M Phase Questions in English

(D) In mitosis,prophase is the longest phase of karyokinesis.
In early prophase,the nuclear membrane and nucleolus start disintegrating.
Cell cytoskeleton,Golgi complex,$ER$,and other organelles also begin to disappear during this stage.

(B) Cell division involves two distinct processes: $1$. Karyokinesis,which is the division of the nucleus,and $2$. Cytokinesis,which is the division of the cytoplasm. Karyokinesis typically precedes cytokinesis to ensure that each daughter cell receives a complete set of genetic material.

(C) Mitosis is the process of somatic cell division,which is divided into four main stages based on the behavior of chromosomes and the nuclear envelope.
The four stages are:
$1.$ Prophase
$2.$ Metaphase
$3.$ Anaphase
$4.$ Telophase
Therefore,the correct answer is four stages.

(A) The anaphase promoting complex $(APC/C)$ is a protein complex that triggers the transition from metaphase to anaphase during the cell cycle.
It is activated by the $M-cdk-cyclin$ complex (also known as $Cyclin B-Cdk1$ or $MPF$ - Maturation Promoting Factor).
Once activated,$APC/C$ tags specific proteins like securin and cyclin $B$ for degradation,which allows the separation of sister chromatids.

(D) Telophase is the final stage of mitosis and is essentially the reverse of prophase.
During this phase,the following events occur:
$1$. Chromosomes arrive at opposite poles and lose their individuality as discrete elements,becoming chromatin material.
$2$. The nuclear membrane $(NM)$ reassembles around each chromosome cluster.
$3$. The nucleolus,Golgi bodies $(GB)$,and endoplasmic reticulum $(ER)$ reform.
Therefore,all the given statements are correct.

(D) During prophase,the following events occur:
$I.$ Chromosomal material condenses to form compact mitotic chromosomes,which is a hallmark of prophase.
$II.$ The assembly of the mitotic spindle is initiated by the microtubules,which are proteinaceous components of the cell cytoplasm.
$III.$ When observed under a microscope,cells at the late stage of prophase do not show cell organelles like the Golgi complex,endoplasmic reticulum,nucleolus,and nuclear envelope.
$IV.$ The nucleolus or nucleoli degenerate and disappear during prophase.
Since all the statements $(I, II, III, IV)$ are correct characteristics of prophase,the correct option is $D$.

(D) Colchicine is an antimitotic drug (alkaloid) obtained from $Colchicum$ (family $Liliaceae$).
It binds to tubulin molecules and prevents their polymerization into microtubules.
The depolymerization of tubulin leads to the disappearance of the mitotic spindle.
As a result,the cell is unable to proceed from metaphase to anaphase,effectively arresting the cell cycle at the metaphase stage.

(B) The correct answer is $CdK_{2} / Cyclin\; B$.
The cell cycle is regulated by enzymes known as cyclin-dependent kinases $(CdKs)$.
$CdKs$ phosphorylate specific amino acids,such as serine and threonine,which either initiate or inhibit cell cycle activities.
Key checkpoints in the cell cycle include:
$1$. The $G_{1}$ checkpoint (entry into $S$ phase) is controlled by $CdK_{4} / Cyclin\; D$ and $CdK_{6} / Cyclin\; D$.
$2$. The $G_{2}$ checkpoint (entry into $M$ phase,involving the Maturation Promoting Factor or $MPF$) is controlled by $CdK_{2} / Cyclin\; B$.
$3$. The Metaphase checkpoint (Spindle assembly checkpoint) is controlled by the degradation of $Cyclin\; B$.

(D) The drug colchicine is obtained from $Colchicum$ $autumnale$.
It inhibits the polymerization of tubulin protein into microtubules.
As a result, it prevents the formation of the spindle apparatus, effectively arresting the cell cycle at the metaphase stage.

