M Phase Questions in English

(B) The nuclear spindle (mitotic spindle) is primarily composed of two types of microtubules:
$1$. Kinetochore microtubules: These attach to the kinetochores of the chromosomes.
$2$. Polar (or non-kinetochore) microtubules: These extend from the poles towards the equator and overlap with each other to maintain the shape of the spindle.
Therefore,the spindle consists of two types of fibres.

(B) Chromosomes that attach to spindle fibers are designated as spindle chromosomes.
The specific site on the centromere where the spindle fibers attach is known as the kinetochore.
Therefore,spindle chromosomes possess a kinetochore for the attachment of spindle fibers during cell division.

(B) The kinetochore is a complex protein structure that forms on the centromere of a chromosome during cell division.
It serves as the attachment site for spindle fibers (microtubules) to pull sister chromatids apart during mitosis and meiosis.
Therefore,the kinetochore is associated with chromosomes.

(C) During cell division,spindle fibres attach to the disc-shaped structures present on the sides of the centromere known as kinetochores.
These structures are essential for the movement of chromosomes towards the poles during anaphase.

(C) The correct answer is $(c)$. During cell division,spindle fibres attach to the kinetochore located on the centromere of the chromosomes.
As the spindle fibres shorten,they pull the chromosomes towards the opposite poles.
This movement results in the chromosomes appearing $'V'$,$'L'$,$'J'$,or $'I'$ shaped depending on the position of the centromere.

(A) Colchicine is a chemical substance derived from the plant $Colchicum \text{ } autumnale$.
It acts as a mitotic poison by binding to tubulin proteins, thereby inhibiting the polymerization of microtubules.
Since microtubules are essential for the formation of spindle fibers during cell division, colchicine prevents the assembly of the spindle apparatus.
This effect is specifically observed during $Metaphase$, as the chromosomes fail to align or separate properly due to the absence of spindle fibers.

(B) The formation of the spindle apparatus begins during the $Prophase$ stage of mitosis. During this phase,the centrosomes,which were duplicated during the $S$ phase of interphase,begin to move towards opposite poles of the cell. As they move,they initiate the assembly of the mitotic spindle,which is composed of microtubules.

(D) $Cyclin$ is a regulatory protein that controls the progression of the cell cycle by activating cyclin-dependent kinases $(CDKs)$.
It plays a crucial role in the regulation of $Mitosis$ (cell division) by ensuring that the cell proceeds through the various checkpoints of the cell cycle correctly.

(C) During cleavage,the zygote undergoes rapid mitotic cell divisions. These divisions are characterized by the absence of the $G_1$ and $G_2$ phases of the cell cycle,meaning the cells do not grow in size between divisions. This specific type of rapid mitotic division,where the cell cycle is restricted primarily to $S$ and $M$ phases,is referred to as closed mitosis.

(B) In meiosis,the cell undergoes two successive divisions.
After the completion of meiosis $I$,two haploid daughter cells are formed.
In the subsequent phase,meiosis $II$,these two cells enter prophase $II$ followed by metaphase $II$.
During metaphase $II$,the chromosomes (each consisting of two sister chromatids) align at the spindle equator in both cells.
Therefore,the correct phase is metaphase $II$.

(B) The correct answer is $B$ (Metaphase).
During the $Metaphase$ stage of mitosis,the chromosomes are highly condensed and aligned at the equatorial plate (metaphase plate).
This maximum condensation makes them clearly visible under a microscope,allowing for the accurate counting and study of their morphology,such as the position of the centromere.

(B) During $Metaphase$, the chromosomes align themselves at the equator of the spindle apparatus, which is known as the metaphase plate. This alignment is a characteristic feature of $Metaphase$ in mitosis, ensuring that each daughter cell receives an identical set of chromosomes.

(A) During the $S$-phase of the cell cycle,$DNA$ replication occurs,resulting in each chromosome consisting of two sister chromatids joined at the centromere. Each chromatid contains one $DNA$ molecule. During anaphase,the centromere splits,and the sister chromatids separate. Each separated chromatid is now considered an independent chromosome. Therefore,an anaphase chromosome (which is a single chromatid) contains exactly $1$ $DNA$ molecule.

(C) During mitosis,the $Anaphase$ stage is characterized by the splitting of the centromere of each chromosome.
As the centromere splits,the two sister chromatids,which were previously held together,separate and begin to move towards opposite poles of the cell.
Therefore,the separation of chromosomes (chromatids) begins at the onset of $Anaphase$.

