Nucleus and Chromosomes Questions in English

(C) Joachim Hammerling performed experiments on the unicellular green alga $Acetabularia$ to demonstrate the role of the nucleus in heredity.
He used two species, $Acetabularia$ $crenulata$ and $Acetabularia$ $mediterranea$.
In his experiments, he performed grafting by exchanging the $stalk$ (which contains the cytoplasm) and the $rhizoid$ (which contains the nucleus) between the two species.
He observed that the characteristics of the cap that regenerated were determined by the species from which the $rhizoid$ (nucleus) was taken, not the $stalk$.
Therefore, the parts exchanged in his experiments were the $rhizoids$ and the $stalks$.

(B) Joachim Hammerling conducted experiments on the unicellular green alga $Acetabularia$ to determine the role of the nucleus in heredity.
He used two species,$Acetabularia$ $crenulata$ and $Acetabularia$ $mediterranea$.
By performing grafting experiments,he observed that the cap morphology (the shape of the umbrella-like structure) was determined by the nucleus present in the rhizoid.
When the nucleus of one species was transplanted into the stalk of another,the regenerated cap always resembled the species from which the nucleus was derived.
This proved that the nucleus contains the genetic information responsible for controlling cellular traits and heredity.

(A) The $Nucleolus$ is a non-membrane-bound structure found within the nucleus of eukaryotic cells.
It is primarily involved in the synthesis of ribosomal $RNA$ $(rRNA)$ and the assembly of ribosomes.
In contrast,$Lysosomes$,$Mitochondria$,and $Chloroplasts$ are all membrane-bound organelles.

(B) The centromere is a specialized $DNA$ sequence of a chromosome that links a pair of sister chromatids. During mitosis,spindle fibers attach to the centromere via the kinetochore. Therefore,the centromere is an integral part of the chromosome structure.

(A) $DNA$ (Deoxyribonucleic acid) is the genetic material of eukaryotic cells. It is primarily located within the nucleus,where it is organized into chromosomes. While small amounts of $DNA$ are also found in mitochondria and chloroplasts (which are organelles in the cytoplasm),the primary and most significant location for the bulk of cellular $DNA$ is the nucleus.

(A) Mature mammalian red blood cells $(RBCs)$ are specialized cells that lose their nucleus and most organelles,including mitochondria and ribosomes,during maturation to maximize space for hemoglobin. Consequently,they do not contain $DNA$. In contrast,an enucleated ovum (if referring to a laboratory-manipulated one) lacks a nucleus,but a normal ovum contains $DNA$. Mature sperm cells contain a haploid nucleus with $DNA$. Hair roots contain living cells with nuclei and $DNA$.

(B) The nucleolus is a non-membrane-bound structure present within the nucleus of eukaryotic cells.
It is the primary site for the active synthesis of ribosomal $RNA$ $(rRNA)$.
After synthesis,these $rRNA$ molecules are assembled with ribosomal proteins to form ribosomal subunits,which are then exported to the cytoplasm for protein synthesis.

(A) Balbiani rings are large puffs found on polytene chromosomes.
These structures are sites of intense gene transcription,where $DNA$ is actively transcribed into $RNA$.
Consequently,they are the primary sites for $RNA$ and protein synthesis within the cell nucleus.

(B) Polytene chromosomes are giant chromosomes that are common to many dipteran flies. They were first discovered by $E.G. Balbiani$ in $1881$ in the salivary glands of $Chironomus$ larvae. These chromosomes are formed by repeated rounds of $DNA$ replication without cell division,a process known as endoreduplication.

(B) The nucleolus is a dense,spherical,non-membrane-bound structure found within the nucleus of eukaryotic cells.
It is the primary site for the transcription of ribosomal $RNA$ $(rRNA)$ genes and the assembly of ribosomal subunits.
Therefore,ribosomal $RNA$ is actively synthesized in the nucleolus.

