Cell Cycle and Cell Division

During $\mathrm{G}_1$ phase the cell is metabolically active and continuously grows but does not replicate its DNA. S-phase marks the phase during which DNA synthesis or replication takes place and amount of DNA per cell doubles.

During the $\mathrm{G}_2$ phase, proteins are synthesised in preparation for mitosis while cell growth continues.

During M phase the actual cell division takes place.

The final stage of meiotic prophase-I is diakinesis. So, at the end of prophase-I the chromosomes are fully condensed and the meiotic spindle is assembled to prepare the homologous chromosomes for separation. So, Statement I is correct.

Meiosis II resembles mitosis as it is an equational division.

Gap1 phase is the first phase of interphase. After Gap1 phase, synthesis phase starts in which DNA replicates, then Gap2 phase occur and finally cell cycle ends with M phase (mitosis).

During mitosis, spindle fibre get attach to the kinetochores of the chromosome and help in the separation of the chromosome.

To understand the correctness of the statements, let's break down the key terms mentioned in each statement.

Statement I: Failure of segregation of chromatids during cell cycle resulting in the gain or loss of whole set of chromosome in an organism is known as aneuploidy.

Aneuploidy refers to a condition in which the number of chromosomes is not an exact multiple of the haploid set. This typically results in the gain or loss of one or more chromosomes, not entire sets. Therefore, the definition provided in the statement is incorrect as it suggests a whole set of chromosomes, which is more aligned with polyploidy.

Statement II: Failure of cytokinesis after anaphase stage of cell division results in the gain or loss of a chromosome is called polyploidy.

Polyploidy is a condition in which an organism has more than two complete sets of chromosomes. This typically occurs due to the failure of cytokinesis, but it does not result in the gain or loss of a single chromosome, rather, it results in the addition of entire sets of chromosomes.

Given the explanations above, the correct answer would be:

Option D: Both Statement I and Statement II are false

The correct answer is Option D: Pachytene. Here's why:

Meiosis is a cell division process that produces gametes (sperm and egg cells) with half the number of chromosomes as the parent cell. During meiosis, homologous chromosomes (pairs of chromosomes with the same genes) undergo a process called crossing over, which involves the exchange of genetic material.

Let's break down the stages of prophase I of meiosis, where recombination occurs:

• Leptotene: Chromosomes condense and become visible.
• Zygotene: Homologous chromosomes begin to pair up, forming structures called synaptonemal complexes.
• Pachytene: This is where crossing over actually takes place. The synaptonemal complexes are fully formed, and the chromosomes are tightly paired. The exchange of genetic material between non-sister chromatids occurs within these complexes.
• Diplotene: The synaptonemal complexes begin to dissolve, and the chromosomes start to separate. However, they remain attached at points called chiasmata, which are the visible sites of crossing over. Recombination is complete at this stage.
• Diakinesis: The chromosomes condense further, and the chiasmata become more prominent. The nuclear envelope breaks down, and the spindle fibers start to form.

Therefore, recombination is completed by the end of Pachytene, where the exchange of genetic material is finished. Although chiasmata are still visible in Diplotene, the actual recombination event is already completed in Pachytene.

Here's a breakdown of the chromosome types and their corresponding centromere positions, helping you match the lists correctly:

Understanding Chromosome Structure

Chromosomes are thread-like structures made of DNA that carry genetic information. The centromere is a constricted region on the chromosome that holds sister chromatids (identical copies of DNA) together during cell division. The location of the centromere determines the shape of the chromosome and is used for classification.

Types of Chromosomes

• Metacentric Chromosome (A): The centromere is located in the middle of the chromosome, resulting in two arms of equal length. (II)
• Sub-metacentric Chromosome (B): The centromere is slightly off-center, leading to one arm slightly longer than the other. (III)
• Acrocentric Chromosome (C): The centromere is located close to the end of the chromosome, creating one very short arm and one very long arm. (IV)
• Telocentric Chromosome (D): The centromere is at the very end of the chromosome, resulting in a single, very long arm. (I)

Matching the Lists

Based on the above explanations, the correct match is:

Option D: A-II, B-III, C-IV, D-I

To match the events in List-I with the stages of Prophase-I (Meiosis-I) in List-II correctly, we need to understand the sequence of events that occur in each stage:

Chiasmata formation: This occurs during the Diplotene stage.

Crossing over: This occurs during the Pachytene stage.

Synaptonemal complex formation: This occurs during the Zygotene stage.

