Class 11 Biology NCERT Notes- Chapter 10: Cell Cycle and Cell Division

Detailed Study Notes – Chapter 10: Cell Cycle and Cell Division (Class 11 Biology, Notes, PDFs, Quizzes, MCQs)

1. The Cell Cycle: An Overview

The cell cycle is the fundamental sequence of events by which a cell duplicates its genome, synthesizes its other constituents, and ultimately divides into two daughter cells. This process of growth and division is a core characteristic of all living organisms, allowing a single cell to form a multicellular structure. The entire cycle must be precisely coordinated to ensure that progeny cells receive intact and correct genetic information.

The duration of the cell cycle varies significantly. A typical human cell in culture divides approximately once every 24 hours, whereas yeast can complete the cycle in about 90 minutes.

The cell cycle is divided into two primary phases:

• Interphase: The preparatory period between two successive M phases. It constitutes more than 95% of the total cell cycle duration. Though once called the “resting phase,” it is a period of intense metabolic activity, cell growth, and DNA replication.
• M Phase (Mitosis Phase): The period of actual cell division, which lasts for about an hour in a typical human cell. It involves nuclear division (karyokinesis) and cytoplasmic division (cytokinesis).

2. Interphase: Preparation for Division

Interphase is subdivided into three distinct phases:

• G₁ Phase (Gap 1): This phase occurs between the end of mitosis (M phase) and the beginning of DNA synthesis (S phase). During G₁, the cell is metabolically active and grows continuously, but its DNA does not replicate.
• S Phase (Synthesis): This is the critical period when DNA synthesis or replication occurs. The amount of DNA per cell doubles (from 2C to 4C). However, the chromosome number remains the same; a diploid (2n) cell remains diploid (2n) after the S phase, but each chromosome now consists of two sister chromatids. In animal cells, the centriole also duplicates in the cytoplasm during this phase.
• G₂ Phase (Gap 2): Following the S phase, the cell enters G₂. During this phase, protein synthesis continues in preparation for mitosis, and cell growth proceeds.

The Quiescent Stage (G₀)

Some cells in adult animals, such as heart cells, do not divide. Other cells only divide occasionally to replace lost or damaged cells. These cells exit the G₁ phase and enter a metabolically active but non-proliferating, inactive stage known as the quiescent stage (G₀). They can re-enter the cell cycle if required by the organism.

3. M Phase: Mitosis (Equational Division)

Mitosis is the most dramatic phase of the cell cycle, involving the reorganization of nearly all cellular components. It is called equational division because the number of chromosomes in the parent and daughter cells remains the same. The process is continuous but is divided into four stages of nuclear division (karyokinesis).

3.1 Karyokinesis: Nuclear Division

• Prophase:
  • This is the first stage, following the G₂ phase.
  • The tangled chromatin material from the S and G₂ phases begins to untangle and condense, forming compact, visible mitotic chromosomes.
  • Each chromosome is composed of two sister chromatids attached at the centromere.
  • The centrosome, which duplicated during interphase, moves toward opposite poles of the cell, radiating microtubules called asters. The asters and spindle fibers form the mitotic apparatus.
  • By the end of prophase, the Golgi complex, endoplasmic reticulum, nucleolus, and nuclear envelope are no longer visible.
• Metaphase:
  • This stage begins with the complete disintegration of the nuclear envelope.
  • Chromosome condensation is complete, making them clearly visible and ideal for studying their morphology.
  • Spindle fibers attach to disc-shaped structures on the centromeres called kinetochores.
  • The chromosomes are moved to the center of the cell and align along the equatorial plane, known as the metaphase plate. Each chromosome’s sister chromatids are connected by their kinetochores to spindle fibers from opposite poles.
• Anaphase:
  • This stage is marked by the simultaneous splitting of the centromere of each chromosome.
  • The sister chromatids separate and are now referred to as daughter chromosomes.
  • These daughter chromosomes begin migrating to opposite poles, with their centromeres leading the way and the arms trailing behind.
• Telophase:
  • This is the final stage of karyokinesis.
  • The chromosomes reach their respective poles, decondense, and lose their individual visibility, forming chromatin material clusters.
  • A new nuclear envelope develops around each chromosome cluster, forming two distinct daughter nuclei.
  • The nucleolus, Golgi complex, and ER reform.