(B) Balbiani rings,also known as chromosomal puffs,are large,localized swellings found on polytene chromosomes in certain insect tissues (e.g.,salivary glands of Chironomus larvae).
These structures represent regions of intense gene activity where the chromatin is decondensed.
This decondensation allows for high levels of transcription,leading to the synthesis of large amounts of $mRNA$ (messenger $RNA$).
Consequently,these $mRNA$ molecules are subsequently used for protein synthesis in the cytoplasm.
Therefore,Balbiani rings are primarily sites of active $RNA$ transcription and subsequent protein synthesis.

(D) Polytene chromosomes are giant chromosomes found in the salivary glands of Drosophila larvae and some other Dipteran insects.
They are formed through a process called endoreduplication,where the $DNA$ undergoes multiple rounds of replication without cell division.
These replicated chromatids remain attached to each other side-by-side,a process known as somatic pairing.
This pairing occurs specifically between homologous chromosomes,resulting in a bundle of many parallel chromatids.

(D) Polytene chromosomes are giant chromosomes found in the salivary glands of Drosophila larvae.
These chromosomes consist of many chromatids that are held together at a common central point.
This central point where all the chromatids of the polytene chromosome converge is known as the $Chromocenter$.

(B) The question asks for a structure that contains more than one but fewer than five chromosomes.
In biological terminology,a single chromosome consists of two chromatids joined at a centromere during the $S$ phase and $G_2$ phase of the cell cycle.
However,the term 'chromosome' itself refers to the entire structure.
Looking at the options,a 'chromosome' is the fundamental unit.
If we consider the structure of a chromosome,it is composed of $DNA$ and proteins.
In certain contexts,such as polytene chromosomes found in salivary glands of Drosophila,the structure contains multiple strands of $DNA$ (chromonemata).
However,based on standard cell biology,the question is likely referring to the structure of a chromosome itself or a specific biological entity.
Given the options,the most appropriate answer in the context of basic cell biology is that a chromosome is the entity that holds genetic material.
Actually,the question is a bit ambiguous,but in many competitive biology contexts,it refers to the fact that a chromosome is a single unit,and the options provided are parts of it.
Re-evaluating: $A$ chromosome is a single structure. The question might be flawed or referring to a specific organism's ploidy.
However,based on standard curriculum,the answer is $B$.

(D) Polytene chromosomes are giant chromosomes found in the salivary glands of Dipteran larvae (e.g.,$Drosophila$).
They are formed by the process of endoreduplication (or endomitosis),where $DNA$ replication occurs repeatedly without cell division,leading to thousands of chromatids aligned side-by-side.
Because of their massive size and the presence of distinct banding patterns (chromomeres),they are extremely useful for gene mapping and studying chromosomal structure.

(A) Polytene chromosomes are giant chromosomes found in the salivary glands of Drosophila (fruit fly) larvae and some other dipteran insects,not in the oocytes of amphibians.
Lampbrush chromosomes are observed in the diplotene stage of meiotic prophase-$I$ in the oocytes of many vertebrates,including amphibians.
Submetacentric chromosomes have a centromere slightly away from the center,resulting in unequal arms,which gives them an $L$-shape during anaphase.
Allosomes are the chromosomes that determine the sex of an organism,commonly known as sex chromosomes.
Therefore,the incorrect match is Polytene chromosomes - Oocytes of amphibians.

(A) The nucleolus is a spherical structure present in the nucleoplasm of the nucleus.
It is not a membrane-bound structure,meaning it lacks a limiting membrane.
Its primary function is the active synthesis of ribosomal $RNA$ $(rRNA)$.
Cells that are actively involved in protein synthesis contain larger and more numerous nucleoli.
Spindle formation is a function of the centrosome/microtubules,not the nucleolus.
Therefore,the statement that it is a site for active ribosomal $RNA$ synthesis is correct.

(B) In a submetacentric chromosome,the centromere is situated slightly away from the middle,resulting in one shorter arm and one longer arm.
By convention,the shorter arm is designated as the $p$-arm (from the French 'petit',meaning small).
The longer arm is designated as the $q$-arm (the letter following $p$ in the alphabet).
Therefore,the shorter and longer arms are referred to as the $p$-arm and $q$-arm respectively.