Terminalisation of chiasmata: This occurs during the Diakinesis stage.

So, matching these correctly:

(A) Chiasmata formation - (III) Diplotene

(B) Crossing over - (I) Pachytene

(C) Synaptonemal complex formation - (IV) Zygotene

(D) Terminalisation of chiasmata - (II) Diakinesis

The correct answer is:

Option A

A-III, B-I, C-IV, D-II

To match List-I with List-II correctly, we need to understand what happens in each phase of the cell cycle:

1. G$_1$ Phase: This is the first phase in interphase. During this phase, cells are metabolically active and proliferate. It fits for "Cells are metabolically active and proliferate".

2. S Phase: This is the phase where DNA replication takes place. It fits for "DNA replication takes place".

3. G$_2$ Phase: During this phase, the cell continues to grow and proteins are synthesized in preparation for mitosis. It fits for "Proteins are synthesized".

4. G$_0$ Phase: This is a resting or quiescent stage where cells remain metabolically active but do not proliferate. It fits for "Quiescent stage with metabolically active cells".

Now, let's match the items:

(A) Cells are metabolically active and proliferate - matches with G$_1$ phase (II)

(B) DNA replication takes place - matches with S phase (IV)

(C) Proteins are synthesized - matches with G$_2$ phase (I)

(D) Quiescent stage with metabolically active cells - matches with G$_0$ phase (III)

Therefore, the correct matching is:

(A) - (II)

(B) - (IV)

(C) - (I)

(D) - (III)

The correct answer is:

Option D

A-II, B-IV, C-I, D-III

During the process of mitosis, spindle fibers play a crucial role in the movement and alignment of chromosomes. These fibers are made of microtubules and are essential for the chromosomes to move accurately into position during cell division. Let's discuss when the spindle fibers attach to the kinetochores of the chromosomes:

• Prophase: This is the initial stage of mitosis, where the chromosomes condense and become visible. However, the spindle fibers have not yet attached to kinetochores. Spindle fibers begin to form, but their attachment to kinetochores happens later.
• Metaphase: This is the stage where the spindle fibers attach to the kinetochores of chromosomes. During metaphase, chromosomes align at the metaphase plate (also known as the equatorial plane of the cell). This alignment is facilitated by the spindle fibers, which pull the chromosomes into position for accurate separation into daughter cells.
• Anaphase: During anaphase, the sister chromatids separate and are pulled apart as the spindle fibers shorten. While spindle fibers are essential during this phase for pulling apart the chromatids, their initial attachment to kinetochores occurs earlier.
• Telophase: This is the final stage of mitosis, where the chromosomes reach the opposite poles of the cell, and the cell starts to re-form its nuclear membranes. Spindle fibers disassemble during this phase.

Based on the description of functions and timing within mitosis, the correct answer for when spindle fibers attach to kinetochores of chromosomes is Option B: Metaphase.

The two statements in question are related to specific stages of meiosis, specifically prophase I. Understanding these stages will help us decide the accuracy of the statements.

Statement I: Chromosomes become gradually visible under light microscope during leptotene stage.

Analysis of Statement I: During the leptotene stage of prophase I in meiosis, chromatin begins to condense, making chromosomes more visible under the light microscope. However, it is important to note that the chromosomes are not fully condensed at this stage. They continue condensing in the later stages (zygotene and pachytene). Therefore, the statement is largely accurate as it reflects the initial visibility of the chromosomes.

Statement II: The beginning of diplotene stage is recognized by dissolution of synaptonemal complex.

Analysis of Statement II: In the diplotene stage of prophase I, the previously formed synaptonemal complexes, which assist in the pairing of homologous chromosomes during the earlier zygotene and pachytene stages, start to dissolve. This dissolution marks the transition into diplotene and is a key event that defines this stage. So, this statement is true.

Given that the analysis shows both statements to be accurate in their description of events during meiosis, the most appropriate choice is:

Option A: Both Statement I and Statement II are true.

