Cell Cycle Questions in English

(C) The cell cycle is divided into two main phases: Interphase and $M$ phase (Mitosis).
Interphase is further subdivided into $G_1$ phase,$S$ phase,and $G_2$ phase.
The $S$ phase (Synthesis phase) is the period during which the replication of nuclear $DNA$ takes place.
During this phase,the amount of $DNA$ per cell doubles,although the chromosome number remains the same.

(B) During the $S$ (synthesis) phase of interphase,$DNA$ replication occurs,resulting in the doubling of the $DNA$ content within the nucleus.
After the $S$ phase,the cell enters the $G_2$ phase.
Since the $DNA$ has already replicated,each chromosome consists of two sister chromatids,meaning there are two $DNA$ molecules per chromosome in the $G_2$ phase.

(D) The correct answer is $D$. Cells that do not divide further exit the $G_1$ phase to enter an inactive stage called the quiescent stage ($G_0$ phase) of the cell cycle.
In this state,cells remain metabolically active but no longer proliferate unless called upon to do so depending on the requirement of the organism.
Therefore,the $G_0$ state represents the exit of cells from the active cell cycle.

(A) The cell cycle consists of two main phases: Interphase and the $M$-phase (Mitotic phase).
Interphase is the period during which the cell prepares for division. It is further divided into $G_1$,$S$,and $G_2$ phases.
During Interphase,the cell is metabolically active and performs its normal cellular functions,including protein synthesis,$RNA$ synthesis,and $DNA$ replication (in the $S$ phase).
In contrast,the $M$-phase is primarily focused on the physical division of the nucleus and cytoplasm,where cellular metabolic activity is significantly reduced.
Therefore,normal cellular activities like protein synthesis occur primarily during Interphase.

(B) The $G_1$ phase (Gap $1$ phase) is the first phase of interphase in the cell cycle.
During this stage,the cell is metabolically active and continuously grows.
It involves the synthesis of various $RNA$ molecules and proteins required for cell growth and preparation for $DNA$ replication.
$DNA$ synthesis occurs specifically during the $S$ phase,not the $G_1$ phase.
Therefore,among the given options,$RNA$ and protein synthesis is the correct phenomenon occurring in the $G_1$ stage.

(D) $DNA$ replication occurs during the $S$-phase (Synthesis phase) of the cell cycle,which is a part of the interphase. During this phase,the amount of $DNA$ per cell doubles,although the chromosome number remains the same. Interphase is the preparatory phase where the cell grows and prepares for division.

(C) In the $S$-phase (synthesis phase) of interphase,$DNA$ replication occurs,which leads to the doubling of the $DNA$ content within the nucleus. This process is referred to as the duplication of chromosomes.

(A) The phase between two successive mitotic divisions is known as the $Interphase$.
Although it is historically referred to as the 'resting stage' because there is no visible cell division occurring,it is actually a period of intense metabolic activity where the cell prepares for division by replicating its $DNA$ and synthesizing necessary proteins and organelles.
Therefore,the correct term used to describe this phase is the resting stage.

(A) The centrosome is an organelle that serves as the main microtubule organizing center $(MTOC)$ of the animal cell. During cell division, the centrosome duplicates and the two centrosomes move to opposite poles of the cell, where they organize the mitotic spindle apparatus. This apparatus is essential for the separation of chromosomes during mitosis. Therefore, the centrosome plays a critical role in initiating and regulating cell division.

(C) The cell cycle is a series of events that take place in a cell leading to its division and duplication of its $DNA$ to produce two daughter cells.
It consists of two main phases: Interphase and $M$-phase (Mitosis).
Interphase is further divided into three sub-phases: $G_1$ phase (Gap $1$),$S$ phase (Synthesis),and $G_2$ phase (Gap $2$).
The sequence of these phases is $G_1 \rightarrow S \rightarrow G_2 \rightarrow M$.
Therefore,the correct sequence is $G_1, S, G_2$ and $M$.