(C) During mitosis,the movement of chromosomes towards the opposite poles is facilitated by the spindle apparatus.
Spindle fibres are composed of microtubules that attach to the kinetochores of the chromosomes.
These fibres contract or shorten,thereby pulling the sister chromatids (chromosomes) towards the respective poles of the cell.

(B) During the $Prophase$ stage of mitosis,the chromatin condenses into compact mitotic chromosomes. Simultaneously,the nucleolus,nuclear envelope,and other organelles like the endoplasmic reticulum and Golgi complex begin to disappear or disintegrate to facilitate the movement of chromosomes.

(D) Mitosis is the process of cell division that results in two genetically identical daughter cells.
It is divided into four main stages:
$1$. Prophase: Chromatin condenses into chromosomes,and the nuclear envelope begins to break down.
$2$. Metaphase: Chromosomes align at the equatorial plate of the cell.
$3$. Anaphase: Sister chromatids separate and move towards opposite poles.
$4$. Telophase: Chromosomes arrive at the poles,and the nuclear envelope reforms.
Therefore,the correct sequence is Prophase $\rightarrow$ Metaphase $\rightarrow$ Anaphase $\rightarrow$ Telophase.

(A) During the $Metaphase$ stage of mitosis,the chromosomes align themselves along the equatorial plane of the cell,which is also known as the metaphase plate. This alignment is facilitated by the spindle fibers attached to the kinetochores of the chromosomes. Therefore,the correct stage is $Metaphase$.

(A) The cell cycle consists of two main phases: Interphase and $M$-phase (Mitosis).
Interphase is the preparatory phase that lasts for the majority of the cell cycle duration.
In contrast,the $M$-phase,which involves the actual division of the cell,is relatively short.
For most eukaryotic cells,the duration of the active mitotic phase ($M$-phase) ranges from approximately $10$ minutes to a few hours,depending on the cell type and organism.

(C) The mitotic index is defined as the ratio of the number of cells undergoing mitosis (dividing cells) to the total number of cells in a given population.
It is a measure of the proliferative activity of a cell population.
Mathematically, it is expressed as: $\text{Mitotic Index} = \frac{\text{Number of cells in mitosis}}{\text{Total number of cells}} \times 100$.

(D) The $M$ phase (Mitotic phase) is the shortest phase of the cell cycle.
It typically occupies only about $1.5 - 10\%$ of the total duration of the cell cycle,while the interphase $(G_1, S, G_2)$ occupies the majority of the time.

(C) Centrioles are cylindrical organelles that play a crucial role in the formation of the spindle apparatus during nuclear division in animal cells.
In higher plants,cell division occurs without centrioles. Instead,they form spindle fibers using microtubule organizing centers (MTOCs) located in the cytoplasm.
Therefore,centrioles are absent in the mitotic cells of higher plants.

(B) During the process of mitosis,the $M$ phase consists of four stages: Prophase,Metaphase,Anaphase,and Telophase.
In Prophase,the nuclear envelope and nucleolus disappear.
In Telophase,which is the final stage of karyokinesis,the chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements.
The nuclear envelope assembles around the chromosome clusters at each pole,and the nucleolus,Golgi complex,and endoplasmic reticulum reform,marking the reappearance of the nuclear membrane.

(C) During $Anaphase$, the centromere of each chromosome splits, and the chromatids (now referred to as daughter chromosomes) separate. These daughter chromosomes are then pulled towards opposite poles of the cell by the shortening of spindle fibers attached to the kinetochores. Therefore, the movement of chromosomes to the poles is the characteristic feature of $Anaphase$.

(C) During the prophase stage of mitosis,the nucleolus,along with the nuclear envelope and the endoplasmic reticulum,disappears.
This process is essential to allow the condensation of chromatin into compact chromosomes and to facilitate the movement of chromosomes during cell division.
While the nuclear envelope and nucleolus break down,the cell wall and cell membrane remain intact throughout the process of mitosis.

(C) In the process of cell division,the division of the nucleus is known as $Karyokinesis$.
$Cytokinesis$ refers to the division of the cytoplasm,which results in the separation of the cell into two daughter cells.
Therefore,$Karyokinesis$ specifically involves the division of the nucleus.