The correct sequence of stages of cell division is

$\mathrm{\mathop {Gap\,1\,phase}\limits_{(E)} \to \mathop {Synthesis\,phase}\limits_{(C)} \to \mathop {Gap\,2\,phase}\limits_{(A)} \to \mathop {Karyokinesis}\limits_{(D)} \to \mathop {Cytokinesis}\limits_{(B)}}$

$ \text { The correct sequence will be } \rightarrow \mathrm{E} \rightarrow \mathrm{C} \rightarrow \mathrm{A} \rightarrow \mathrm{D} \rightarrow \mathrm{B} $

The matching between the sub-phases of Prophase I and their specific characters can be understood as follows:

List I (Sub Phases of Prophase I):

• Leptotene: This is the first stage where chromosomes start to become visible under the microscope as thin, thread-like structures. Hence, it is characterized by chromosomes looking like thin threads.
• Zygotene: During this stage, synapsis begins, which is the pairing of homologous chromosomes. This corresponds to the formation of the synaptonemal complex.
• Pachytene: At this stage, crossing over occurs where genetic material is exchanged between homologous chromosomes. Appearance of recombination nodules, which are essential for crossing over, characterizes this stage.
• Diakinesis: This is the final stage of prophase I where chiasmata (the site of crossing over) move to the ends of the chromosomes, a process known as terminalization. Hence, this phase is marked by the completion of terminalization of chiasmata.

List II (Specific Characters):

• I. Synaptonemal complex formation - Characteristic of Zygotene.
• II. Completion of terminalisation of chiasmata - Characteristic of Diakinesis.
• III. Chromosomes look like thin threads - Characteristic of Leptotene.
• IV. Appearance of recombination nodules - Characteristic of Pachytene.

Matching the sub-phases to their characters:

• A (Diakinesis) matches with II (Completion of terminalisation of chiasmata).
• B (Pachytene) matches with IV (Appearance of recombination nodules).
• C (Zygotene) matches with I (Synaptonemal complex formation).
• D (Leptotene) matches with III (Chromosomes look like thin threads).

Thus, the correct answer is Option C:

A-II, B-IV, C-I, D-III.

The dissolution of the synaptonemal complex occurs during :

Option B : Diplotene

Diplotene is the stage of meiosis during which the synaptonemal complex, a structure that holds homologous chromosomes together, dissolves, and the chromosomes begin to move apart, although they remain attached at chiasmata (points of crossing over).

The doubling of the number of chromosomes can be achieved by disrupting mitotic cell division soon after DNA replication has occurred and before the separation of sister chromatids. This stage of mitosis is the metaphase, where chromosomes align in the center of the cell, prior to separation in anaphase.

If mitosis is disrupted after this point, sister chromatids cannot separate, leading to a doubling of the chromosome number in the resulting cells.

So, the correct answer is :

Option D : Metaphase.

The centromere of each chromosome splits during the anaphase stage of both mitosis and meiosis.

In mitosis, this happens during anaphase, when sister chromatids separate and move to opposite poles of the cell.

In meiosis, the centromere splits during anaphase II, which is similar to anaphase of mitosis, and sister chromatids separate.

So, the correct answer is :

Option D : Anaphase, Anaphase II.

The cell cycle comprises several stages through which a cell passes during its life, from its formation to its division into two daughter cells.

Option A : M (Mitotic phase) - This is the phase where the cell divides.

Option B : G₂ (Gap 2) - This is the phase of the cell cycle following DNA replication (S phase) and preceding mitosis (M phase). During this time, the cell will continue to grow and produce proteins necessary for cell division.

Option C : G₁ (Gap 1) - This phase follows cell division (M phase) and precedes DNA replication (S phase). In this phase, the cell grows and monitors its environment to determine whether it should initiate DNA synthesis.

Option D : G₀ (Gap 0) - This is a stage where cells are neither dividing nor preparing to divide. Instead, they are performing their designated functions and are in a state of dormancy or quiescence.

So, the quiescent stage of the cell cycle is :

Option D : G₀.

1. Leptotene (D) : This is the first stage of Prophase I. During this stage, the chromosomes begin to condense and become visible under a microscope.




2. Zygotene (A) : In this stage, homologous chromosomes begin to pair up, a process known as synapsis. This pairing forms structures known as bivalents or tetrads, which allow for crossing-over (exchange of genetic material) to occur.




3. Pachytene (B) : This is the stage when crossing-over occurs. Recombination nodules appear where the crossing-over has occurred. These are the physical manifestation of genetic recombination, which increases genetic diversity.




4. Diplotene (E) : At this point, the homologous chromosomes begin to separate, but remain attached at points called chiasmata. This separation of chromosomes is called desynapsis.




5. Diakinesis (C) : This is the final stage of Prophase I. The chiasmata move to the ends of the chromosomes, and the nuclear membrane breaks down, signaling the end of Prophase I.

So, the correct order of the stages is: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis. Hence, the correct answer is Option C: (D), (A), (B), (E), (C).

The correct answer is Option B : Anaphase II.