(A) Chromosomes that result from the separation of sister chromatids during cell division are called daughter chromosomes.
During the $Anaphase$ stage of mitosis,the centromeres split,and the paired sister chromatids separate to form individual daughter chromosomes.
Each daughter chromosome then migrates,with the centromere leading,toward the opposite poles of the cell.
At the end of cell division,these chromosomes are partitioned into two distinct daughter cells.

(D) The $M$ phase (Mitotic phase) is considered the most dramatic period of the cell cycle because it involves the complex process of nuclear division (karyokinesis) and cytoplasmic division (cytokinesis).
During this phase,significant structural changes occur,including the condensation of chromatin into visible chromosomes,the alignment of chromosomes at the equatorial plate,and the separation of sister chromatids to opposite poles,which are clearly observable under a microscope.

(B) Mitosis is divided into four main stages: Prophase,Metaphase,Anaphase,and Telophase.
Interphase is the preparatory phase of the cell cycle.
The first stage of mitosis that occurs immediately after the completion of the $G_2$ phase of interphase is Prophase.
During Prophase,chromatin condenses into visible chromosomes and the nuclear envelope begins to disintegrate.

(A) During the $Prophase$ stage of the cell cycle,the chromatin material,which exists as a loose network of $DNA$ and proteins in the interphase nucleus,begins to undergo condensation. This process transforms the chromatin into compact,distinct,and thread-like structures known as chromosomes.

(C) During the late $Prophase$ stage of mitosis, the condensation of chromatin material is completed, and the mitotic spindle begins to organize from the centrosomes, which move towards opposite poles of the cell.

(D) During the process of mitosis,the $Prophase$ stage is characterized by the condensation of chromatin into compact mitotic chromosomes. As $Prophase$ progresses,the nucleolus and the nuclear envelope begin to disintegrate and eventually disappear by the end of this phase,allowing the spindle fibers to interact with the chromosomes.

(B) During the $M$ phase of the cell cycle,specifically in metaphase,chromosomes reach their maximum level of condensation.
This process of condensation,which begins in prophase,is completed by metaphase.
As a result,the chromosomes appear shortest and thickest during this stage,making them the most suitable for microscopic study of chromosome morphology.

(C) During $Metaphase$,the chromosomes condense and move to the center of the cell.
They align themselves along the equatorial plane of the cell,which is known as the $Metaphase$ plate or equatorial plate.

(C) Statement $(a)$ is correct: The complete disintegration of the nuclear envelope marks the transition from prophase to metaphase,which is the second phase of mitosis.
Statement $(b)$ is incorrect: $A$ metaphase chromosome consists of two sister chromatids held together at the centromere.

(A) During $Metaphase$,chromosomes reach their maximum level of condensation.
They become the shortest and thickest in this stage,which makes them clearly visible under a microscope.
Therefore,the morphology of chromosomes is best studied during $Metaphase$.

(B) The kinetochore is a complex,trilamellar proteinaceous structure located on the surface of the centromere. During cell division,spindle microtubules attach to the kinetochore to facilitate the movement of chromosomes toward the poles.

(D) During $Anaphase$, the centromere of each chromosome splits, and the chromatids (now referred to as chromosomes of the future daughter nuclei) separate.
These chromatids are pulled towards opposite poles by the shortening of spindle fibers attached to the kinetochores.
This poleward movement is characteristic of $Anaphase$ and is often referred to as anaphasic movement.

(A) During $Anaphase$,the centromere of each chromosome splits,allowing the sister chromatids to separate. These separated chromatids,now referred to as daughter chromosomes,are pulled towards opposite poles of the cell by the spindle fibers.

(A) During Telophase,the chromosomes that have reached their respective poles decondense and lose their individuality. The individual chromosomes can no longer be seen as discrete elements,and the chromatin material tends to collect in a mass of the chromatin network.

(D) During $Telophase$,the chromosomes reach their respective poles and decondense. Since the proper division and distribution of $DNA$ has been completed,the spindle apparatus is no longer required and therefore disappears.