(C) The cell cycle consists of two main phases: Interphase and $M$-phase (Mitosis). Interphase is further divided into $G_1$,$S$,and $G_2$ phases.
In a typical eukaryotic cell cycle (e.g.,human cells taking $24$ hours),the $G_1$ phase lasts about $8-10$ hours,the $S$ phase lasts about $6-8$ hours,and the $G_2$ phase lasts about $3-4$ hours.
Calculating the percentage for the $G_2$ phase: $(3/24) \times 100 \approx 12.5\%$ and $(4/24) \times 100 \approx 16.6\%$.
Therefore,the $G_2$ phase occupies approximately $12$ to $16\%$ of the total cell cycle duration.

(D) In human cells, the cell cycle lasts approximately $24$ hours. The $M$ phase (mitosis) is relatively short, lasting about $1$ hour. The remaining $23$ hours are occupied by the interphase, which consists of $G_1$, $S$, and $G_2$ phases.

(C) During the cell cycle,the $S$ phase (Synthesis phase) is the period of interphase where $DNA$ replication occurs.
In this phase,the amount of $DNA$ per cell doubles,although the number of chromosomes remains the same.
Therefore,each chromosome consists of two sister chromatids after the $S$ phase.

(C) The decision for a cell to undergo division is primarily made during the $G_1$ phase of the cell cycle.
This point is known as the restriction point or 'Start' point.
Once a cell passes this checkpoint,it is committed to $DNA$ replication and subsequent cell division.
If the cell does not receive the necessary signals to divide,it may exit the cell cycle and enter the $G_0$ phase (quiescent stage).

(D) The replication of centrioles occurs during the $S$-phase of $Interphase$. During this phase,the cell prepares for division by duplicating its $DNA$ and organelles,including the centrosomes which contain the centrioles. As the cell enters $Prophase$,these replicated centrosomes move toward opposite poles of the cell to organize the mitotic spindle.

(D) The nucleocytoplasmic ratio (or Kern-Plasma ratio) is a critical factor in cell regulation. When the nucleus is disproportionately large compared to the cytoplasm,it often indicates that the cell is preparing for division. $A$ high nuclear-to-cytoplasmic ratio is a characteristic feature of cells that are actively engaged in or preparing for the cell cycle,particularly as the cell grows and the nucleus prepares for $DNA$ replication and subsequent division.

(A) Cell fusion experiments (heterokaryon studies) demonstrate that the cytoplasm of a cell in a specific phase of the cell cycle contains factors that can regulate the progression of the nucleus of another cell.
$1$. When an $S$-phase cell is fused with a $G_1$-phase cell,the $G_1$ nucleus is induced to enter the $S$-phase and begin $DNA$ replication,proving that $S$-phase cytoplasm contains $DNA$ replication activators.
$2$. When an $M$-phase cell is fused with a $G_1$-phase cell,the $G_1$ nucleus is forced to undergo premature chromosome condensation,as the $M$-phase cytoplasm contains maturation-promoting factors $(MPF)$.

(A) The $G_1$ phase (Gap $1$ phase) is the first phase of interphase. During this phase,the cell is metabolically active and continuously grows. It synthesizes various enzymes,structural proteins,and $RNA$ required for $DNA$ replication. $DNA$ polymerase is one of the essential enzymes synthesized during the $G_1$ phase to prepare the cell for the $S$ phase (Synthesis phase),where $DNA$ replication occurs. Histones and tubulin proteins are primarily synthesized during the $S$ phase and $G_2$ phase,respectively.

(A) The cell cycle consists of two main phases: Interphase and $M$ phase (Mitosis).
Interphase is further divided into $G_1$,$S$,and $G_2$ phases.
During the $S$ phase (Synthesis phase),the replication of $DNA$ takes place.
Along with $DNA$ replication,the synthesis of histone proteins occurs in the nucleus to facilitate the packaging of the newly formed $DNA$ into chromatin.
Therefore,both $DNA$ and histone proteins are synthesized during the $S$ phase.

(D) The correct answer is $D$. In the somatic cell cycle,the interphase is divided into $G_1$,$S$,and $G_2$ phases. The $S$-phase (Synthesis phase) is the stage where $DNA$ replication occurs,resulting in the duplication of the $DNA$ content of the cell. $G_1$ phase precedes $S$-phase,and $G_2$ phase follows $S$-phase but precedes the mitotic phase ($M$-phase).

