Cell Cycle and Cell Division - Class 11 Biology - Chapter 10 - Notes, NCERT Solutions & Extra Questions
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Extra Questions - Cell Cycle and Cell Division | NCERT | Biology | Class 11
The cells of meristems are characterized by:
A. continuous division
B. large vacuoles
C. chloroplasts
D. all of the above
The correct answer is A. continuous division.
Meristematic cells, which are involved in plant growth and development, are primarily characterized by their ability to divide continuously. These cells typically exhibit several key features:
Cell walls that are thin and primarily composed of cellulose.
Dense cytoplasm along with large, prominent nuclei.
Absence of vacuoles, which contrasts with mature plant cells that often contain large vacuoles.
Lack of intercellular spaces, contributing to a compact cell structure.
Although chloroplasts are essential for photosynthesis, they are generally present in specialized tissues such as parenchyma and collenchymaāreferred to as chlorenchymaānot in meristematic cells. Therefore, options B and C are not correct for meristematic cells, focusing the correct choice on option A.
These processes are necessary for the complete development of male gametophyte from Pollen mother cell:
A) One meiotic and two mitotic divisions.
B) One meiotic cell division and one mitotic cell division.
C) Two meiotic cell divisions and one mitotic cell division.
D) Two mitotic cell divisions.
The correct answer is Option A: One meiotic and two mitotic divisions.
This option accurately describes the process required for the development of the male gametophyte from the Pollen mother cell. Specifically, one meiotic division is needed to produce haploid cells from the diploid Pollen mother cell. Following this, two mitotic divisions are necessary to form the mature male gametophyte.
What is the cell division type occurring during the formation of a sperm?
The type of cell division involved in the formation of sperm is called spermatogenesis. This process occurs in the male gonad from the spermatogonial stem cells. It comprises two distinct stages of cell division: mitosis and meiosis.
The process starts with spermatogonia, which divide mitotically to produce primary spermatocytes. These primary spermatocytes then undergo the first meiotic division, known as Meiosis I, which results in two secondary spermatocytes. Each secondary spermatocyte subsequently undergoes Meiosis II to produce two spermatids, totaling four spermatids per each primary spermatocyte. These spermatids develop into mature sperm cells, also referred to as spermatozoa. Therefore, the primary spermatocyte initially gives rise to four sperm cells through a combination of both mitotic and meiotic divisions.
Cell wall synthesis is the final step in the formation of daughter cells in many bacteria.
A) True
B) False
The correct answer is A) True.
Bacteria replicate through a process known as binary fission, which requires precise timing and spatial organization to ensure that each daughter cell receives a proper set of genetic materials, primarily DNA. Within this process, the bacterial cell duplicates its DNA and divides the copies to opposite ends of the cell. The crucial final step in this sequence is cell wall synthesis. This step is essential for separating the newly formed daughter cells, ensuring that they each have their complete cell structures and are capable of surviving and growing independently.
Which stages of cell division do the following figures $A$ and $B$ represent, respectively?
A
B
A Metaphase, Telophase
B Telophase, Metaphase
C Late Anaphase, Prophase
D Prophase, Anaphase
The correct answer is C: Late Anaphase, Prophase.
Figure A illustrates Late Anaphase. In this stage, the sister chromatids separate and move towards opposite ends (poles) of the cell. The spindle fibers that are not attached to chromatids extend, helping to elongate the cell. At the conclusion of anaphase, each pole has its own complete set of chromosomes.
Figure B represents Prophase. During this stage, centrioles migrate to opposite sides of the nucleus and the chromosomes begin to condense and shorten.
For additional clarity:
Metaphase is the cell division phase that comes after prophase and before anaphase, characterized by chromosomes attaching to the spindle fibers.
Telophase is the final phase of cell division that occurs between anaphase and interphase, during which the chromatids or chromosomes move to opposite ends of the cell, resulting in the formation of two nuclei.
Once mitosis is complete, the cells go through a phase in which the cytoplasm in the cells increases. What is this phase called?
A S phase
B R phase
C G1 phase
D $G_2$ phase
The correct answer is C G1 phase.
G1 phase is crucial because it enables the new cells, formed after mitosis, to grow and increase in size to match the size of the parent cells. This period of growth is fundamental for the cells to function efficiently and prepare for the next cell cycle.
Select the correct statement about $\mathrm{G}_{1}$ phase:
A. Cell is metabolically inactive.
B. DNA in the cell does not replicate.
C. It is not a phase of synthesis of macromolecules.
D. Cell stops growing.
The correct answer is B: DNA in the cell does not replicate.