3.2 Cytokinesis: Cytoplasmic Division

Cytokinesis is the division of the cytoplasm, which follows karyokinesis and completes the process of cell division.

• In Animal Cells: A furrow appears in the plasma membrane, which gradually deepens and joins in the center, dividing the cell into two.
• In Plant Cells: Due to the rigid cell wall, a different mechanism is used. A precursor called the cell plate forms in the center of the cell and grows outward to meet the existing lateral walls. This cell plate represents the middle lamella between the two new cells.

In some cases, karyokinesis is not followed by cytokinesis, resulting in a multinucleate condition known as a syncytium (e.g., liquid endosperm in coconut).

3.3 Significance of Mitosis

• Growth: Mitosis is responsible for the growth of multicellular organisms from a single zygote.
• Cell Repair and Replacement: It constantly replaces old or damaged cells, such as those in the upper layer of the epidermis, the lining of the gut, and blood cells.
• Maintaining Nucleo-cytoplasmic Ratio: Cell growth disturbs this ratio, and division restores it.
• Genetic Stability: It produces diploid daughter cells with a genetic complement identical to the parent cell.
• Plant Growth: Mitotic divisions in meristematic tissues (apical and lateral cambium) allow plants to grow throughout their lives.

4. Meiosis (Reduction Division)

Meiosis is a specialized type of cell division that occurs in diploid cells destined to form gametes. It reduces the chromosome number by half, producing haploid daughter cells. This ensures that when two gametes fuse during fertilization, the diploid chromosome number is restored in the offspring.

4.1 Key Features of Meiosis

• It involves two sequential cycles of division, Meiosis I and Meiosis II, but only one cycle of DNA replication.
• It involves the pairing of homologous chromosomes and crossing over (recombination) between their non-sister chromatids.
• It results in the formation of four haploid cells.

4.2 Meiosis I

This division separates homologous chromosomes.

• Prophase I: This is a longer and more complex phase than mitotic prophase, subdivided into five stages:
  • Leptotene: Chromosomes begin to compact and become visible.
  • Zygotene: Homologous chromosomes pair up in a process called synapsis, forming a structure called the synaptonemal complex. The paired chromosomes are called a bivalent or a tetrad.
  • Pachytene: The four chromatids of each bivalent become distinct. Crossing over, the exchange of genetic material between non-sister chromatids of homologous chromosomes, occurs at sites called recombination nodules. This process is mediated by the enzyme recombinase.
  • Diplotene: The synaptonemal complex dissolves. The homologous chromosomes begin to separate but remain attached at the sites of crossing over, forming X-shaped structures called chiasmata.
  • Diakinesis: The final stage, marked by the terminalisation of chiasmata. Chromosomes are fully condensed, the meiotic spindle assembles, and the nucleolus and nuclear envelope disappear.
• Metaphase I: The bivalent chromosomes align on the equatorial plate. Microtubules from opposite poles attach to the kinetochores of the homologous chromosomes.
• Anaphase I: The homologous chromosomes separate and move to opposite poles. The sister chromatids remain attached at their centromeres.
• Telophase I: The nuclear membrane and nucleolus reappear. Cytokinesis follows, resulting in a dyad of cells (two haploid cells).
• Interkinesis: A short-lived stage between Meiosis I and Meiosis II where there is no DNA replication.

4.3 Meiosis II

This division is similar to mitosis and separates sister chromatids.

• Prophase II: Initiated immediately after cytokinesis. The chromosomes become compact again, and the nuclear membrane disappears.
• Metaphase II: The chromosomes align at the equator. Microtubules from opposite poles attach to the kinetochores of the sister chromatids.
• Anaphase II: The centromere of each chromosome splits, allowing the sister chromatids to separate and move to opposite poles.
• Telophase II: The chromosomes are enclosed in a new nuclear envelope. Cytokinesis follows, resulting in a tetrad of cells (four haploid daughter cells).