(D) During $Telophase$,the chromosomes that reached their respective poles decondense and lose their individuality.
Individual chromosomes can no longer be seen,and chromatin material tends to collect in a mass of chromatin threads.
This process is the reverse of $Prophase$.

(B) Telophase is the final stage of mitosis. During this phase,the chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements. The nuclear envelope reassembles around the chromosome clusters at each pole,leading to the formation of two daughter nuclei.

(C) The Maturation Promoting Factor $(MPF)$ is a complex consisting of mitotic cyclin and $cdc2$ kinase. It acts as a key regulatory switch that triggers the transition of a cell from the $G_2$ phase into the $M$ (Mitosis) phase by phosphorylating various target proteins involved in nuclear envelope breakdown,chromosome condensation,and spindle assembly.

(B) In higher plants,the cell division is characterized by the absence of centrioles. Since centrioles are responsible for the formation of astral rays,the mitosis in plants is termed as anastral mitosis. In contrast,animal cells contain centrioles,which lead to the formation of astral rays (amphiastral mitosis).

(B) In mitosis,the $Prophase$ is the longest phase because it involves complex processes like chromatin condensation,nuclear envelope breakdown,and spindle formation.
$Anaphase$ is the shortest phase because it involves the rapid separation of sister chromatids towards opposite poles.

(A) The correct match is: Reformation of $ER$ and golgi complex - Telophase.
- $ER$ (Endoplasmic Reticulum) and Golgi complex reform during Telophase.
- The invisible phase of the cell cycle is Interphase,not Metaphase.
- Polar movement of chromatids occurs during Anaphase,not $S$-phase.
- Recombination nodules are formed during Pachytene,not Zygotene.

(A) During the early prophase of mitosis,the chromatin material condenses into long,thin,thread-like structures that appear tangled,resembling a ball of wool. This specific appearance is referred to as the spireme stage.

(B) In mitosis,chromosomes align at the equator to form a single metaphasic plate. The term 'homologous pair' is specific to meiosis $I$,where bivalents align to form a double metaphasic plate. Therefore,stating that homologous pairs align at the metaphasic plate during general metaphase (implying mitosis) is incorrect.

(A) The complete disintegration of the nuclear envelope is the characteristic feature that marks the transition from prophase to metaphase.
During metaphase,the chromosomes are clearly visible under a microscope and consist of two sister chromatids held together at the centromere.
Therefore,both the Assertion and the Reason are correct,and the Reason provides a correct description of the state of chromosomes during this phase.

(B) During cell division,specifically in metaphase,the spindle fibers attach to the $kinetochore$ of the chromosome.
$Kinetochores$ are complex protein structures that assemble on the $centromere$ of a chromosome.
They serve as the site of attachment for spindle microtubules,which are responsible for the movement of chromosomes during mitosis and meiosis.
Therefore,the correct option is $B$.

(A) The cell cycle is divided into two main phases: Interphase and $M$ phase (Mitosis phase).
Interphase lasts for more than $95\%$ of the total duration of the cell cycle.
Consequently,the $M$ phase,which involves actual cell division,occupies a very small portion of the total cell cycle,typically less than $5\%$ of the total time.

(A) During the prophase stage of mitosis,the $DNA$ has already replicated during the $S$-phase of interphase.
Each chromosome consists of two sister chromatids held together at a single centromere.
Each chromatid has its own kinetochore,which is a protein structure on the centromere where spindle fibers attach.
Therefore,for a single chromosome in prophase:
- Number of centromeres = $1$
- Number of chromatids = $2$
- Number of kinetochores = $2$
Thus,the correct sequence is $1, 2, 2$.

(B) During $Metaphase$,the chromosomes are condensed to their maximum extent and align themselves at the equator of the cell,which is known as the $Metaphase$ plate. This alignment is essential for the equal distribution of sister chromatids to the daughter cells. Therefore,the correct phase is $Metaphase$.