(A) Polytene chromosomes are giant chromosomes found in the salivary glands of Drosophila larvae.
They are formed through a process called endoreduplication or endomitosis,where the $DNA$ replicates repeatedly without the cell dividing.
Because of this repeated replication of $DNA$ strands,these chromosomes contain a very high amount of $DNA$ and appear as thick,banded structures.
Therefore,the Assertion is correct,and the Reason correctly explains why they have a high amount of $DNA$.

(C) The non-membranous nucleoplasmic structures present in the nucleus are known as nucleoli (singular: nucleolus).
These structures are not bounded by any membrane.
The nucleolus is the primary site for the active synthesis of ribosomal $RNA$ $(rRNA)$.
Ribosomal proteins are imported from the cytoplasm and assembled with $rRNA$ within the nucleolus to form ribosomal subunits.

(N/A) Nuclear pores are complex proteinaceous structures formed by the fusion of the inner and outer nuclear membranes. These pores appear as tiny holes or gaps in the nuclear envelope.
Their primary function is to regulate the movement of molecules between the nucleus and the cytoplasm. They allow the selective transport of essential molecules such as $RNA$,ribosomal subunits,and proteins in both directions,ensuring proper cellular communication and gene expression.

(N/A) centromere is a primary constriction present on the chromosomes where the chromatids are held together.
Chromosomes are divided into four types based on the position of the centromere:
$(i)$ Metacentric chromosome: The chromosome in which the centromere is present in the middle and divides the chromosome into two equal arms.
$(ii)$ Sub-metacentric chromosome: The chromosome in which the centromere is slightly away from the middle region,resulting in one arm being slightly longer than the other.
$(iii)$ Acrocentric chromosome: The chromosome in which the centromere is located close to one of the terminal ends,resulting in one extremely long arm and one extremely short arm.
$(iv)$ Telocentric chromosome: The chromosome in which the centromere is located at one of the terminal ends.

(N/A) $ \Rightarrow $ The nucleus as a cell organelle was first described by Robert Brown in $1831$. Later, the material of the nucleus stained by basic dyes was given the name chromatin by Flemming.
$ \Rightarrow $ The interphase nucleus contains highly extended and elaborate nucleoprotein fibres called chromatin, nuclear matrix, and one or more spherical bodies called nucleoli.
$ \Rightarrow $ The nuclear envelope consists of two parallel membranes with a space between them ($10$ to $50 \, nm$) called the perinuclear space. This forms a barrier between the materials present inside the nucleus and the cytoplasm.
$ \Rightarrow $ The outer membrane usually remains continuous with the endoplasmic reticulum and also bears ribosomes on its surface.
$ \Rightarrow $ At a number of places, the nuclear envelope is interrupted by minute pores, which are formed by the fusion of its two membranes. These nuclear pores are the passages through which the movement of $RNA$ and protein molecules takes place in both directions between the nucleus and the cytoplasm.
$ \Rightarrow $ Nucleoli: These are spherical structures present in the nucleoplasm. The content of the nucleolus is continuous with the rest of the nucleoplasm. It is a site for active ribosomal $RNA$ synthesis. Large and more numerous nucleoli are present in cells actively carrying out protein synthesis.

(N/A) $\Rightarrow$ The nucleus as a cell organelle was first described by Robert Brown in $1831$. Later,the material of the nucleus stained by basic dyes was given the name chromatin by Flemming.
$\Rightarrow$ The interphase nucleus has highly extended and elaborate nucleoprotein fibres called chromatin,nuclear matrix,and one or more spherical bodies called nucleoli.
$\Rightarrow$ The nuclear envelope consists of two parallel membranes with a space between them ($10$ to $50 \ nm$) called the perinuclear space. This forms a barrier between the materials present inside the nucleus and those of the cytoplasm.
$\Rightarrow$ The outer membrane usually remains continuous with the endoplasmic reticulum and also bears ribosomes on it.
$\Rightarrow$ At a number of places,the nuclear envelope is interrupted by minute pores,which are formed by the fusion of its two membranes. These nuclear pores are the passages through which the movement of $RNA$ and protein molecules takes place in both directions between the nucleus and the cytoplasm.
$\Rightarrow$ Nucleoli: These are spherical structures present in the nucleoplasm. The content of the nucleolus is continuous with the rest of the nucleoplasm. It is a site for active ribosomal $RNA$ synthesis.
$\Rightarrow$ Large and more numerous nucleoli are present in cells actively carrying out protein synthesis.