4.4 Significance of Meiosis

• Conservation of Chromosome Number: By producing haploid gametes, meiosis ensures the specific chromosome number of a species is maintained across generations in sexually reproducing organisms.
• Genetic Variation: Crossing over during Prophase I creates new combinations of genes, increasing genetic variability in a population. This variation is crucial for the process of evolution.

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Q&A Section

Short-Answer Questions (25 Questions)

1. What is the definition of the cell cycle?
2. Compare the duration of the cell cycle in a typical human cell versus a yeast cell.
3. What are the two basic phases of the cell cycle, and what percentage of the cycle does interphase typically occupy?
4. Describe the key activities that occur during the G₁ phase of interphase.
5. Explain what happens to the amount of DNA and the chromosome number in a diploid cell during the S phase.
6. What is the G₀ phase, and what is an example of cells that enter this stage?
7. Why is mitosis referred to as “equational division”?
8. List three characteristic events that mark the completion of prophase in mitosis.
9. What is the metaphase plate, and what happens to chromosomes at this location during metaphase?
10. What are kinetochores and what is their function during mitosis?
11. Describe the two key events that characterize the anaphase stage of mitosis.
12. What cellular structures reform during telophase?
13. How does cytokinesis differ in animal cells compared to plant cells?
14. What is a syncytium, and how is it formed?
15. State two significant contributions of mitosis to a multicellular organism.
16. What is the primary outcome of meiosis in terms of chromosome number and cell count?
17. Name the five sub-stages of Prophase I in the correct order.
18. Define synapsis and identify the stage of Prophase I in which it occurs.
19. What is crossing over, and which enzyme mediates this process?
20. What are chiasmata, and in which stage of Prophase I do they become visible?
21. What is the main difference between Anaphase I of meiosis and anaphase of mitosis?
22. What is interkinesis? Does DNA replication occur during this stage?
23. In what way does Meiosis II resemble a normal mitotic division?
24. What happens during Anaphase II that allows sister chromatids to separate?
25. Explain the two main points regarding the significance of meiosis.

Multiple-Choice Questions (20 Questions)

1. In a human cell with a 24-hour cell cycle, the M phase lasts for approximately: a) 23 hours b) 12 hours c) 1 hour d) 90 minutes
2. DNA replication occurs during which phase of the cell cycle? a) G₁ phase b) G₂ phase c) M phase d) S phase
3. If a diploid cell (2n) with DNA content 2C enters the S phase, what will be its chromosome number and DNA content after completion of the S phase? a) 2n and 2C b) 4n and 4C c) 2n and 4C d) n and 2C
4. Cells that are metabolically active but no longer proliferate are said to be in the: a) G₁ phase b) S phase c) G₀ phase d) G₂ phase
5. The condensation of chromosomal material is initiated during: a) Anaphase b) Metaphase c) Telophase d) Prophase
6. The morphology of chromosomes is most easily studied during: a) Prophase b) Metaphase c) Anaphase d) Interphase
7. Spindle fibers attach to what structures on the chromosomes? a) Centrosomes b) Kinetochores c) Chromatids d) Asters
8. In plant cells, cytokinesis is achieved by the formation of a: a) Cleavage furrow b) Cell plate c) Syncytium d) Bivalent
9. Meiosis involves how many cycles of nuclear division and DNA replication? a) One nuclear division, one DNA replication b) Two nuclear divisions, two DNA replications c) Two nuclear divisions, one DNA replication d) One nuclear division, two DNA replications
10. The pairing of homologous chromosomes during Meiosis I is called: a) Crossing over b) Synapsis c) Condensation d) Terminalisation
11. Crossing over occurs between non-sister chromatids during which stage? a) Leptotene b) Zygotene c) Pachytene d) Diplotene
12. The X-shaped structures that become visible after the dissolution of the synaptonemal complex are called: a) Bivalents b) Kinetochores c) Recombination nodules d) Chiasmata
13. What separates during Anaphase I of meiosis? a) Sister chromatids b) Homologous chromosomes c) Centromeres d) Spindle fibers
14. The stage between the two meiotic divisions is known as: a) Interphase II b) Cytokinesis I c) Interkinesis d) Prophase II
15. Meiosis II begins with the splitting of the centromere during: a) Prophase II b) Metaphase II c) Anaphase II d) Telophase II
16. The final result of meiosis is the formation of: a) Two diploid daughter cells b) Two haploid daughter cells c) Four diploid daughter cells d) Four haploid daughter cells
17. Genetic variability in sexually reproducing organisms is increased by: a) Cytokinesis b) Mitosis c) DNA replication in the S phase d) Crossing over in meiosis
18. In animal cells, the centriole duplicates during the: a) G₁ phase b) S phase c) G₂ phase d) M phase
19. Which of the following cellular components disappear at the end of prophase? a) Plasma membrane and cytoplasm b) Chromosomes and centrosomes c) Nucleolus and nuclear envelope d) Spindle fibers and kinetochores
20. A multinucleate condition arising from karyokinesis without cytokinesis is called: a) A dyad b) A tetrad c) A syncytium d) A chiasma

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Answer Keys

Short-Answer Questions: Answer Key

1. The cell cycle is the sequence of events by which a cell duplicates its genome, synthesizes other cell constituents, and eventually divides into two daughter cells.
2. A typical human cell completes its cycle in approximately 24 hours, while a yeast cell can complete its cycle much faster, in about 90 minutes.
3. The two basic phases are Interphase and M Phase. Interphase lasts for more than 95% of the duration of the cell cycle.
4. During the G₁ phase, the cell is metabolically active and grows continuously. It is the interval between mitosis and the initiation of DNA replication.
5. During the S phase, the amount of DNA per cell doubles (from 2C to 4C). However, the chromosome number remains the same (e.g., 2n).
6. The G₀ phase is an inactive, quiescent stage where cells are metabolically active but do not divide. Heart cells are an example of cells that enter this stage.
7. Mitosis is called equational division because the number of chromosomes in the daughter cells is the same as in the parent cell.
8. The completion of prophase is marked by: condensed chromosomes appearing as two chromatids attached at a centromere; centrosomes moving to opposite poles; and the disappearance of the nuclear envelope, nucleolus, Golgi, and ER.
9. The metaphase plate is the plane at the equator of the cell where all chromosomes align during metaphase. This alignment is facilitated by spindle fibers attached to both poles.
10. Kinetochores are small disc-shaped structures at the surface of the centromeres. They serve as the sites of attachment for spindle fibers to the chromosomes.
11. The two key events of anaphase are the splitting of the centromeres and the subsequent movement of the separated chromatids (now daughter chromosomes) to opposite poles.
12. During telophase, the nuclear envelope, nucleolus, Golgi complex, and ER reform around the chromosome clusters at each pole.
13. In animal cells, cytokinesis occurs via a furrow in the plasma membrane that deepens to divide the cell. In plant cells, a cell plate forms in the center and grows outward to create a new cell wall.
14. A syncytium is a cell with multiple nuclei. It is formed when karyokinesis (nuclear division) is not followed by cytokinesis (cytoplasmic division).
15. Two significant contributions are the growth of multicellular organisms and the repair/replacement of cells (e.g., skin, gut lining, blood cells).
16. Meiosis results in four haploid daughter cells, with the chromosome number reduced by half compared to the parent cell.
17. The five sub-stages are Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.
18. Synapsis is the process of association where homologous chromosomes pair together. This occurs during the Zygotene stage of Prophase I.
19. Crossing over is the exchange of genetic material between two non-sister chromatids of homologous chromosomes. The enzyme involved is called recombinase.
20. Chiasmata are the X-shaped structures that are the sites of crossovers between homologous chromosomes. They become visible during the Diplotene stage.
21. In Anaphase I of meiosis, homologous chromosomes separate, but sister chromatids remain joined. In anaphase of mitosis, the centromeres split and sister chromatids separate.
22. Interkinesis is the short stage between Meiosis I and Meiosis II. No DNA replication occurs during this stage.
23. Meiosis II resembles mitosis because it involves the separation of sister chromatids, resulting in daughter cells with the same number of chromosomes as the cells that entered the division (in this case, haploid).
24. Anaphase II begins with the simultaneous splitting of the centromere of each chromosome, which allows the sister chromatids to separate and move to opposite poles.
25. The two main points are: (1) it conserves the specific chromosome number of a species across generations, and (2) it increases genetic variability in a population through crossing over, which is important for evolution.

Multiple-Choice Questions: Answer Key

1. c) 1 hour
2. d) S phase
3. c) 2n and 4C
4. c) G₀ phase
5. d) Prophase
6. b) Metaphase
7. b) Kinetochores
8. b) Cell plate
9. c) Two nuclear divisions, one DNA replication
10. b) Synapsis
11. c) Pachytene
12. d) Chiasmata
13. b) Homologous chromosomes
14. c) Interkinesis
15. c) Anaphase II
16. d) Four haploid daughter cells
17. d) Crossing over in meiosis
18. b) S phase
19. c) Nucleolus and nuclear envelope
20. c) A syncytium

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Essay Questions and Answers

1. Describe the entire sequence of events that occurs during the Interphase of the cell cycle.

Answer: Interphase is the preparatory period between cell divisions and constitutes over 95% of the cell cycle. It is divided into three phases: G₁ (Gap 1), S (Synthesis), and G₂ (Gap 2). In the G₁ phase, the cell is metabolically active and undergoes continuous growth, synthesizing proteins and organelles; however, its DNA is not replicated. Following G₁, the cell enters the S phase, which is defined by DNA synthesis or replication. During this crucial stage, the amount of DNA per cell doubles (from 2C to 4C), but the chromosome number remains constant (2n). In animal cells, the centrioles also duplicate in the cytoplasm. Finally, the cell enters the G₂ phase, where cell growth continues and proteins are synthesized in preparation for the upcoming M phase (mitosis). This orderly progression ensures the cell is fully prepared for division.

2. Explain the key differences and similarities between Prophase of Mitosis and Prophase I of Meiosis.

Answer: Both Prophase of Mitosis and Prophase I of Meiosis involve the condensation of chromosomes, the formation of the spindle apparatus, and the breakdown of the nuclear envelope and nucleolus. However, Prophase I is significantly longer and more complex due to events not seen in mitosis. The key difference is the behavior of homologous chromosomes. In Prophase I, homologous chromosomes undergo synapsis (pairing) to form bivalents, a process absent in mitotic prophase. Furthermore, within these bivalents, crossing over occurs between non-sister chromatids during the pachytene sub-stage, leading to genetic recombination. This pairing and exchange of genetic material is the foundation of meiosis’s role in creating genetic diversity and does not happen in mitosis.

3. Compare and contrast cytokinesis in plant and animal cells.

Answer: Cytokinesis is the division of the cytoplasm that follows nuclear division. In animal cells, which lack a rigid cell wall, cytokinesis is achieved by the formation of a cleavage furrow in the plasma membrane. This furrow, driven by a contractile ring of microfilaments, gradually deepens from the outside in and ultimately joins in the center, pinching the cell into two daughter cells. In contrast, plant cells possess a rigid, inextensible cell wall that prevents furrowing. Therefore, they undergo cytokinesis by forming a cell plate in the center of the cell. This plate, a precursor to the new cell wall, grows outward from the middle towards the existing lateral walls, eventually fusing with them to completely separate the cytoplasm and form two distinct cells.

4. Detail the events of Anaphase and Telophase in mitosis.

Answer: Anaphase is characterized by the separation of sister chromatids. At its onset, the centromere holding each pair of sister chromatids together splits simultaneously. The separated chromatids, now considered individual daughter chromosomes, begin to migrate towards the two opposite poles of the cell, pulled by the shortening spindle fibers. The centromere of each chromosome leads the way, with the arms trailing behind. Telophase begins as the chromosomes arrive at their respective poles. In this final stage of karyokinesis, the chromosomes decondense and lose their distinct identities, reverting to a mass of chromatin. A new nuclear envelope develops around each of the two chromosome clusters, forming two daughter nuclei. Concurrently, the nucleolus, Golgi complex, and endoplasmic reticulum reform within each new nucleus.

5. Explain the entire process of Meiosis II, from Prophase II to Telophase II.

Answer: Meiosis II is the second division in meiosis and closely resembles a normal mitotic division. It begins with the products of Meiosis I, which are two haploid cells. In Prophase II, which is much simpler than Prophase I, the chromosomes re-condense, and the nuclear envelope breaks down. During Metaphase II, the chromosomes align individually along the equatorial plate, with the kinetochores of the sister chromatids facing opposite poles and attaching to spindle fibers. The key event of Anaphase II is the splitting of the centromeres, which allows the sister chromatids to separate and move to opposite poles of the cell. Finally, in Telophase II, the chromosomes arrive at the poles and a nuclear envelope reforms around each of the four groups. Cytokinesis follows, resulting in the formation of a tetrad of cells, meaning four haploid daughter cells are produced.

6. What is the significance of mitosis in the life of an organism? Provide at least three distinct examples.

Answer: Mitosis, or equational division, plays several vital roles. Firstly, it is the basis for the growth of multicellular organisms from a single-celled zygote into a complex individual, by increasing the total number of cells. Secondly, mitosis is essential for cell repair and replacement. For example, the cells of the upper layer of the epidermis, the cells lining the gut, and blood cells are constantly being replaced through mitotic division. Thirdly, in plants, mitotic divisions in specific tissues—the apical and lateral meristems—are responsible for the continuous growth of the plant throughout its life. It also helps restore the nucleo-cytoplasmic ratio when a cell grows too large.

7. What is crossing over? Describe when and how it occurs and explain its biological importance.

Answer: Crossing over is the exchange of genetic material between the non-sister chromatids of two homologous chromosomes. This process occurs during the pachytene sub-stage of Prophase I in meiosis. During this stage, homologous chromosomes are paired in a bivalent structure, and specific sites called recombination nodules appear. At these sites, an enzyme-mediated process, involving the enzyme recombinase, facilitates the breaking and rejoining of chromatid segments. The biological importance of crossing over is immense; it leads to the recombination of genetic material, creating new combinations of alleles on the chromosomes. This is a primary source of genetic variation in a population, which is the raw material for natural selection and evolution.

8. Distinguish between Anaphase of Mitosis and Anaphase I of Meiosis.

Answer: The primary distinction between Anaphase of Mitosis and Anaphase I of Meiosis lies in what separates and moves to the poles. In Anaphase of Mitosis, the centromeres split, and the sister chromatids of each chromosome are pulled apart. These separated chromatids, now called daughter chromosomes, migrate to opposite poles. In contrast, during Anaphase I of Meiosis, the centromeres do not split. Instead, the homologous chromosomes of each bivalent separate and move to opposite poles. Each chromosome that moves to a pole still consists of two sister chromatids joined at the centromere. This separation of homologous chromosomes is what reduces the chromosome number from diploid to haploid.

9. Why is meiosis referred to as “reduction division,” and how does this process contribute to the life cycle of sexually reproducing organisms?

Answer: Meiosis is called reduction division because it reduces the chromosome number by half, from a diploid state (2n) to a haploid state (n). This reduction is accomplished during Meiosis I, when homologous chromosomes are segregated into two different cells. This process is fundamental to the life cycle of sexually reproducing organisms. It ensures the production of haploid gametes (e.g., sperm and egg). When these haploid gametes fuse during fertilization, the diploid chromosome number is restored in the zygote, thereby maintaining the specific chromosome count for that species across generations. Without this reduction, the chromosome number would double with each generation.

10. Describe the structure of a chromosome during Metaphase of Mitosis and Metaphase I of Meiosis.

Answer: During Metaphase of Mitosis, a single chromosome is at its most condensed state. It consists of two identical sister chromatids joined together by a single centromere. The chromosome aligns individually at the metaphase plate, and spindle fibers from opposite poles attach to the kinetochores on either side of the centromere. During Metaphase I of Meiosis, the structure is a bivalent (or tetrad). This structure consists of a pair of homologous chromosomes aligned together at the metaphase plate. Each homologous chromosome within the pair still consists of two sister chromatids. Therefore, the bivalent is composed of four chromatids in total. Microtubules from one pole attach to the kinetochore of one homologous chromosome, while microtubules from the opposite pole attach to the kinetochore of the other homologous chromosome.

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Glossary of Key Terms

Term	Definition
Anaphase	The stage of mitosis or meiosis when chromosomes move toward opposite poles of the cell. In mitosis and meiosis II, sister chromatids separate; in meiosis I, homologous chromosomes separate.
Asters	Microtubules that radiate out from each centrosome during prophase.
Bivalent	A pair of synapsed homologous chromosomes, consisting of four chromatids, formed during Prophase I of meiosis. Also called a tetrad.
Cell Cycle	The sequence of events by which a cell duplicates its genome, synthesizes its other constituents, and eventually divides into two daughter cells.
Cell Plate	A precursor structure formed in the center of a plant cell during cytokinesis that grows outward to form a new cell wall between the daughter cells.
Centromere	The region of a chromosome where the two sister chromatids are attached.
Chiasmata	X-shaped structures visible during the diplotene stage of Prophase I, representing the sites where crossing over has occurred between homologous chromosomes.
Chromatid	One of two identical sister parts of a duplicated chromosome.
Crossing Over	The exchange of genetic material between non-sister chromatids of homologous chromosomes during Pachytene of Prophase I.
Cytokinesis	The division of the cytoplasm to form two separate daughter cells, which usually occurs after karyokinesis.
Diakinesis	The final stage of Prophase I, marked by terminalisation of chiasmata and breakdown of the nuclear envelope.
Diplotene	The stage of Prophase I following pachytene, where the synaptonemal complex dissolves and chiasmata become visible.
Equational Division	A term for mitosis, as the chromosome number of the parent cell is conserved in the daughter cells.
G₀ Phase	A quiescent, non-dividing stage that cells can enter from the G₁ phase. Cells in G₀ are metabolically active but do not proliferate.
G₁ Phase (Gap 1)	The first growth phase of the interphase in the cell cycle, occurring between mitosis and the S phase.
G₂ Phase (Gap 2)	The second growth phase of the interphase, occurring after the S phase and before mitosis.
Homologous Chromosomes	A pair of chromosomes (one from each parent) that are similar in shape and size and carry genes for the same traits.
Interkinesis	The short-lived stage between Meiosis I and Meiosis II, during which DNA replication does not occur.
Interphase	The preparatory phase of the cell cycle between two successive M phases, consisting of G₁, S, and G₂ phases.
Karyokinesis	The division of a cell’s nucleus.
Kinetochores	Small, disc-shaped protein structures at the surface of the centromeres that serve as attachment points for spindle fibers.
Leptotene	The first stage of Prophase I, during which chromosomes begin to condense and become visible.
M Phase	The phase of the cell cycle where actual cell division (mitosis or meiosis) occurs.
Meiosis	A specialized type of cell division that reduces the chromosome number by half, creating four haploid cells from one diploid cell.
Metaphase	The stage of cell division in which chromosomes align at the equatorial plate of the cell.
Metaphase Plate	The plane in the center of the cell where chromosomes align during metaphase.
Mitosis	A process of nuclear division in eukaryotic cells that results in two daughter nuclei each having the same number of chromosomes as the parent nucleus.
Pachytene	The stage of Prophase I where crossing over occurs between homologous chromosomes.
Prophase	The first stage of cell division, during which the chromosomes become visible as paired chromatids and the nuclear envelope disappears.
Recombinase	The enzyme that mediates the process of crossing over.
Reduction Division	A term for Meiosis I, as it reduces the chromosome number from diploid to haploid.
S Phase (Synthesis)	The phase of the cell cycle in which DNA is replicated.
Spindle Fibres	Microtubule structures that are involved in moving chromosomes during cell division.
Synapsis	The pairing of homologous chromosomes during the zygotene stage of Prophase I.
Synaptonemal Complex	A protein structure that forms between homologous chromosomes during synapsis and is thought to mediate crossing over.
Syncytium	A multinucleate cell that can result from multiple nuclear divisions without accompanying cytokinesis.
Telophase	The final stage of mitosis or meiosis, in which the separated chromosomes arrive at the poles and new nuclear envelopes form.
Tetrad	A structure containing four chromatids that forms during Meiosis I (another name for a bivalent).
Zygotene	The stage of Prophase I during which synapsis of homologous chromosomes occurs.