(C) During the process of mitosis, the $Prophase$ stage is divided into early and late phases.
In $Early \text{ prophase}$, the chromatin condenses into chromosomes.
In $Late \text{ prophase}$ (also known as prometaphase), the nuclear envelope (nuclear membrane) and the nucleolus disappear completely to allow the spindle fibers to attach to the kinetochores of the chromosomes.
Therefore, the nuclear membrane disappears in $Late \text{ prophase}$.

(A) During the metaphase stage of cell division,all chromosomes align at the equatorial plate (metaphase plate).
Chromosomes at this stage are highly condensed to facilitate their movement and segregation.
An acrocentric chromosome,like all other chromosomes,will be in its condensed form and positioned at the equator of the cell,attached to the spindle fibers via its kinetochore.

(C) Colchicine is an alkaloid that acts as a mitotic poison.
It binds to the protein tubulin,thereby preventing the polymerization of microtubules.
Since spindle fibres are composed of microtubules,colchicine inhibits the formation of the spindle apparatus during cell division.
As a result,the chromosomes fail to separate during anaphase,leading to a state where the cell contains double the number of chromosomes (polyploidy) without undergoing cytokinesis.

(A) Spindle fibres are composed of microtubules,which are structural components of the cytoskeleton.
These microtubules are primarily made up of a protein called tubulin.
Therefore,spindle fibres are proteinaceous in nature.

(B) Mitosis is classified based on the presence or absence of asters.
In animal cells,centrioles form asters (star-shaped structures) around the spindle poles,which is known as astral mitosis.
In higher plants,centrioles are absent,and therefore,no asters are formed during spindle formation. This type of division is known as anastral mitosis.

(A) During $Anaphase$,the centromere of each chromosome splits,and the chromatids separate. These chromatids,now referred to as chromosomes,move toward opposite poles. Additionally,the spindle poles move farther apart. The reformation of the $Golgi$ body and $Endoplasmic$ $Reticulum$ $(ER)$ occurs during $Telophase$,not $Anaphase$. Therefore,option $A$ is not true for $Anaphase$.

(B) The $G_2$ phase (Gap $2$ phase) is the period after $DNA$ replication ($S-$ phase) and before the onset of mitosis ($M$ phase).
During this phase,the cell continues to grow and synthesizes essential proteins,such as tubulin,which are required for the formation of spindle fibres during mitosis.
Therefore,the correct phase for the synthesis of proteins for spindle fibre formation is the $G_2$ phase.

(A) During cell division,spindle fibers attach to the $kinetochore$,which is a disc-shaped structure present on the surface of the centromeres of chromosomes. This attachment allows the chromosomes to move and align at the equatorial plate during metaphase.

(A) Colchicine is a chemical agent that inhibits the formation of the spindle apparatus during cell division (mitosis).
Because the spindle fibers do not form,the sister chromatids fail to separate during anaphase.
However,the replication of $DNA$ and the duplication of chromosomes continue normally.
As a result,the cell enters the next interphase with double the number of chromosomes,turning a $2n$ (diploid) cell into a $4n$ (tetraploid) cell.

(C) In the $S$ phase of the cell cycle,$DNA$ replication occurs,resulting in each chromosome consisting of two sister chromatids.
During metaphase,these chromosomes align at the equatorial plate.
At the onset of anaphase,the centromere of each chromosome splits,and the sister chromatids separate.
Once separated,each chromatid is considered an independent chromosome.
Therefore,at the anaphase stage,each chromosome consists of only one chromatid.

(C) The centromere is the region of the chromosome that acts as the attachment site for spindle fibres during cell division.
It contains a protein complex called the kinetochore,which binds to the microtubules of the spindle fibres.
This attachment is essential for the proper alignment and subsequent movement of chromosomes towards the opposite poles during anaphase.
Therefore,the centromere is directly involved in the movement of chromosomes.

(D) During the $M$ phase (Mitosis),specifically in the metaphase stage,the condensation of chromosomes reaches its maximum level.
This high degree of condensation makes the chromosomes clearly visible under a light microscope.
Additionally,the centromeres become distinct and are easily observable during this phase,which is essential for the proper alignment and segregation of chromosomes.

