Nucleus and Chromosomes Questions in English

(A) Centromeres consist of a complex combination of proteins and $DNA$. They are essential to the division and ensure the accurate segregation of chromosomes.
Metacentric chromosomes have the centromere in the centre,such that both sections are of equal length.
Human chromosome $1$ and $3$ are metacentric.

(A) Metacentric chromosomes have the centromere in the middle,dividing the chromosome into two equal arms.
Submetacentric chromosomes have the centromere slightly away from the middle,resulting in one shorter arm and one longer arm.
Acrocentric chromosomes have the centromere situated almost near the tip,forming one extremely short arm and one very long arm.
Telocentric chromosomes have the centromere located at the terminal tip of the chromosome.
Therefore,the correct matching is: $A-III, B-IV, C-II, D-I$.

(C) The nucleus was discovered by Robert Brown in $1831$. He observed it in the cells of orchid roots.

(C) The $Nucleolus$ is a spherical structure present inside the nucleus that is not membrane-bound. It is the primary site for the synthesis of ribosomal $RNA$ $(rRNA)$ and the assembly of ribosomal subunits.
$Mesosomes$ $\rightarrow$ Infoldings of the plasma membrane in prokaryotes involved in $DNA$ replication,respiration,and cell wall formation.
$Nucleoplasm$ $\rightarrow$ The dense,protein-rich fluid (matrix) enclosed within the nuclear envelope.
$DNA$ $\rightarrow$ The hereditary genetic material found within the nucleus.

(A) The term 'chromatin' was coined by Walther Flemming,who discovered and described the thread-like structures within the nucleus that take up basic dyes.
$A$. Flemming: Discovered chromatin.
$B$. Schleiden: Proposed the plant cell theory.
$C$. Schwann: Proposed the animal cell theory.
$D$. Robert Brown: Discovered the nucleus.

(B) Chromatin is the nucleoprotein fiber found in the nucleus of eukaryotic cells during interphase. It is essentially composed of $DNA$ and basic proteins called histones,along with some non-histone chromosomal proteins and $RNA$.

(D) The chromatids of a chromosome are held together at a primary constriction called the centromere.
Centrosome $\rightarrow$ An organelle that serves as the main microtubule organizing center $(MTOC)$ of the animal cell.
Centriole $\rightarrow$ $A$ cylindrical structure composed mainly of the protein tubulin,found in most eukaryotic cells.
Satellite $\rightarrow$ $A$ small fragment of a chromosome separated from the main body by a secondary constriction.

(C) Chromosomes are classified based on the position of the centromere:
$1$. Metacentric: The centromere is located in the middle of the chromosome,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 one end,resulting in one extremely short arm and one very long arm.
$4$. Telocentric: The centromere is located at the terminal end of the chromosome.
Therefore,chromosomes with the centromere at the terminal end are called Telocentric.

(D) chromosome typically possesses a primary constriction,also known as the centromere,which is essential for the attachment of spindle fibers during cell division.
In addition to the primary constriction,some chromosomes exhibit a secondary constriction at a constant location.
This secondary constriction is often referred to as the nucleolar organizer region $(NOR)$.
The small fragment of the chromosome beyond this secondary constriction is called a satellite.
Therefore,satellite chromosomes possess both a primary constriction (centromere) and a secondary constriction.

(C) karyotype is the complete set of chromosomes in an organism. $A$ diagrammatic representation or a photograph of these chromosomes,arranged in a standard sequence (usually by size and centromere position),is known as an Idiogram.

(B) The nucleolus is a non-membrane-bound structure present within the nucleus. It is not separated from the nucleoplasm by any membrane or envelope. Therefore,the statement that it is separated from the nucleoplasm by a nuclear envelope is incorrect.

(A) Based on the structure of chromosomes:
$A$ points to the terminal,small,rounded fragment known as the Satellite.
$B$ points to the Secondary constriction (which is often associated with the Nucleolar Organizer Region,$NOR$).
$C$ points to the shorter arm of the chromosome (p-arm).
$D$ points to the longer arm of the chromosome (q-arm).
Therefore,the correct labeling is $A$ - Satellite,$B$ - Secondary constriction,$C$ - Short arm,$D$ - Long arm.