During Interphase ā a significantly extended time where a cell is preparing for division ā it experiences distinct subphases, which are the $\mathrm{G}_1$, $\mathrm{S}$, and $\mathrm{G}_2$ phases.
In the $\mathrm{G}_1$ phase, the cell is metabolically active, engaging in the synthesis of various cellular contents, leading to growth in cell size. This includes the production of macromolecules such as RNA and proteins. It is crucial to note that during this phase, there is no DNA replication.
The actual doubling of DNA content occurs during the $\mathrm{S}$ phase (Synthesis phase).
Following this, in the $\mathrm{G}_2}$ phase, the cell synthesizes proteins necessary for cell division.
Therefore, the statement B is correct as it emphasizes that DNA replication does not take place during the $\mathrm{G}_1$ phase.
Which is the first step of cell division?
A. Division of nucleus
B. Division of genetic material
C. Division of cytoplasm
D. Division of the entire cell
The correct answer is B. Division of genetic material.
Cell division is the process by which a parent cell divides into two daughter cells. The first crucial step in this process involves the division of genetic material within the nucleus. This ensures that each new nucleus will possess an equal amount of genetic information. Following this, the nucleus itself divides. Subsequently, the cytoplasm divides, finally leading to the division of the entire cell into two separate daughter cells.
In mitosis, the number of chromosomes is maintained in the cell.
A) True
B) False
The correct answer is A) True.
In mitosis, the process involves an equal division that maintains the number of chromosomes within the cell. This contrasts with meiosis, where the chromosome number is reduced to half.
During G1 phase of cell cycle, cell increases
A) supply of oxygen
B) supply of carbon dioxide
C) supply of water
D) supply of proteins
The correct answer is D) supply of proteins.
The G1 phase is one of the stages of the cell cycle in interphase, which also includes the S phase and G2 phase. The primary function of the G1 phase is to prepare the cell for DNA replication in the subsequent S phase. During this time, the cell increases in size and is actively involved in the synthesis of mRNA and proteins, specifically histones, which are essential for DNA synthesis. Thus, the focus on the supply of proteins is crucial during the G1 phase of the cell cycle.
How many times does the Krebs cycle turn per glucose molecule?
A. once
B. twice
C. three times
D. four times
The correct answer is B. twice.
The Krebs cycle turns twice per glucose molecule. This is attributed to the fact that during glycolysis, one molecule of glucose is broken down into 2 molecules of pyruvic acid. Each of these molecules of pyruvic acid then enters the Krebs cycle separately, leading to two complete turns of the cycle for each original glucose molecule.
Chromosome number in the daughter cells after meiosis is -
A) 1/2
B) 1/3
C) 1/4
D) 1/5
Meiosis is a type of cell division that reduces the chromosome number by half. This reduction is crucial for sexual reproduction and contributes to genetic diversity.
During the process of meiosis:
A single cell divides twice to produce four daughter cells.
Each of these daughter cells contains half the chromosome number of the original parent cell.
In humans, for example, each cell initially contains 46 chromosomes (23 pairs). After meiosis, each daughter cell will have 23 chromosomes - precisely half the number of chromosomes, making these cells haploid.
From the provided information and explanation, it's clear that after meiosis, the chromosome number in the daughter cells is specifically reduced to half compared to the original parent cell. Therefore, the correct answer to the question "Chromosome number in the daughter cells after meiosis is" should be:
A) 1/2
This reflects the haploid nature of the daughter cells post-meiosis, carrying half the chromosome number of the diploid parent cell.
DNA replication (synthesis) occurs in
A. G1-phase
B. S-phase
C. G2 phase
D. M phase
DNA replication, also known as synthesis of DNA, takes place during cell cycle events. The cell cycle includes different phases, primarily divided into interphase and the mitotic phase. Interphase itself is divided into three parts: G1 phase, S-phase, and G2 phase.
G1 Phase: This is the first part of interphase, where the cell grows and prepares for DNA replication. However, actual DNA synthesis does not occur here.
S-Phase (Synthesis Phase): This is the critical phase where DNA replication happens. During S-phase, each chromosome's DNA molecule is duplicated, which is essential for genetic material to be properly distributed to daughter cells during cell division. The term S-phase stands for synthesis because this is when DNA synthesis occurs.
G2 Phase: Following the S-phase, the cell enters G2. This phase involves further cell growth and preparation for mitosis (M phase), but no DNA replication occurs here.