(D) The $centromere$ is a specialized $DNA$ sequence of a chromosome that links a pair of sister chromatids. During cell division, the $kinetochore$ (a protein complex) assembles on the $centromere$. Spindle fibers attach to the $kinetochore$, which is essential for the movement of chromosomes towards the opposite poles during $anaphase$ of mitosis and meiosis.

(B) During mitosis,the centromere of each chromosome splits,allowing the two sister chromatids to separate.
These separated chromatids are then pulled towards opposite poles of the cell.
This movement is known as anaphasic movement,which occurs specifically during the Anaphase stage.

(B) Colchicine is an alkaloid that acts as a mitotic poison.
It inhibits the polymerization of tubulin proteins,which prevents the formation of spindle fibres during cell division.
As a result,the chromosomes fail to separate during anaphase,leading to polyploidy,which is effectively the duplication of chromosomes within the cell.

(A) Colchicine is an alkaloid obtained from $Colchicum$ $autumnale$ (family $Liliaceae$).
It acts as a spindle poison by inhibiting the formation of spindle fibers during cell division.
This prevents the separation of sister chromatids,leading to the failure of cytokinesis and resulting in polyploidy,which is an increase in the chromosome number.

(D) The alkaloid extracted from $Colchicum$ $autumnale$ is known as $Colchicine$.
$Colchicine$ is a mitotic poison that inhibits the formation of spindle fibers during cell division.
By preventing the separation of chromosomes during anaphase, it leads to the doubling of the chromosome number in a cell, a condition known as $Polyploidy$.
Therefore, $Colchicine$ is widely used in plant breeding to induce $Polyploidy$ in plants.

(B) Colchicine is an alkaloid that inhibits the formation of spindle fibers during cell division (mitosis).
By preventing the separation of sister chromatids during anaphase,it leads to the failure of cytokinesis.
As a result,a diploid cell $(2n)$ undergoes $DNA$ replication but fails to divide,doubling its chromosome number to become tetraploid $(4n)$.

(B) During the $G_2$ phase of the cell cycle,the cell contains replicated $DNA$.
Since $DNA$ replication occurs during the $S$ phase,the sister chromatids are present in the $G_2$ phase.
These sister chromatids serve as templates for the repair of $DNA$ damage (such as double-strand breaks) via homologous recombination,ensuring that the genetic information is accurately passed to the daughter cells.

(B) Spindle fibers are essential structures formed during cell division (mitosis and meiosis) that help in the movement and segregation of chromosomes.
These fibers are composed of microtubules, which are cylindrical structures made of the protein $\text{Tubulin}$.
Specifically, $\text{Tubulin}$ dimers ($\alpha$ and $\beta$ subunits) polymerize to form the microtubule filaments that constitute the spindle apparatus.

(A) The cell cycle is divided into two main phases: Interphase and $M$ phase (Mitosis).
Interphase is further divided into three sub-phases: $G_1$ phase,$S$ phase,and $G_2$ phase.
$1$. $G_1$ phase (Gap $1$) is the interval between mitosis and $DNA$ replication.
$2$. $S$ phase (Synthesis) is the period during which $DNA$ replication takes place.
$3$. $G_2$ phase (Gap $2$) is the interval between the completion of $DNA$ replication ($S$ phase) and the beginning of mitosis ($M$ phase).
Therefore,the $G_2$ phase occurs between the $S$ phase and the $M$ phase.

(A) Mitosis is divided into four main stages: Prophase,Metaphase,Anaphase,and Telophase.
Prophase is the first and longest phase of mitosis.
During this phase,the chromatin condenses into visible chromosomes,the nuclear envelope begins to disintegrate,and the mitotic spindle starts to form.
Because these complex structural changes take significant time,Prophase lasts longer than the subsequent stages of mitosis.

(B) The $M$ phase (Mitosis phase) represents the phase where the actual cell division occurs.
It starts with the division of the nucleus,which is known as $Karyokinesis$.
$Karyokinesis$ is followed by the division of the cytoplasm,known as $Cytokinesis$.
Therefore,the $M$ phase begins with $Karyokinesis$.

(B) In mineralogy,a cluster of crystals that radiates from a central point,often resembling a star-like shape,is known as a $Druse$ or sometimes referred to as a star-shaped crystal aggregate. However,specifically,a $Druse$ refers to a surface covered in small crystals. In the context of biological or geological terminology,the term $Aster$ is used for star-shaped structures (like in cell division),but for mineral clusters,$Druse$ is the appropriate term.