(N/A) $ \Rightarrow $ Chromatin contains $DNA$, some basic proteins called histones, some non-histone proteins, and also $RNA$. $A$ single human cell has approximately $2 \, m$ long thread of $DNA$ distributed among its $46$ chromosomes ($23$ pairs).
$ \Rightarrow $ Every chromosome essentially has a primary constriction or the centromere, on the sides of which disc-shaped structures called kinetochores are present.
$ \Rightarrow $ Based on the position of the centromere, chromosomes are classified into four types:
$ \Rightarrow $ Metacentric: The metacentric chromosome has a middle centromere forming two equal arms of the chromosome.
$ \Rightarrow $ Sub-metacentric: The sub-metacentric chromosome has a centromere slightly away from the middle of the chromosome, resulting in one shorter arm and one longer arm.
$ \Rightarrow $ Acrocentric: In the case of an acrocentric chromosome, the centromere is situated close to its end, forming one extremely short and one very long arm.
$ \Rightarrow $ Telocentric: $A$ telocentric chromosome has a terminal centromere.

(D) Based on the position of the centromere,chromosomes are classified into $4$ types:
$1$. Metacentric: The centromere is in the middle,forming two equal arms.
$2$. Sub-metacentric: The centromere is slightly away from the middle,resulting in one shorter arm and one longer arm.
$3$. Acrocentric: The centromere is situated close to the end,forming one extremely short arm and one very long arm.
$4$. Telocentric: The centromere is located at the terminal end of the chromosome.

(N/A) $ \Rightarrow $ The nucleus as a cell organelle was first described by Robert Brown in $1831$. Later, the material of the nucleus stained by basic dyes was named chromatin by Flemming.
$ \Rightarrow $ The interphase nucleus contains highly extended and elaborate nucleoprotein fibres called chromatin, nuclear matrix, and one or more spherical bodies called nucleoli.
$ \Rightarrow $ The nuclear envelope consists of two parallel membranes with a space between them ($10$ to $50 \ nm$) called the perinuclear space.
$ \Rightarrow $ This forms a barrier between the materials present inside the nucleus and the cytoplasm. The outer membrane usually remains continuous with the endoplasmic reticulum and also bears ribosomes on it.
$ \Rightarrow $ At a number of places, the nuclear envelope is interrupted by minute pores, which are formed by the fusion of its two membranes. These nuclear pores are the passages through which the movement of $RNA$ and protein molecules takes place in both directions between the nucleus and cytoplasm.
$ \Rightarrow $ Nucleoli: The nucleoli are spherical structures present in the nucleoplasm. The content of the nucleolus is continuous with the rest of the nucleoplasm. It is a site for active ribosomal $RNA$ synthesis. Large and more numerous nucleoli are present in cells actively carrying out protein synthesis. Thus, the nucleus acts as the control center of the cell by regulating gene expression and protein synthesis.

(B) In $1831$,the Scottish botanist Robert Brown discovered the nucleus within the cell. He observed this structure while studying orchid cells and named it the 'nucleus' (Latin for 'kernel'). This discovery was a significant milestone in cell biology,as it identified the central organelle responsible for controlling cellular activities.

(N/A) The nucleolus is the primary site for $rRNA$ synthesis.
It acts as a center for producing the components of ribosomes.
It connects $rRNA$ with proteins to form the subunits of ribosomes.
These subunits are then transported into the cytoplasm,where they assemble to perform protein synthesis.

(N/A) $\Rightarrow$ Histones are basic proteins rich in positively charged amino acids,specifically lysine and arginine.
$\Rightarrow$ They are found in the nuclei of eukaryotic cells and are responsible for the packaging of $DNA$ into structural units called nucleosomes.
$\Rightarrow$ Function: Histones possess positively charged ends that attract the negatively charged phosphate backbone of $DNA$,allowing the $DNA$ to wrap around the histone octamer.
$\Rightarrow$ This packaging is essential for condensing $DNA$ into chromatin,which helps in fitting the long $DNA$ molecules into the small volume of the nucleus and plays a crucial role in gene regulation.