(A) The $G_1$ phase (Gap $1$ phase) is the first phase of the interphase in the cell cycle. During this phase,the cell is metabolically active and continuously grows. It involves the synthesis of various $RNA$ molecules and proteins required for cell growth and preparation for the subsequent $S$ phase (Synthesis phase),where $DNA$ replication occurs. Therefore,$RNA$ and proteins are synthesized during the $G_1$ phase.

(A) Neurons are highly specialized cells that lose their ability to divide once they reach maturity. After birth,most neurons in the human brain do not undergo mitosis and remain in the $G_0$ phase (quiescent stage) of the cell cycle throughout the life of the organism. In contrast,glial cells,epithelial cells,and liver cells retain the capacity to divide and regenerate under specific conditions.

(C) Cleavage is a series of rapid mitotic divisions of the zygote. During cleavage,the $G_1$ and $G_2$ phases of the cell cycle are typically absent or significantly shortened,leading to a very short Interphase. Since the rate of cleavage depends on how quickly the cell can divide,a shorter Interphase allows for a faster rate of division. Therefore,the rate of cleavage is inversely proportional to the duration of the Interphase.

(C) The total number of cells produced is $128$.
Let the number of divisions (cleavages) be $n$.
The formula for the number of cells produced after $n$ divisions is $2^n$.
Given $2^n = 128$.
Since $2^7 = 128$,the number of divisions $n = 7$.

(B) The correct answer is $B$. Two sister chromatids are held together at a specific region called the centromere. The centromere is the primary constriction of the chromosome. During cell division,the kinetochore,a disc-shaped protein structure,is assembled on the surface of the centromere,which serves as the attachment site for spindle fibers.

(C) The cell cycle is divided into two main phases: Interphase and $M$-phase (Mitosis).
Interphase is further divided into $G_1$,$S$,and $G_2$ phases.
During the $S$ phase (Synthesis phase),the amount of $DNA$ per cell doubles.
If the initial amount of $DNA$ is denoted as $2C$,then after the $S$ phase,it becomes $4C$,although the number of chromosomes remains the same.
Therefore,$DNA$ replication occurs specifically during the $S$ phase.

(C) The cell cycle consists of interphase and the $M$ phase. Interphase is further divided into $G_1$,$S$,and $G_2$ phases.
During the $S$ phase (Synthesis phase),the replication of $DNA$ occurs.
If the initial amount of $DNA$ in the cell is denoted as $2C$,then after the $S$ phase,the amount of $DNA$ becomes $4C$.
However,the number of chromosomes remains the same during this phase.

(C) 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.
During the $S$ phase (Synthesis phase),the replication of $DNA$ takes place.
In this phase,the amount of $DNA$ per cell doubles,although the number of chromosomes remains the same.

(B) The cell cycle is divided into two main phases: Interphase and $M$ phase (Mitosis/Meiosis).
Interphase is further divided into three sub-phases: $G_1$ phase,$S$ phase,and $G_2$ phase.
During the $S$ phase (Synthesis phase),the cell replicates its $DNA$.
If the initial amount of $DNA$ in the cell is denoted as $2C$,then after $S$ phase,the amount of $DNA$ becomes $4C$,although the number of chromosomes remains the same.
Therefore,the correct phase for $DNA$ replication is the $S$ phase.

(A) The cell cycle is divided into two main phases: Interphase and $M$-phase (Mitosis).
Interphase is further divided into $G_1$,$S$,and $G_2$ phases.
The $G_1$ phase is known as the first gap phase or the biosynthetic phase.
During the $G_1$ phase,the cell is metabolically active and continuously grows in size by synthesizing proteins,$RNA$,and various organelles,but it does not replicate its $DNA$.
Therefore,the $G_1$ phase is referred to as the biosynthetic phase.

(A) Ribonuclease is an enzyme that degrades $RNA$. During the cell cycle,the synthesis of various proteins and enzymes required for mitosis occurs during the $G_1$ and $G_2$ phases of Interphase. Since $RNA$ is essential for protein synthesis,the application of Ribonuclease disrupts the metabolic activities necessary for the cell to progress through Interphase. Therefore,it inhibits the cell cycle during Interphase.