(B) The Nucleolus is a non-membrane-bound structure present within the nucleus.
It is formed at specific sites on certain chromosomes known as the Nucleolar Organiser Regions $(NOR)$.
These regions contain the genes for ribosomal $RNA$ $(rRNA)$ synthesis,which are essential for the assembly of ribosomes.
Therefore,the correct answer is the Nucleolar organiser.

(C) Lampbrush chromosomes are large chromosomes found in the oocytes of many vertebrates and invertebrates. They possess lateral loops that are sites of intense $RNA$ synthesis,making them transcriptionally active.
Informosomes are complexes of $mRNA$ and proteins found in the cytoplasm of eukaryotic cells,particularly in oocytes. They serve as a storage form of $mRNA$ that is translated later during development. However,the statement that they can be used in the future for embryo development is scientifically vague and not a standard biological fact in this context. Therefore,the Reason is considered incorrect or irrelevant to the Assertion.

(D) The Assertion is incorrect because a telocentric chromosome has only one arm,not two unequal arms. $A$ chromosome with two unequal arms is called a submetacentric chromosome.
The Reason is also incorrect because in a telocentric chromosome,the centromere is situated at the terminal end,not close to the end. $A$ chromosome with the centromere close to the end is called an acrocentric chromosome.
Therefore,both the Assertion and the Reason are incorrect.

(C) The centromere is the region where the two sister chromatids of a chromosome are joined together. This region appears as a narrowed or constricted area under a microscope,which is known as the primary constriction.

(C) The eukaryotic $DNA$ is organized in the form of chromosomes.
Chemically,a chromosome consists of nucleic acids ($DNA$ and $RNA$),histone proteins,and non-histone proteins.

(C) Chromosomes are composed of chromatin,which consists of $DNA$ and basic proteins called histones.
$DNA$ molecules wrap around histone octamers to form nucleosomes,which further condense to form the structure of a chromosome.

(A) In acrocentric chromosomes,the centromere is situated close to its end,resulting in one very short arm and one very long arm.

(B) The shape of a chromosome is primarily determined by the position of the centromere (also known as the primary constriction). Based on the position of the centromere,chromosomes can be classified as metacentric,submetacentric,acrocentric,or telocentric.

(C) Polytene chromosomes radiate as $5$ long and $1$ short arm from a deeply staining and more or less amorphous structure called the chromocentre.
This chromocentre is formed by the fusion of the centromeric regions of all the chromosomes and,in males,the entire $Y$-chromosome.

(C) chromosome is classified based on the position of the centromere:
$1$. Metacentric: The centromere is in the middle,resulting in two equal arms.
$2$. Submetacentric: The centromere is slightly away from the middle,resulting in one shorter and one longer arm.
$3$. Acrocentric: The centromere is situated close to the end,forming one extremely short and one very long arm.
$4$. Telocentric: The centromere is at the terminal end of the chromosome.
Therefore,when the centromere is in the middle of two equal arms,it is called Metacentric.

(A) Nuclear pores are minute openings in the nuclear envelope formed by the fusion of its two membranes.
These pores act as channels that facilitate the bidirectional movement of $RNA$ and protein molecules between the nucleus and the cytoplasm.

(B) In a metacentric chromosome,the centromere is located exactly in the middle of the chromosome.
This positioning divides the chromosome into two equal arms of the same length.
During anaphase,such chromosomes typically appear $V$-shaped.

(B) Sometimes,a few chromosomes possess non-staining secondary constrictions at a constant location.
This gives the appearance of a small fragment called a satellite.
Chromosomes $13, 14, 15, 21,$ and $22$ are known as satellite chromosomes ($SAT$-chromosomes).

(D) The classification of chromosomes is based on the position of the centromere:
$1$. Metacentric chromosome: The centromere is in the middle,resulting in two equal arms. This matches $(III)$.
$2$. Acrocentric chromosome: The centromere is situated close to the end,forming one extremely short and one very long arm. This matches $(I)$.
$3$. Sub-metacentric chromosome: The centromere is slightly away from the middle,forming one shorter arm and one longer arm. This matches $(IV)$.
$4$. Telocentric chromosome: The centromere is at the terminal end. This matches $(II)$.
Therefore,the correct matching is $(a)-(iii), (b)-(i), (c)-(iv), (d)-(ii)$.

(B) The nucleus was first described by the Scottish botanist $Robert \ Brown$ in $1831$. He observed it while studying the cells of orchid roots. Robert Hooke discovered the cell,while Anton van Leeuwenhoek was the first to observe living cells.