M Phase: During this phase, the actual cell division (mitosis) takes place. This phase does not involve DNA replication.
Considering the choices provided and understanding the stages of the cell cycle, the correct answer where DNA replication occurs is:
B. S-phase
Which of the following is not a divisional stage:
A. Telophase
B. Prophase
C. Metaphase
D. Interphase
The stages of cell division (either in mitosis or meiosis) include Prophase, Metaphase, Anaphase, and Telophase. Each of these stages is part of the active division process wherein the chromosomes divide and the cell prepares to split into daughter cells.
Interphase, which is also listed among the options, is in fact not a stage of division itself but rather a preparatory phase where the cell grows, duplicates its DNA, and prepares for mitosis or meiosis.
Therefore, option D (Interphase) is the correct answer because it is not a divisional stage but rather a stage of preparation for division.
Which of the following plant hormones induces cell division?
A) Auxin
B) Gibberellin
C) Ethylene
D) Cytokinin.
Auxin is primarily involved in the growth regulation of plants, influencing cell elongation primarily in root and shoot tips rather than cell division.
Gibberellins are plant hormones that mainly promote elongation in parts of the plants such as stems and assist in the growth processes, not specifically in initiating new cell division.
Ethylene is known for its role in the aging process of plants and in the ripening of fruits, rather than affecting cell division processes directly.
Cytokinins, however, are crucial for cell division. They stimulate cells to divide and form new cells and tissues, thus facilitating the overall growth and development of the plant.
So, the correct answer is: D) Cytokinin
Cytokinin is the hormone that specifically induces cell division, leading to the formation of new tissues and supporting the further growth of the plant.
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Ask Chatterbot AINCERT Solutions - Cell Cycle and Cell Division | NCERT | Biology | Class 11
What is the average cell cycle span for a mammalian cell?
The average cell cycle span for a mammalian cell is approximately 24 hours.
Distinguish cytokinesis from karyokinesis.
Cytokinesis and karyokinesis are two distinct processes in cell division:
Karyokinesis: This refers to the division of the nucleus. It involves the separation of the duplicated chromosomes into two daughter nuclei. Karyokinesis is subdivided into four stages: prophase, metaphase, anaphase, and telophase.
Cytokinesis: This refers to the division of the cytoplasm. It occurs after karyokinesis and results in the formation of two separate daughter cells. In animal cells, a furrow forms in the plasma membrane that deepens to split the cell, while in plant cells, a cell plate forms to divide the cell.
Describe the events taking place during interphase.
Interphase, which is often referred to as the "resting phase," is actually a period of vigorous cellular activity in preparation for cell division. The events during interphase can be divided into three phases:
Gā Phase (Gap 1):
Cell growth: The cell grows in size.
Metabolic activity: The cell is metabolically active and synthesizes various enzymes and nutrients necessary for DNA replication.
Preparation for DNA replication: Although DNA replication does not occur in this phase, the cell prepares the necessary machinery for it.
S Phase (Synthesis):
DNA replication: The cell's DNA is replicated, resulting in the doubling of DNA content from 2C to 4C.
Centrosome duplication: In animal cells, the centrosomes also duplicate.
Gā Phase (Gap 2):
Continued cell growth: The cell continues to grow.
Protein synthesis: Proteins and other molecules required for mitosis are synthesized.
Preparation for mitosis: The cell accumulates energy and completes the formation of components required for mitosis.
During interphase, the cell ensures that DNA replication and cell growth occur in a coordinated manner to facilitate the accurate division and formation of progeny cells.
What is $\mathrm{G}_{\mathrm{o}}$ (quiescent phase) of cell cycle?
The $\mathrm{G}_{0}$ phase (quiescent phase) is an inactive stage of the cell cycle where cells exit the $\mathrm{G}_{1}$ phase. Here, the cells remain metabolically active but stop proliferating. They do not divide unless called on to do so, based on the organism's requirements, such as in response to injury or cell death. Examples include heart cells and other cells that divide only occasionally.
Why is mitosis called equational division?
Mitosis is called equational division because it results in the production of two genetically identical daughter cells with the same number of chromosomes as the parent cell. In other words, the chromosome number remains unchanged, ensuring that both the parent cell and the daughter cells have the same genetic content.
Name the stage of cell cycle at which one of the following events occur:
(i) Chromosomes are moved to spindle equator.
(ii) Centromere splits and chromatids separate.
(iii) Pairing between homologous chromosomes takes place.
(iv) Crossing over between homologous chromosomes takes place.
