTP) is a molecule containing a nucleotide bound to three phosphates. These NTPs include ATP, GTP, CTP, UTP. They generally provide energy and the phosphate group for phosphorylation. They are stored in the nucleus.
Thus, the correct answer is option (D), 'CTP and all the above'.

Ribonucleic acid occurs in the nucleus and cytoplasm. The DNA molecules are transcribed inside the nucleus to form RNA. The RNA undergoes post-translational modification and forms different types of RNA which leave the nucleus by nuclear pore and undergo translation to form protein.

• Robert Brown: Robert Brown, a Scottish botanist, discovered the cell nucleus in 1831 while studying orchids under a microscope. He observed a central structure within the cells which he named the nucleus.


• Significance: Brown's discovery of the nucleus was a crucial milestone in the field of cell biology as it provided insight into the organization and function of cells.


• Structure: The nucleus is a membrane-bound organelle found in eukaryotic cells and contains the cell's genetic material in the form of DNA. It plays a vital role in controlling cellular activities and gene expression.


• Function: The nucleus is responsible for regulating gene expression, DNA replication, and cell division. It acts as the control center of the cell, coordinating various cellular processes.

The chromatin material which takes darker stain in interphase is called Heterochromatin. Heterochromatin is a tightly packed form of DNA, which comes in different varieties. These varieties lie on a continuum between the two extremes of constitutive and facultative heterochromatin.

• Some chromosomes have secondary constrictions which are non-staining, apart from the centromere.
• They give the appearance of a small fragment.
• This small fragment is known as a satellite.

• Nucleus Structure:




• The nucleus is a two-layered structure.


• It consists of an inner and outer nuclear membrane.


• The nuclear membrane is made up of two lipid bilayers.




• Functions of Nucleus:




• The nucleus houses the genetic material of the cell, including DNA and RNA.


• It is responsible for controlling the activities of the cell by regulating gene expression.


• The nucleus is involved in processes such as DNA replication, transcription, and mRNA processing.




• Importance of Nucleus:




• The nucleus is essential for the growth, development, and reproduction of the cell.


• It plays a crucial role in maintaining the integrity of genetic information.


• Any abnormalities in the nucleus can lead to various genetic disorders and diseases.

• Every chromosome has a primary constriction known as the centromere.
• The sides of the centromere contains disc shaped structures known as kinetochores.

• A single human cell has approximately two metre long thread of DNA.
• This DNA is distributed among its forty-six (twenty-three pairs) chromosomes.
• Since, the oocyte is haploid, it contains 23 chromosomes.

DNase catalyses the cleavage of the phosphodiester linkage in the DNA molecule and thus facilitates the degradation of the DNA.

The nucleolus is the largest structure present in the nucleus of eukaryotic cells where it primarily serves as the site of ribosome synthesis and assembly. Nucleoli also have other important functions like the assembly of signal recognition particles and playing a role in the cell's response to stress. Nucleoli are made of proteins and RNA and form around
the specific chromosomal regions.

The nucleolar chromatin contains the gene for rRNA and ribonucleoprotein. The nucleolus is the largest structure present in the nucleus of eukaryotic cells where it primarily serves as the site of ribosome synthesis and assembly. Nucleoli also have other important functions like the assembly of signal recognition particles and playing a role in the cell's response to stress. Nucleoli are made of proteins and RNA and form around specific chromosomal regions.

In metacentric centromere is located at centre of chromosome and therefore arm ratio is 1:1

To determine which statement about the nucleus is incorrect, let's analyze each statement:

1. In plant cells, the nucleus lies towards the periphery due to the large nucleus that occupies almost the entire region of the cell:

   • This is incorrect. In plant cells, the nucleus is pushed towards the periphery not because of a large nucleus, but due to the presence of a large central vacuole that occupies most of the cell's interior.

2. Nucleus helps in reproduction and acts as the control centre of the cell:

   • This is correct. The nucleus contains genetic material and controls cell activities, including reproduction

3. Nucleoplasm condenses and groups together to form chromosomes:

   • This is correct. The nucleoplasm contains chromatin, which condenses and organizes into chromosomes during cell division.

4. Nucleus comprises of nuclear membrane, nuclear pores, nucleolus, nucleoplasm, and chromatin:
           This is correct. The nucleus includes all these components.

Therefore, the incorrect statement is:

1. In plant cells, the nucleus lies towards the periphery due to the large nucleus that occupies almost the entire region of the cell

• Nuclear membrane is a double layered covering of nucleus which is separated by a perinuclear space.
• Nuclear pores allow the transfer of materials and disrupt the continuity of nuclear membrane.
• Nucleoplasm is also termed as “Karyolymph” and act as site for synthesis of DNA and RNA.
• Chromatin contains RNA, DNA, histone and non – histone proteins.

Chromosomes are the carrier of genetic information in form of DNA. The secondary constrictions are used as a point of identification of chromosomes. These are always constant in their positions and hence, can be used as markers that identify particular chromosomes. A satellite chromosome or SAT chromosome has a chromosome segment that is separated from the main body of the chromosome by such a secondary constriction.
So, the correct answer is option C.

• Chromosomes are rod shaped structures that condenses together when the cell is about divide.
• Chromosomes are composed of DNA, RNA and proteins which act as repository for inheritance.
• Kinetochores are disc like structures that are present on the sides of centromeres.
• In telocentric chromosomes, centromere is present in the end of the chromatids.

Linker DNA is double-stranded DNA in between two nucleosome cores that, in association with histone H1, holds the cores together. Linker DNA connects to histone H1 and histone H1 sits on the nucleosome core. Nucleosome is technically the consolidation of a nucleosome core and one adjacent linker DNA; however, the term nucleosome is used freely for solely the core. Linker DNA may be degraded by endonucleases.

Chromatin is made of nucleoprotein. Chromatin is a complex of macromolecules found in cells, consisting of DNA, protein and RNA. The primary functions of chromatin are to pack DNA into a smaller volume to fit in the cell, to reinforce the DNA macromolecule to allow mitosis, to prevent DNA damage, and to control gene expression and DNA replication. The primary protein components of chromatin are histones that compact the DNA.