(D) Chromatin consists of $DNA$ and basic proteins called histones.
Because $DNA$ is acidic in nature due to the presence of phosphate groups,chromatin is acidic.
To stain acidic components,a basic stain (such as acetocarmine or basic fuchsin) is used.
Therefore,$P$ is acidic and $Q$ is basic.

(C) The term 'chromatin' was coined and the structure was described by the German biologist $Walther \ Flemming$ in $1880$. He observed the thread-like structures within the nucleus that stained intensely with basic dyes during cell division. Therefore,the correct option is $C$.

(A) The nuclear envelope consists of two parallel membranes with a space between them called the perinuclear space. This space acts as a barrier between the materials present inside the nucleus and those of the cytoplasm. The width of this perinuclear space is typically $10$ to $55 \ nm$.

(C) The image shows a eukaryotic nucleus,which contains a nuclear envelope,nucleolus,and nucleoplasm.
Eukaryotic cells (Fungi,Animals,and Plants) possess a well-defined nucleus with a nuclear membrane.
Bacteria are prokaryotic organisms and lack a membrane-bound nucleus.
Therefore,the structure shown is not found in bacteria.

(D) The nucleolus is the site of ribosomal $RNA$ $(rRNA)$ synthesis and ribosome assembly. Since proteins are synthesized by ribosomes,cells that require high rates of protein synthesis (such as secretory cells or rapidly growing cells) need a large number of ribosomes. To meet this demand,these cells often exhibit a prominent,large nucleolus or multiple nucleoli to facilitate the rapid production of ribosomal subunits. Therefore,both $A$ and $B$ are correct characteristics of such cells.

(A) The nucleolus is a spherical structure present inside the nucleus of eukaryotic cells.
It is not bounded by any membrane.
It is the primary site for the active synthesis of ribosomal $RNA$ $(rRNA)$.
These $rRNA$ molecules are essential for the assembly of ribosomes,which are the protein-synthesizing machinery of the cell.

(B) The primary constriction of a chromosome is known as the centromere.
On the sides of the centromere,disc-shaped structures are present,which are called kinetochores.
These kinetochores are proteinaceous structures that serve as the attachment sites for spindle fibers during cell division.

(D) The classification of chromosomes is based on the position of the centromere:
$1$. Metacentric $(P)$: The centromere is in the middle,forming two equal arms $(IV)$.
$2$. Sub-metacentric $(Q)$: The centromere is slightly away from the middle,resulting in one shorter and one longer arm $(I)$.
$3$. Acrocentric $(R)$: The centromere is situated close to the end,forming one extremely short and one extremely long arm $(II)$.
$4$. Telocentric $(S)$: The centromere is at the terminal end of the chromosome $(III)$.
Therefore,the correct matching is $P-IV, Q-I, R-II, S-III$.

(B) Some chromosomes have non-staining secondary constrictions at a constant location. This gives the appearance of a small fragment called the $Satellite$. These chromosomes are known as $SAT$-chromosomes.

(B) The image shows a 'beads-on-a-string' structure,which is characteristic of chromatin when viewed under an electron microscope. This structure represents the nucleosomes,which are the repeating units of chromatin,consisting of $DNA$ wrapped around a histone octamer. While the entire structure is chromatin,the specific repeating unit shown is the nucleosome.

(C) The 'beads-on-string' structure is a characteristic appearance of chromatin under an electron microscope.
In this structure,the $DNA$ molecule is wrapped around a core of histone proteins to form units called nucleosomes.
These nucleosomes are connected by linker $DNA$,giving the appearance of beads on a string.
Therefore,the chromatin fiber is the structure that exhibits this specific morphology.

(D) Chromatin is the complex of $DNA$ and proteins found in the eukaryotic cell nucleus.
It consists of $DNA$,basic histone proteins,and non-histone chromosomal proteins.
Histones are positively charged proteins that help in the packaging of negatively charged $DNA$ into nucleosomes.
Non-histone proteins are involved in various functions such as $DNA$ replication,transcription,and chromatin remodeling.
Therefore,chromatin contains all of the mentioned components.