(B) During cell division,spindle fibers attach to the $kinetochores$,which are disc-shaped structures present on the surface of the $centromere$ of each chromosome.
While the $centromere$ is the primary constriction point of the chromosome,the specific protein complex where the spindle fibers actually bind is the $kinetochore$.

(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 due to the shortening of spindle fibers attached to the kinetochores.
Therefore, the correct phase is $Anaphase$.

(D) Telophase is often referred to as the reverse prophase because the events that occur during this stage are essentially the reverse of those that occur during prophase.
During prophase,the nuclear envelope breaks down,the nucleolus disappears,and chromosomes condense.
Conversely,during telophase,the nuclear envelope reforms around the chromosome clusters,the nucleolus reappears,and the chromosomes decondense into chromatin.

(D) Telophase is the final stage of mitosis. During this phase,the chromosomes that have reached their respective poles decondense and lose their individuality. The nuclear envelope reforms around each cluster of chromosomes,resulting in the formation of two daughter nuclei at opposite poles of the cell. Thus,the correct answer is $D$.

(B) During $Metaphase$,the chromosomes are highly condensed and aligned at the equatorial plate (metaphase plate).
This stage provides the most clear view of the chromosome morphology,size,and structure,making it the ideal phase for karyotyping and chromosomal analysis.

(C) The stage shown is Metaphase.
During Metaphase,the chromosomes align at the equatorial plate (metaphase plate) of the cell.
The spindle fibers from both poles are attached to the kinetochores of the chromosomes.
This alignment is a characteristic feature of Metaphase.

(A) The provided image shows a cell where the nuclear envelope is breaking down (indicated by the dashed line) and chromatin is condensing. This is characteristic of the Prophase stage of mitosis. During Prophase,the chromatin material condenses into compact mitotic chromosomes,and the nuclear envelope begins to disintegrate.

(D) Telophase is the final stage of mitosis. During this phase,the events that occurred during prophase are reversed. Chromosomes reach the opposite poles and lose their individual identity (they decondense into chromatin fibers). The nuclear envelope,nucleolus,Golgi complex,and $ER$ (endoplasmic reticulum) reform. Option $D$ is incorrect because these organelles do not disappear; rather,they reappear or reform during telophase.

(D) The bipolar spindle,which is essential for chromosome segregation during cell division,is primarily composed of microtubules. Microtubules are dynamic polymers made of the protein tubulin. Therefore,the spindle apparatus is essentially a proteinaceous structure.

(A) The process of cell division involves several stages.
During the $Prophase$ stage,the chromatin material,which is usually diffuse in the interphase nucleus,begins to condense into compact mitotic chromosomes.
This condensation is a hallmark of the initiation of $Prophase$ in mitosis.
Therefore,the correct answer is $Prophase$.

(C) The $M$ phase (Mitosis) consists of four main stages that occur in a specific sequence:
$1$. Prophase: Chromatin condenses into chromosomes and the nuclear envelope begins to disintegrate.
$2$. Metaphase: Chromosomes align at the equatorial plate.
$3$. Anaphase: Sister chromatids separate and move towards opposite poles.
$4$. Telophase: Chromosomes reach the poles and the nuclear envelope reforms.
Therefore,the correct sequence is Prophase $\rightarrow$ Metaphase $\rightarrow$ Anaphase $\rightarrow$ Telophase.

(B) In higher plants,centrioles are absent. The spindle fibers,which form the bipolar spindle during cell division,are composed of microtubules. These microtubules are organized by the cytoskeleton components present in the cytoplasm. Therefore,the bipolar spindle is formed from the cytoskeleton.

(C) During $Anaphase$,the centromeres split and the chromatids separate. The spindle fibers attached to the kinetochores shorten,pulling the daughter chromosomes towards opposite poles. This shortening of spindle fibers is a characteristic feature of $Anaphase$.