(N/A) The genomic $DNA$ (nuclear $DNA$) is replicated and distributed through a highly regulated process (mitosis/meiosis),ensuring that every cell of a given species contains a constant and identical amount of genetic material.
In contrast,extrachromosomal $DNA$,such as plasmids in bacteria or $DNA$ in mitochondria and chloroplasts,does not follow the same strict segregation pattern.
Plasmids are small,circular,extrachromosomal $DNA$ molecules that can be gained or lost through horizontal gene transfer (conjugation,transformation,transduction).
Similarly,the number of organelles like mitochondria varies depending on the metabolic activity of the cell. Since these organelles contain their own $DNA$,their variable copy number leads to variation in the total amount of extrachromosomal $DNA$ among different members of a population.

(D) Chromosomes are composed of chromatin,which consists of $DNA$ and proteins. To visualize them under a microscope,specific stains are used that bind to the $DNA$ or chromatin structure.
Commonly used stains for chromosomes include $Acetocarmine$,$Giemsa$ stain,and $Orcein$.
$Acetocarmine$ is frequently used for staining plant chromosomes,while $Giemsa$ stain is widely used in cytogenetics for $G-banding$ of chromosomes.
Therefore,all the listed options are correct stains used for coloring chromosomes.

(N/A)

Euchromatin	Heterochromatin
$(1)$ The region of chromatin that is loosely packed and stains light is referred to as euchromatin.	$(1)$ The region of chromatin that is densely packed and stains dark is referred to as heterochromatin.
$(2)$ It is transcriptionally active chromatin.	$(2)$ It is transcriptionally inactive chromatin.

(D) The diploid number of chromosomes $(2n)$ in maize $(Zea \ mays)$ is $20$.
$A$ haploid cell $(n)$ contains half the number of chromosomes of a diploid cell.
Therefore,the number of chromosomes in a haploid cell of maize is $20 / 2 = 10$.

(B) The correct matches for the number of chromosomes in somatic cells are as follows:
$(a)$ Human: Humans have $46$ chromosomes in their somatic cells $(2n = 46)$. Thus, $(a-4)$.
$(b)$ Apple: Apple cells contain $34$ chromosomes $(2n = 34)$. Thus, $(b-3)$.
$(c)$ Maize: Maize cells contain $20$ chromosomes $(2n = 20)$. Thus, $(c-2)$.
$(d)$ Rice: Rice cells contain $24$ chromosomes $(2n = 24)$. Thus, $(d-1)$.
Therefore, the correct sequence is $(a-4, b-3, c-2, d-1)$.

(B) The nucleus is a membrane-bound organelle found in eukaryotic cells.
Its size varies depending on the cell type,but a typical nucleus has a diameter of approximately $10^{-6} \ m$ (or $1 \ \mu m$).
This is equivalent to $1000 \ nm$.

(A, B) In eukaryotic cells,such as plant and animal cells,$DNA$ is organized into long,linear strands associated with histone proteins to form chromosomes.
Viroids consist of short,circular,single-stranded $RNA$ molecules without a protein coat.
Lactobacillus is a prokaryote where $DNA$ exists as a circular chromosome,which is not organized into the complex linear strands found in eukaryotes.
Therefore,both plant and animal cells contain $DNA$ organized as large,linear strands within the nucleus.

(C) Chromatin is the complex of $DNA$ and proteins found in the eukaryotic nucleus.
Under an electron microscope,chromatin appears as 'beads-on-string' structures.
The 'beads' are known as nucleosomes.
Nucleosomes are the repeating structural units of chromatin,formed by the wrapping of $DNA$ around a core of eight histone proteins (histone octamer).
Therefore,nucleosomes are the fundamental repeating units of chromatin.

(B) The 'beads-on-a-string' structure refers to the appearance of chromatin under an electron microscope.
This structure is formed by the wrapping of $DNA$ around histone octamers to form nucleosomes.
These nucleosomes are connected by linker $DNA$,giving the appearance of beads on a string.
This organization is characteristic of the chromatin found within the nucleus of eukaryotic cells.