(D) In the cell cycle,the $G_1$ phase represents the first gap phase where the cell grows and prepares for $DNA$ replication.
During the $S$ phase (Synthesis phase),$DNA$ replication occurs.
Although the amount of $DNA$ doubles (from $2C$ to $4C$),the number of chromosomes remains the same.
Since the cell is diploid $(2n)$ in the $G_1$ phase,it remains diploid $(2n)$ in the $S$ phase because the sister chromatids are still attached at the centromere,and the chromosome count does not change.

(A) During the $S$-phase (Synthesis phase) of the cell cycle,$DNA$ replication occurs.
Along with $DNA$ replication,the synthesis of histone proteins also takes place to form new nucleosomes,which are essential for packaging the newly synthesized $DNA$.

(A) The $G_0$ phase,also known as the quiescent stage,is a state where cells exit the cell cycle.
$I.$ It is indeed a quiescent stage.
$II.$ The cell cycle is arrested as the cells do not proceed to the $S$ phase.
$III.$ Cells in $G_0$ do not proliferate but remain metabolically active,performing their specific functions.
$IV.$ Cells in $G_0$ can re-enter the cell cycle and divide in response to specific stimuli (e.g.,injury or growth factors).
Since all statements $I, II, III,$ and $IV$ are correct,the correct option is $A$.

(D) The cell cycle consists of two main phases: Interphase and $M$-phase (Mitosis).
Interphase is further divided into $G_1$,$S$,and $G_2$ phases.
Once a cell passes the $G_1$ checkpoint (also known as the restriction point),it is committed to $DNA$ replication and subsequent cell division.
Therefore,the $S-$ phase (Synthesis phase) is the stage where $DNA$ replication occurs,but the commitment to divide is finalized at the end of the $G_1$ phase,making the cell enter the $S-$ phase.

(A) The cell cycle consists of two main phases: $Interphase$ and $M$-phase (Mitosis/Meiosis).
$Interphase$ is often called the 'resting phase' because the cell is not actively dividing,although it is metabolically very active.
During $Interphase$,the cell prepares for division by replicating its $DNA$ and synthesizing proteins,but these changes are not visible as distinct structural changes (like chromosome condensation) under a light microscope.
In contrast,$Prophase$,$Metaphase$,and $Anaphase$ are stages of the $M$-phase where visible changes such as chromosome condensation,alignment,and separation occur,which can be observed under a microscope.

(A) During the $G_1$ phase (Gap $1$ phase),the cell undergoes significant growth and metabolic activity. The cell synthesizes various proteins,enzymes,and organelles required for $DNA$ replication. As a result of this intense metabolic activity and the accumulation of cellular components,the nucleus increases in size to accommodate the increased transcriptional activity and preparation for the upcoming $S$ phase.

(C) When a cell is small,it has a high surface area to volume ratio,which facilitates efficient exchange of materials. During the $G_1$ phase of the cell cycle,the cell grows in size and is metabolically very active,preparing for $DNA$ replication. As the cell grows,its metabolic activity increases to support the synthesis of proteins and organelles required for division.

(C) The cell cycle consists of $G_1$,$S$,$G_2$,and $M$ phases.
During the $S$ phase (Synthesis phase),$DNA$ replication occurs.
Since $DNA$ is wrapped around histone proteins to form chromatin,the synthesis of histone proteins must occur simultaneously with $DNA$ replication to ensure that the newly formed $DNA$ can be packaged into nucleosomes.
Therefore,histone proteins are synthesized during the $S$ phase.

(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.
During the $G_1$ phase (Gap $1$),the cell grows and prepares for $DNA$ replication.
The $S$ phase (Synthesis phase) is the period during which $DNA$ replication occurs.
Therefore,the $G_1$ phase is the phase that occurs before $DNA$ synthesis ($S$ phase).