During meiosis, the division of the centromere occurs in Anaphase II. At this stage, the sister chromatids of each chromosome (which are attached at the centromere) are pulled apart and move toward opposite poles of the cell. This is similar to what happens in anaphase of mitosis. In contrast, during Anaphase I of meiosis, homologous chromosomes are separated but the centromeres do not divide, meaning the sister chromatids stay together.

M phase or mitosis is the phase where the actual cell division occurs. Mitosis is also called equational division.

During G₂ phase DNA synthesis stops but cell synthesis RNA, proteins, etc. for next phase.

Quiescent stage is inactive phase in which non-dividing cells enters.

G₁ phase is the interval between mitosis and initiation of DNA replication.

Therefore, option (B) is correct.

In oocytes of some vertebrates, diplotene lasts for months or years. This stage is referred as dictyotene stage.

Correct sequence of events during Prophase I of meiosis is : (b) → (a) → (c) → (e) → (d)

(b) Chromosomes become gradually visible under microscope.

(a) Synapsis of homologues chromosomes.

(c) Crossing over between non-sister chromatids of homologous chromosomes.

(e) Dissolution of synaptonemal complex.

(d) Terminalisation of chiasmata.

Bivalent or tetrad formation is called synapsis which is accompanied by the formation of complex structure called synaptonemal complex.

The question asks which statement is incorrect with respect to the metaphase stage of cell division.

Option D is incorrect because metaphase chromosomes are not made up of four sister chromatids held together by the centromere. Instead, metaphase chromosomes consist of two sister chromatids held together by the centromere. The sister chromatids are identical copies of a single chromosome that have been replicated during the S phase of the cell cycle.

Now, let's look at the other options, which are correct :

Option A : Chromosomes lie at the equator of the cell - During metaphase, the chromosomes align along the equatorial plane (also known as the metaphase plate) in the middle of the cell. This arrangement ensures that each daughter cell will receive an equal number of chromosomes during cell division.

Option B : Complete disintegration of the nuclear envelope takes place - In both mitosis and meiosis, the nuclear envelope breaks down during prophase and is completely disintegrated by the time the cell reaches metaphase. This allows the spindle fibers to attach to the chromosomes and facilitate their alignment at the equator.

Option C : Chromosomes are highly condensed - During metaphase, the chromosomes are at their highest level of condensation, which allows them to be more easily visualized under a microscope. This condensation also aids in the proper alignment and segregation of chromosomes during cell division.

Pairing of homologous chromosomes occurs during prophase I of meiosis.

Coiling and condensation of chromatids, spindle fibres attachment to the kinetochores and movement of centrioles towards opposite poles occur in both mitosis and meiosis.

Pachytene stage of meiosis is characterized by the appearance of recombination nodules, the sites at which crossing over occurs between non sister chromatids of homologous chromosomes.

In metacentric chromosome, centromere is in the middle of the chromosomes. Acrocentric chromosome has centromere close to the end of the chromosome. In submetacentric chromosome, centromere is slightly away from the middle of the chromosome. Telocentric chromosome has terminal centromere.

Meiosis involves two sequential cycles of nuclear and cell division called meiosis-I and meiosis-II but only single cycle of DNA replication.

The stage between two meiotic divisions is called interkinesis and is generally short lived and involves no DNA replication.

Spindle fibres attach to the kinetochores of chromosomes.

Kinetochores are the disc shaped structures present on sides of primary constriction or centromere of chromosomes.

To match the stages of meiosis in Column I with their characteristic features in Column II, we need to understand each stage and its key features :

1. Pachytene (A): This is a stage in Prophase I of meiosis where crossing-over takes place. Homologous chromosomes are already paired, and genetic recombination occurs.




2. Metaphase I (B): At this stage, chromosomes align at the equatorial plate. This is a characteristic feature of metaphase in both mitosis and meiosis.




3. Diakinesis (C): This is the final stage of Prophase I in meiosis. During diakinesis, terminalisation of chiasmata occurs, which means that the chiasmata move to the ends of the chromatids.




4. Zygotene (D): This stage is characterized by the pairing (synapsis) of homologous chromosomes. It's one of the early stages of Prophase I in meiosis.

Based on this understanding, the matches would be:

• A (Pachytene) with (iii) Crossing-over takes place.


• B (Metaphase I) with (iv) Chromosomes align at the equatorial plate.


• C (Diakinesis) with (ii) Terminalisation of chiasmata.


• D (Zygotene) with (i) Pairing of homologous chromosomes.