(B) In the nucleus,chromatin is organized into two types based on its staining intensity during interphase:
$1$. Euchromatin: This is the loosely packed and lightly stained region of chromatin. It is transcriptionally active and contains genes that are expressed.
$2$. Heterochromatin: This is the densely packed and darkly stained region of chromatin. It is transcriptionally inactive and remains tightly coiled.
Therefore,the loosely stained region is known as Euchromatin.

(D) In the nucleus,chromatin is organized into two types based on its density and staining properties:
$1$. Euchromatin: This is loosely packed chromatin that stains lightly and is transcriptionally active.
$2$. Heterochromatin: This is densely packed chromatin that stains darkly and is transcriptionally inactive.
Therefore,the chromatin that is densely packed and stains darkly is known as Heterochromatin.

(A) In the nucleus,chromatin is organized into two types based on its staining intensity and transcriptional activity.
$1$. $Euchromatin$: This is the loosely packed region of chromatin that stains lightly. It is transcriptionally active because the $DNA$ is accessible to $RNA$ polymerase and other transcription factors.
$2$. $Heterochromatin$: This is the densely packed region of chromatin that stains darkly. It is transcriptionally inactive because the $DNA$ is tightly coiled and inaccessible to the transcriptional machinery.
Therefore,$Euchromatin$ is the structure active for transcription.

(A) The $K.I.$ (Karyoplasmic Index) is defined as the ratio of the volume of the nucleus to the volume of the cytoplasm $(K.I. = V_n / V_c)$.
In cancerous cells,the nucleus becomes enlarged and hyperchromatic,leading to an increase in the nuclear volume relative to the cytoplasm.
Therefore,cancerous cells exhibit a high $K.I.$ compared to normal cells.

(A) The biosynthesis of ribosomal $RNA$ $(rRNA)$ occurs in the nucleolus. The nucleolus is a non-membrane-bound structure present within the nucleus,which serves as the primary site for $rRNA$ transcription and the assembly of ribosomal subunits.

(A) The Assertion is true because the nucleus in plasma cells is eccentric,rounded,and exhibits a characteristic 'cart-wheel' appearance.
The Reason is also true because this 'cart-wheel' appearance is specifically caused by the arrangement of heterochromatin in peripheral clumps along the nuclear membrane.
Therefore,both Assertion and Reason are true,and the Reason is the correct explanation of the Assertion.

(A) Based on the position of the centromere,chromosomes are classified as follows:
$A$: Telocentric chromosome (centromere at the terminal end).
$B$: Acrocentric chromosome (centromere close to the end,forming one very short and one very long arm).
$C$: Submetacentric chromosome (centromere slightly away from the middle,forming unequal arms).
$D$: Metacentric chromosome (centromere in the middle,forming two equal arms).
Therefore,the correct sequence is $A-$ Telocentric,$B-$ Acrocentric,$C-$ Submetacentric,$D-$ Metacentric.

(B) The figure shows the structure of a metacentric chromosome $(C)$.
$A$ represents the satellite,which is a small fragment of the chromosome separated by a secondary constriction.
$B$ represents the secondary constriction,which is a constant location on the chromosome.
Metacentric chromosomes have the centromere in the middle,resulting in two equal arms.
Human chromosomes $1$ and $3$ are examples of metacentric chromosomes.

(C) The cellular structure $Y$ (nucleus) helps in the transfer of genetic material from one generation to another.
Nucleus is a dense organelle present in most eukaryotic cells.
It is typically a single rounded structure bounded by a double membrane,containing the genetic material $(DNA)$ which acts as the blueprint for inheritance.

(B) Nucleolus is a small rounded body within a resting nucleus that contains $RNA$ and proteins.
It is usually associated with a specific chromosomal site and is involved in ribosomal $RNA$ $(rRNA)$ synthesis and the formation of ribosomes.

(A) satellite is a small chromosomal segment that is separated from the main body of the chromosome by a secondary constriction.
In humans,it is usually associated with the short arm of an acrocentric chromosome.
These segments play a vital role in the formation of the nucleolus after cell division.
In humans,chromosomes number $13, 14, 15, 21$,and $22$ are examples of $SAT$ (satellite) chromosomes.