(i) Metaphase - Chromosomes are moved to the spindle equator.
(ii) Anaphase - Centromere splits and chromatids separate.
(iii) Zygotene of Prophase I (during Meiosis I) - Pairing between homologous chromosomes takes place.
(iv) Pachytene of Prophase I (during Meiosis I) - Crossing over between homologous chromosomes takes place.
Describe the following:
(a) synapsis
(b) bivalent
(c) chiasmata
Draw a diagram to illustrate your answer.
(a) Synapsis
Synapsis refers to the homologous chromosomes pairing together during zygotene stage of prophase I in meiosis. This pairing process is called synapsis and is crucial for crossing over.
(b) Bivalent
A bivalent (or tetrad) is the structure formed by two homologous chromosomes that have paired during synapsis in prophase I of meiosis. Each bivalent consists of a pair of chromosomes, each with two sister chromatids, thus a total of four chromatids.
(c) Chiasmata
Chiasmata (singular: chiasma) are the X-shaped structures that are seen during the diplotene stage of prophase I where crossing over has occurred. These points of contact are where genetic material is exchanged between the chromatids of homologous chromosomes.
How does cytokinesis in plant cells differ from that in animal cells?
Cytokinesis in plant cells and animal cells differs primarily in the mechanism by which the cytoplasm is divided:
In Animal Cells:
Furrowing occurs, where a cleavage furrow forms.
The furrow deepens gradually.
Eventually, the furrow joins at the center, dividing the cell cytoplasm into two daughter cells.
In Plant Cells:
Due to the presence of an inextensible cell wall, plant cells cannot use furrowing.
Instead, a cell plate forms at the center of the dividing cell.
The cell plate grows outward to meet the existing lateral walls, forming a new cell wall.
The precursor to the new cell wall is called the cell-plate, which represents the middle lamella between the adjacent cells.
Thus, while animal cells use a cleavage furrow to achieve cytokinesis, plant cells form a cell plate that develops into separate cell walls.
Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size.
Example where the four daughter cells from meiosis are equal in size:
Plants: In plants, particularly in pollen formation (male gametogenesis), the four microspores produced from meiosis are typically equal in size. These microspores will eventually develop into pollen grains.
Example where the four daughter cells from meiosis are unequal in size:
Animals (Oogenesis): In many animals, during oogenesis (female gametogenesis), meiosis produces one large ovum (egg) and three smaller polar bodies. The polar bodies are significantly smaller in size and generally do not participate in fertilization.
Distinguish anaphase of mitosis from anaphase I of meiosis.
Anaphase of Mitosis:
Chromatid Separation: During mitotic anaphase, the centromeres split, and the sister chromatids separate becoming individual daughter chromosomes.
Chromosome Movement: The separated chromatids, now individual chromosomes, move to opposite poles of the cell.
Anaphase I of Meiosis:
Homologous Chromosome Separation: Unlike mitosis, anaphase I involves the separation of homologous chromosomes, not sister chromatids.
Chromatid Cohesion: During anaphase I, the sister chromatids remain attached at their centromeres and move together to the same pole.
In essence, mitotic anaphase separates sister chromatids, while meiosis I anaphase separates homologous chromosomes.
List the main differences between mitosis and meiosis.
Feature | Mitosis | Meiosis |
---|---|---|
Type of Division | Equational Division: Produces two genetically identical diploid daughter cells. | Reductional Division: Produces four genetically diverse haploid daughter cells. |
Purpose | Growth and Repair: Facilitates growth, cell repair, and asexual reproduction. | Sexual Reproduction: Produces gametes (sperm and egg cells) and introduces genetic diversity. |
Number of Divisions | One Division: Involves one cycle of nuclear and cell division. | Two Divisions: Involves two sequential cycles of nuclear and cell division (meiosis I and meiosis II). |
Daughter Cells | Two Daughter Cells: Resulting cells are genetically identical to the parent cell. | Four Daughter Cells: Resulting cells have half the chromosome number and are genetically diverse. |
Chromosome Number | Same as Parent Cell: Maintains the diploid number of chromosomes (2n). | Half of Parent Cell: Reduces the chromosome number by half, producing haploid cells (n). |
Homologous Chromosomes | No Pairing: Homologous chromosomes do not pair or undergo recombination. | Pairing and Recombination: Homologous chromosomes pair up and may exchange genetic material (crossing over). |
Phases | Four Phases: Prophase, Metaphase, Anaphase, Telophase. | Eight Phases: Prophase I, Metaphase I, Anaphase I, Telophase I, Prophase II, Metaphase II, Anaphase II, Telophase II. |
Genetic Variation | No Variation: Produces genetically identical cells. | Increases Variation: Through crossing over and independent assortment, increases genetic variability. |
Occurrence | Most Somatic Cells: Occurs in body cells, excluding gametes. | Germ Cells: Occurs in cells destined to form gametes (sperm and eggs). |
These differences highlight the distinct roles and processes of mitosis and meiosis in the life cycle of organisms.