(B) The nuclear envelope is a double-membrane structure that encloses the nucleus in eukaryotic cells.
It consists of two parallel membranes,the inner and outer nuclear membranes.
The space between these two membranes is known as the perinuclear space (or perinuclear cisterna).
The width of this perinuclear space typically ranges from $10$ to $50 \text{ nm}$,which is equivalent to $100$ to $500 \mathring A$ $(1 \text{ nm} = 10 \mathring A)$.
Therefore,the correct option is $B$.

(D) Chromatin is the complex of $DNA$ and proteins that makes up chromosomes in eukaryotic cells.
It primarily consists of $DNA$ wrapped around histone proteins to form nucleosomes.
In addition to histones,chromatin also contains non-histone chromosomal proteins,which are involved in the packaging and regulation of gene expression.
$RNA$ is also associated with chromatin as it is synthesized during transcription.
Therefore,chromatin is composed of $DNA, RNA$,histone proteins,and non-histone proteins.

(B) Chromosomes are classified based on the position of the centromere:
$1$. $Metacentric$: 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 an extremely short arm and a very long arm.
$4$. $Telocentric$: The centromere is at the terminal end of the chromosome.
Therefore,when the centromere is slightly away from the middle,it is a $Sub-metacentric$ chromosome.

(C) The nucleolus is a dense region found inside the nucleus that is not surrounded by a membrane.
Its primary function is the synthesis of ribosomal $RNA$ $(rRNA)$ and the assembly of ribosomes.

(B) chromosome typically has a primary constriction called the centromere.
Some chromosomes possess an additional non-staining secondary constriction at a constant location.
The small fragment of the chromosome located beyond this secondary constriction is known as the $Satellite$ (or $SAT$-chromosome).
This region is often associated with the nucleolar organizer region $(NOR)$.

(D) Every chromosome essentially possesses a primary constriction,which is also known as the centromere.
This region is essential for the attachment of spindle fibers during cell division.
While some chromosomes may also have a secondary constriction,the primary constriction (centromere) is a universal feature of all chromosomes.
Therefore,both options $(2)$ and $(3)$ refer to the same structure.

(C) The classification of chromosomes based on the position of the centromere is as follows:
$1$. Metacentric: The centromere is in the middle,forming two equal arms. This corresponds to statement $(i)$.
$2$. Submetacentric: The centromere is slightly away from the middle,resulting in one shorter arm and one longer arm. This corresponds to statement $(iv)$.
$3$. Acrocentric: The centromere is close to the end,forming one extremely short and one very long arm. This corresponds to statement $(iii)$.
$4$. Telocentric: The centromere is at the terminal position. This corresponds to statement $(ii)$.
Therefore,the correct matching is: Metacentric $\rightarrow(i)$,submetacentric $\rightarrow(iv)$,Acrocentric $\rightarrow(iii)$,Telocentric $\rightarrow(ii)$.

(C) The $Feulgen$ reaction is a staining technique discovered by $Robert$ $Feulgen$ in $1924$. It is used to identify chromosomal material or $DNA$ in cell specimens. The reaction specifically stains the deoxyribose sugar of $DNA$ in the $chromatin$ of the cell nucleus. Therefore,$Feulgen$ demonstrated that $DNA$ is present in the $chromatin$ of the nucleus.

(C) The correct matching is as follows:
$1$. Centromere $(c)$: It is the primary constriction where two sister chromatids are held together.
$2$. Secondary constriction $(d)$: It is a region other than the primary constriction where the nucleolus is organized.
$3$. Kinetochore $(a)$: These are disc-shaped structures present on the sides of the centromere where spindle fibres attach during cell division.
$4$. Chromatonema $(b)$: It refers to the long,unbranched,slender,and highly coiled $DNA$ thread within the chromosome.
Thus,the correct sequence is $1-c, 2-d, 3-a, 4-b$.

(D) Based on the position of the primary constriction,also known as the centromere,chromosomes are classified into $4$ types:
$1$. Metacentric: Centromere is in the middle.
$2$. Sub-metacentric: Centromere is slightly away from the middle.
$3$. Acrocentric: Centromere is close to one end.
$4$. Telocentric: Centromere is at the terminal end.

(D) The $centromere$ is a specialized $DNA$ sequence of a chromosome that links a pair of sister chromatids.
During cell division, it appears as a primary constriction on the chromosome where the kinetochore is assembled.
It is essential for the proper segregation of chromosomes during mitosis and meiosis.