(A) The $K.I$ (Karyoplasmic Index) is defined as the ratio of the volume of the nucleus $(V_n)$ to the volume of the cytoplasm $(V_c)$,expressed as $K.I = V_n / V_c$.
During cell growth,the volume of the cytoplasm increases significantly as the cell synthesizes proteins and organelles,while the volume of the nucleus remains relatively stable.
Since the denominator $(V_c)$ increases while the numerator $(V_n)$ remains constant,the overall value of the $K.I$ decreases.
When the $K.I$ falls below a critical threshold,it triggers the cell to undergo division to restore the balance between the nucleus and the cytoplasm.

(D) The cell cycle consists of two main phases: $Interphase$ and $M-phase$ (Mitotic phase).
$Interphase$ is often called the 'resting phase' or 'preparatory phase',but it is metabolically the most active phase of the cell cycle.
During $Interphase$,the cell grows,replicates its $DNA$,and synthesizes proteins and organelles required for cell division.
Therefore,$Interphase$ is considered the most active phase.

(C) The cell cycle was divided into four distinct phases, namely $G_1$ (Gap $1$), $S$ (Synthesis), $G_2$ (Gap $2$), and $M$ (Mitosis), by Alma Howard and Stephen $R$. Pelc in $1953$.
They conducted experiments using radioactive phosphorus $(^{32}P)$ to study the timing of $DNA$ synthesis in the cell cycle of Vicia faba root meristem cells.

(B) In the cell cycle,the $S$-phase (Synthesis phase) is responsible for $DNA$ replication.
During the $S$-phase,the amount of $DNA$ per cell doubles (from $2C$ to $4C$),but the number of chromosomes remains the same.
Since the $G_2$-phase occurs after the $S$-phase and before mitosis,the cell contains double the amount of $DNA$ compared to the initial $G_1$-phase.
Therefore,if the initial amount of $DNA$ in the $G_1$-phase is considered as $1$ unit (or $2C$),the amount in the $G_2$-phase is $2$ units (or $4C$ relative to the original $DNA$ content per chromosome set).

(B) The cell cycle is divided into two main phases: Interphase and $M$-phase (Mitosis).
Interphase is further divided into three stages: $G_1-$phase,$S-$phase,and $G_2-$phase.
$G_1-$phase (Gap $1$) is the interval between mitosis and the initiation of $DNA$ replication ($S-$phase).
During this phase,the cell is metabolically active and continuously grows in size but does not replicate its $DNA$.

(A) The cell cycle consists of interphase ($G_1$,$S$,and $G_2$ phases) and the $M$ phase (mitosis).
During the $G_1$ phase (Gap $1$),the cell grows,and there is active synthesis of $RNA$ and proteins required for cell growth and preparation for $DNA$ replication.
During the $S$ phase (Synthesis),$DNA$ replication occurs.
During the $G_2$ phase (Gap $2$),the cell continues to grow,and there is further synthesis of $RNA$ and proteins (such as tubulin for spindle fibers) required for mitosis.
Therefore,$RNA$ and protein synthesis occur in both $G_1$ and $G_2$ phases.

(C) In the human body,certain specialized cells exit the cell cycle and enter the $G_0$ phase (quiescent stage) where they do not divide further.
Cardiac muscle cells $(a)$ and neurons $(c)$ are highly specialized cells that lose the ability to divide once they reach maturity.
Skeletal muscle cells $(b)$ are also generally considered post-mitotic in adults,though they have limited regenerative capacity through satellite cells; however,in the context of standard biological questions regarding cells that do not undergo division,$a$ and $c$ are the primary examples.
Therefore,the correct answer is $a$ and $c$.

(C) The cell cycle is divided into two main phases: Interphase and $M$-phase (Mitosis).
Interphase is further subdivided into $G_1$ phase,$S$ phase (Synthesis phase),and $G_2$ phase.
During the $S$ phase of Interphase,the amount of $DNA$ per cell doubles.
If the initial amount of $DNA$ is denoted as $2C$,it increases to $4C$ during this phase,although the chromosome number remains the same.

(A) Some cells in the adult animal do not appear to exhibit division (e.g.,heart cells) and many other cells divide only occasionally,as needed to replace cells that have been lost because of injury or cell death.
These cells that do not divide further exit the $G_1$ phase to enter an inactive stage called the quiescent stage $(G_0)$ of the cell cycle.
Cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism.