Cell Cycle And Cell Division Class 11 Biology Chapter 10 Notes

Cell Cycle and Cell Division Class 11 Biology Chapter 10 Notes

Cell Cycle

Importance of Cell Division:
- Essential process in all living organisms.
- Involves DNA replication and cell growth.
- Ensures coordinated processes for correct division and intact genome transmission.
Definition of Cell Cycle:
- The sequence of events that includes genome duplication, synthesis of cell components, and cell division.
- Ensures the formation of two daughter cells.
Key Components of the Cell Cycle:
1. Cell Growth:
  - Ongoing process involving cytoplasmic enlargement.
2. DNA Replication:
  - Occurs during a specific stage in the cell cycle.
  - Ensures the duplication of genetic material.
3. Cell Division:
  - Process where a cell splits into two daughter cells.
  - Involves the distribution of replicated chromosomes to daughter nuclei.
Genetic Control:
- Events in the cell cycle are regulated by genetic mechanisms.
- Ensures the precise timing and coordination of each phase.

Phases of the Cell Cycle

Overview:
- Eukaryotic cell cycles vary in duration among organisms and cell types.
- Divided into two main phases: Interphase and M Phase (Mitosis).
Interphase:
- Predominates the cell cycle, lasting more than 95% of the time.
- Further divided into three phases:
  1. G1 Phase (Gap 1):
    - Occurs between mitosis and DNA replication.
    - Cell is metabolically active and grows without DNA replication.
  2. S Phase (Synthesis):
    - DNA synthesis and replication occur.
    - DNA content doubles (from 2C to 4C), but chromosome number remains the same.
  3. G2 Phase (Gap 2):
    - Protein synthesis for mitosis preparation.
    - Cell growth continues.
M Phase (Mitosis Phase):
- Represents the phase of actual cell division.
- Involves two key processes:
  1. Karyokinesis: Nuclear division, separating daughter chromosomes.
  2. Cytokinesis: Cytoplasm division, completing cell division.
G0 Phase (Quiescent Stage):
- Some adult animal cells (e.g., heart cells) and others divide only as needed.
- Cells exit G1 phase into an inactive G0 stage.
- Metabolically active but do not proliferate unless required.
Mitotic Division in Animals:
- Typically seen in diploid somatic cells.
- Exceptions include haploid cell division, as in male honey bees.
Mitotic Divisions in Plants:
- Plants exhibit mitotic divisions in both haploid and diploid cells.
- Alternation of generations in plants involves the following stages with mitosis in haploid cells:
  - Gametophyte Stage: Haploid generation with mitotic divisions to produce gametes (e.g., pollen and ovule).
  - Sporophyte Stage: Diploid generation with mitotic divisions to produce spores (e.g., fern sporophytes).

M Phase (Mitosis Phase)

The most dramatic phase of the cell cycle.
Major reorganization of cell components occurs.
Called “equational division” because the number of chromosomes in parent and progeny cells remains the same.

Mitosis Stages (Karyokinesis):
- Mitosis is divided into four stages, but it’s important to note that it’s a continuous process without clear-cut boundaries between these stages.
1. Prophase:
  - The first stage of mitosis.
  - Chromosomes condense and become visible.
  - Nuclear envelope begins to break down.
  - Spindle fibers form and extend from pole to pole.
2. Metaphase:
  - Chromosomes align at the cell’s equatorial plane (metaphase plate).
  - Spindle fibers attach to the centromeres of each chromosome.
  - Chromosomes are now maximally condensed.
3. Anaphase:
  - Sister chromatids are separated and pulled toward opposite poles.
  - Centromeres split, allowing each chromatid to become an individual chromosome.
  - Ensures that each daughter cell receives an identical set of chromosomes.
4. Telophase:
  - Marks the final stage of mitosis.
  - Chromatids (now individual chromosomes) arrive at opposite poles.
  - Chromosomes begin to decondense and return to their extended form.
  - A new nuclear envelope forms around each set of chromosomes.
  - The division of the cytoplasm (cytokinesis) typically begins during or after telophase.

Prophase (Mitosis Phase)

Position in the Cell Cycle:
- Prophase is the first stage of karyokinesis in mitosis and follows the S and G2 phases of interphase.
Key Events in Prophase:
1. Chromosomal Condensation:
  - Chromosomal material begins to condense.
  - Distinct chromosomes become visible as chromatin fibers coil and tangle.
  - Chromosomes are composed of two chromatids attached at the centromere.
2. Centrosome Duplication and Movement:
  - Centrosome, which duplicated during the interphase (S phase), starts to move toward opposite poles of the cell.
  - Each centrosome radiates microtubules called asters.
  - The combined microtubules and spindle fibers form the mitotic apparatus.
Completion of Prophase:
- Prophase is marked by the following characteristic events:
  - Chromosomal material condenses to form compact mitotic chromosomes with distinct chromatids.
  - Centrosomes move towards opposite poles, contributing to spindle formation.
  - The two asters (microtubule arrays) and spindle fibers together constitute the mitotic apparatus.
  - Golgi complexes, endoplasmic reticulum, nucleolus, and the nuclear envelope are no longer visible in cells at the end of prophase when viewed under a microscope.

Metaphase (Mitosis Phase)

Commencement of Metaphase:
- Metaphase is the second phase of mitosis and begins with the complete disintegration of the nuclear envelope.
- Chromosomes are dispersed throughout the cell’s cytoplasm.
Chromosomal Condensation:
- During metaphase, chromosomal condensation is complete.
- Chromosomes are now highly visible and can be easily studied under a microscope.
- Each metaphase chromosome consists of two sister chromatids connected by the centromere.
Kinetochore Structure:
- Small disc-shaped structures, called kinetochores, are found on the surface of the centromeres of metaphase chromosomes.
- Kinetochore serves as the site of attachment for spindle fibers, which are formed by the spindle apparatus.
Chromosome Alignment:
- Metaphase is characterized by all the chromosomes aligning at the cell’s equatorial plane, known as the metaphase plate.
- Each chromatid of a chromosome is connected by its kinetochore to spindle fibers extending from one pole, and its sister chromatid is connected to spindle fibers from the opposite pole.
- This alignment ensures equal distribution of chromosomes to the daughter cells during the subsequent stages of mitosis.
Key Features of Metaphase:
- Spindle fibers attach to the kinetochores of chromosomes.
- Chromosomes move to the spindle equator and align along the metaphase plate, held in place by spindle fibers extending to both poles.

Anaphase (Mitosis Phase)

Initiation of Anaphase:
- Anaphase is the third phase of mitosis and follows metaphase.
- It begins with a critical event where each chromosome, aligned at the metaphase plate, is split simultaneously.
Key Events in Anaphase:
- Centromere Split and Chromatid Separation:
  - The centromeres of each chromosome split, allowing the two sister chromatids to separate.
  - These chromatids are now referred to as daughter chromosomes.
- Chromatid Movement:
  - Daughter chromosomes, previously sister chromatids, commence their migration toward opposite poles of the cell.
  - As chromosomes move away from the equatorial plate, the centromere of each chromosome remains directed towards the pole.
  - The arms of the chromosome trail behind the centromere.
Characteristics of Anaphase:
- Anaphase is characterized by the following key features:
  - The centromeres split, resulting in the separation of chromatids.
  - Daughter chromosomes (formerly sister chromatids) move toward opposite poles.
  - Centromeres lead the way as chromatids migrate.

Telophase (Mitosis Phase)

Commencement of Telophase:
- Telophase is the final stage of karyokinesis in mitosis, following anaphase.
- It signifies the near completion of the process of cell division.
Key Events in Telophase:
- Chromosomal Decondensation:
  - In telophase, the chromosomes that have migrated to their respective poles start to decondense.
  - They lose their individuality, and individual chromosomes are no longer visible.
- Chromosome Clustering:
  - Chromatin material begins to collect at each of the two opposite spindle poles.
  - The chromosomes cluster together, and their identity as discrete elements is lost.
- Nuclear Envelope Formation:
  - A new nuclear envelope starts to develop around the chromosome clusters at each pole.
  - This process leads to the formation of two daughter nuclei, each containing a set of chromosomes.
- Reformation of Cellular Structures:
  - In telophase, cellular structures that were disassembled during earlier stages, such as the nucleolus, golgi complex, and endoplasmic reticulum, reform.
  - The reformation of these structures prepares the cell for its return to normal functioning.
Characteristics of Telophase:
- Telophase is characterized by:
  - Chromosomes clustering at opposite spindle poles and losing their individuality.
  - The development of a nuclear envelope around the chromosome clusters, resulting in the formation of two daughter nuclei.
  - The reformation of cellular structures that were temporarily disassembled.

Cytokinesis

Cytokinesis is the final step of the cell division process, following karyokinesis (mitosis), where the cell is physically divided into two daughter cells.
It ensures that each daughter cell receives a portion of the cytoplasm, organelles, and other cell contents.

Animal Cell Cytokinesis:
- In animal cells, cytokinesis is achieved through the formation of a cleavage furrow in the plasma membrane.
- The furrow gradually deepens and eventually meets at the center, leading to the physical separation of the cell cytoplasm into two daughter cells.
Plant Cell Cytokinesis:
- Plant cells have a relatively inextensible cell wall, so cytokinesis occurs through a different mechanism.
- In plant cells, wall formation starts in the center of the cell and grows outward to meet the existing lateral walls.
- It begins with the formation of a simple precursor known as the “cell plate,” which represents the middle lamella between the walls of two adjacent cells.
- As the cell plate expands and the new cell wall forms, it ultimately divides the cytoplasm into two distinct daughter cells.
Distribution of Organelles:
- During cytokinesis, organelles such as mitochondria and plastids are distributed between the two daughter cells.
Exceptions and Multinucleate Conditions:
- In some organisms, karyokinesis is not followed by cytokinesis, resulting in a multinucleate condition.
- A syncytium, such as the liquid endosperm in coconuts, forms where multiple nuclei coexist within a shared cytoplasm.

Significance of Mitosis

Mitosis is a fundamental process in cell biology and plays several crucial roles in the life of an organism. Here are some of the key significance of mitosis:

Maintenance of Ploidy:
- Mitosis ensures that the ploidy level of the daughter cells is identical to that of the parent cell. This is essential for the stability of the genetic complement.
- In some lower plants and certain social insects, haploid cells also divide by mitosis.
Growth of Multicellular Organisms:
- Mitosis is responsible for the growth and development of multicellular organisms.
- As an organism grows, the number of cells increases through mitotic divisions.
Restoration of Nucleo-Cytoplasmic Ratio:
- As cells grow, the ratio between the nucleus and the cytoplasm may become imbalanced.
- Mitosis helps restore the nucleo-cytoplasmic ratio by dividing the cell into two smaller, but equally proportioned, daughter cells.
Cell Repair:
- Mitosis plays a crucial role in cell repair and tissue maintenance.
- Cells that are damaged, old, or worn out can be replaced through mitotic divisions.
- For example, the cells of the upper layer of the epidermis, the lining of the gut, and blood cells are continuously replaced through mitosis.
Plant Growth and Development:
- In plants, mitotic divisions in meristematic tissues (such as the apical and lateral cambium) result in continuous growth throughout the life of the plant.
- These meristematic tissues are responsible for the formation of new cells, leading to the elongation and branching of plant structures.
Asexual Reproduction:
- Mitosis is the basis of asexual reproduction in some organisms.
- Organisms like hydra and some fungi reproduce asexually through mitotic division.
Tissue Regeneration:
- Mitosis is essential for tissue regeneration and wound healing in organisms.
- In response to injuries, cells can undergo mitotic divisions to replace damaged or lost tissue.
Genetic Stability:
- Mitosis ensures that the genetic information is stably maintained and passed on to daughter cells.
- It plays a crucial role in maintaining genetic fidelity during cell division.

Meiosis

Meiosis is a specialized form of cell division that is essential for sexual reproduction in eukaryotes. It results in the formation of haploid daughter cells, which are necessary for the fusion of gametes during fertilization. Here are the key features and phases of meiosis:

Production of Haploid Daughter Cells:
- Meiosis reduces the chromosome number by half, resulting in the formation of haploid daughter cells.
- Haploid cells have a single set of chromosomes, which is essential for sexual reproduction.
Phases of Meiosis:
- Meiosis involves two sequential cycles of nuclear and cell division, known as meiosis I and meiosis II.
- There is only one cycle of DNA replication before meiosis begins.
Meiosis I:
- Meiosis I starts after parental chromosomes have replicated during the S phase.
- During meiosis I, homologous chromosomes pair up and exchange genetic material through recombination between non-sister chromatids.
- At the end of meiosis I, two haploid daughter cells are produced, each with a unique combination of genetic material.
Meiosis II:
- Meiosis II follows meiosis I and resembles a mitotic division but with haploid cells.
- In meiosis II, sister chromatids of each chromosome are separated, resulting in the formation of four haploid cells.
Key Events in Meiosis:
- Prophase I (Meiosis I): Homologous chromosomes pair up and undergo recombination. The nuclear envelope may break down.
- Metaphase I (Meiosis I): Homologous chromosome pairs align at the metaphase plate.
- Anaphase I (Meiosis I): Homologous chromosomes are pulled apart and move to opposite poles.
- Telophase I (Meiosis I): Two haploid daughter cells are formed, each with half the original chromosome number.
- Prophase II (Meiosis II): Chromosomes condense again, and a new spindle apparatus is formed in each haploid cell.
- Metaphase II (Meiosis II): Chromosomes align at the metaphase plate in each haploid cell.
- Anaphase II (Meiosis II): Sister chromatids are separated and move to opposite poles in each haploid cell.
- Telophase II (Meiosis II): Four haploid daughter cells are produced, each with a unique combination of genetic material.
Role in Sexual Reproduction:
- Meiosis ensures the formation of haploid gametes, which are crucial for sexual reproduction.
- Fertilization, the fusion of two gametes, restores the diploid phase, completing the life cycle of sexually reproducing organisms.

1. Meiosis I

Meiosis I is the first division in the meiotic process, which is essential for the formation of haploid cells. It consists of several phases, each with specific events:

Prophase I:
- Leptotene: Chromosomes gradually become visible under a light microscope, and compaction continues.
- Zygotene: Chromosomes start pairing together in a process called synapsis. Paired chromosomes are homologous chromosomes.
- Pachytene: The four chromatids of each bivalent chromosome become distinct and appear as tetrads. Recombination nodules form, sites where crossing over occurs between non-sister chromatids.
- Diplotene: Synaptonemal complex dissolves, and recombined homologous chromosomes tend to separate from each other, except at the sites of crossovers. These sites are called chiasmata, and they represent genetic recombination points.
- Diakinesis: Chiasmata are terminalized, and chromosomes fully condense. The meiotic spindle assembles, the nucleolus disappears, and the nuclear envelope breaks down, transitioning to metaphase I.
Metaphase I:
- Bivalent chromosomes align on the equatorial plate.
- Microtubules from opposite spindle poles attach to the kinetochores of homologous chromosomes.
Anaphase I:
- Homologous chromosomes separate and move to opposite poles.
- Sister chromatids remain associated at their centromeres.
Telophase I:
- The nuclear membrane and nucleolus reappear.
- Cytokinesis occurs, dividing the cell into two daughter cells, known as dyads.
- Although the chromosomes may undergo some dispersion, they do not reach the extended state of the interphase nucleus.
- The stage between meiotic divisions is called interkinesis, which is typically short-lived and involves no DNA replication.
- Interkinesis is followed by prophase II, a simpler prophase compared to prophase I.

2. Meiosis II

Meiosis II is the second division in the meiotic process, following meiosis I. It is initiated immediately after cytokinesis at the end of meiosis I. Meiosis II is similar to mitosis in that it results in the separation of sister chromatids. Here are the key phases of meiosis II:

Prophase II:
- Meiosis II begins immediately after cytokinesis following meiosis I.
- Unlike the complex prophase of meiosis I, prophase II resembles a typical mitotic prophase.
- The nuclear envelope disappears, and chromosomes condense again.
Metaphase II:
- Chromosomes align at the equator of the cell, similar to metaphase in mitosis.
- Microtubules from opposite poles of the spindle apparatus attach to the kinetochores of sister chromatids, just as in mitosis.
Anaphase II:
- Anaphase II begins with the simultaneous splitting of the centromere of each chromosome, which holds the sister chromatids together.
- This separation allows the sister chromatids to move toward opposite poles of the cell, guided by the shortening of microtubules attached to their kinetochores.
Telophase II:
- Meiosis concludes with telophase II.
- The two groups of chromosomes are once again enclosed by a nuclear envelope in each of the resulting daughter cells.
- Cytokinesis follows, leading to the formation of four haploid daughter cells. Each daughter cell contains a unique combination of genetic material, similar to the end of meiosis I.

Significance of Meiosis

Meiosis is a crucial process in sexually reproducing organisms, and its significance lies in several key aspects:

Conservation of Chromosome Number:
- Meiosis ensures that the specific chromosome number characteristic of each species is maintained across generations.
- Even though meiosis reduces the chromosome number by half, this reduction is essential to produce haploid gametes for sexual reproduction.
- When two gametes fuse during fertilization, the diploid chromosome number is restored in the zygote, preserving the species-specific chromosome number.
Genetic Variability:
- Meiosis leads to an increase in genetic variability in the population of organisms.
- During meiosis, homologous chromosomes undergo recombination through the process of crossing over. This exchange of genetic material between homologous chromosomes results in new combinations of alleles.
- The random assortment of chromosomes during meiosis further contributes to genetic diversity. This means that each gamete is unique in terms of the genetic information it carries.
Importance for Evolution:
- Variations generated through meiosis are crucial for the process of evolution.
- Genetic diversity resulting from meiosis provides a pool of different genetic combinations within a population.
- This diversity allows for natural selection and adaptation to changing environments over generations, leading to the evolution of species.
Elimination of Harmful Mutations:
- Meiosis can help eliminate potentially harmful mutations.
- In some cases, meiosis may detect and segregate deleterious mutations, reducing their impact on the overall genetic makeup of the population.
Adaptive Advantage:
- Meiosis contributes to the adaptive advantage of sexual reproduction.
- The genetic variability generated by meiosis allows for a greater potential to adapt to environmental changes, compared to asexual reproduction, which results in genetically identical offspring.