What is the significance of meiosis?
Significance of Meiosis
Meiosis is crucial for several reasons:
Conservation of Chromosome Number: Meiosis maintains the specific chromosome number of each species across generations by reducing the chromosome number by half in the production of gametes. This is essential because it ensures that when gametes fuse during fertilization, the resulting zygote has the correct diploid chromosome number.
Genetic Variability: Meiosis increases genetic variability in the population. This is achieved through the processes of crossing over (exchange of genetic material between homologous chromosomes) and independent assortment of chromosomes. Such variability is important for the adaptation and evolution of species.
Production of Haploid Cells: Meiosis produces haploid cells (gametes), which are necessary for sexual reproduction. This ensures that when two gametes fuse, the offspring have the correct diploid chromosome number.
Role in Evolution: The genetic variations that result from meiosis contribute to evolutionary processes by introducing new gene combinations into a population. These variations can lead to traits that provide a survival advantage, driving natural selection.
Thus, meiosis is essential for sexual reproduction, genetic diversity, and the continuation of species.
Discuss with your teacher about
(i) haploid insects and lower plants where cell-division occurs, and
(ii) some haploid cells in higher plants where cell-division does not occur.
(i) Haploid Insects and Lower Plants Where Cell-Division Occurs
Haploid Insects:
Male Honey Bees (Drones): These insects are haploid (having a single set of chromosomes) and they multiply by mitosis.
Lower Plants:
Fungi and Algae: In many of these organisms, haploid cells can undergo mitosis. For example, the spores produced by these organisms are typically haploid and can divide mitotically.
Bryophytes (Mosses and Liverworts): The gametophytic stage is haploid, and cell division occurs by mitosis.
(ii) Haploid Cells in Higher Plants Where Cell-Division Does Not Occur
Pollen Grains: In flowering plants (angiosperms), pollen grains are the male gametophytes and are haploid. Each pollen grain generally does not divide further once it is formed.
Egg Cells: The female gametophyte in higher plants, such as the ovule in seed plants, also contains haploid cells. These cells typically do not divide by mitosis but rather participate in fertilization.
Can there be mitosis without DNA replication in 'S' phase?
No, there cannot be mitosis without DNA replication in the 'S' phase. During the cell cycle, DNA replication occurs specifically in the 'S phase' (Synthesis phase). This ensures that the cell has a complete set of duplicated chromosomes before it enters mitosis. Without DNA replication, the cell would not be able to evenly distribute identical genetic material to the daughter cells, undermining the fundamental goal of mitosis, which is to create two genetically identical cells from one parent cell.
Can there be DNA replication without cell division?
Yes, DNA replication can occur without cell division. This typically happens during the interphase of the cell cycle, specifically during the S phase (Synthesis phase). During interphase, the cell prepares for division by both growing and replicating its DNA. However, this does not necessarily lead to immediate cell division. Additionally, some cells may enter a state called the quiescent stage (Gā phase), where they remain metabolically active but do not proceed to cell division.
Analyse the events during every stage of cell cycle and notice how the following two parameters change
(i) number of chromosomes ( $\mathrm{N}$ ) per cell
(ii) amount of DNA content (C) per cell
Analysis of Cell Cycle Stages
10.1.1 Phases of Cell Cycle
Interphase
Gā Phase:
Number of Chromosomes (N): Remains diploid (2N for humans).
Amount of DNA Content (C): Each cell has the normal DNA content (2C).
S Phase:
Number of Chromosomes (N): Remains diploid (2N).
Amount of DNA Content (C): DNA replicates, so the DNA content doubles (from 2C to 4C).
Gā Phase:
Number of Chromosomes (N): Remains diploid (2N).
Amount of DNA Content (C): The cell has double the normal DNA content after replication (4C).
M Phase
Prophase:
Number of Chromosomes (N): Diploid (2N).
Amount of DNA Content (C): 4C, because DNA replication has occurred but chromosomes have not separated.
Metaphase:
Number of Chromosomes (N): Diploid (2N).
Amount of DNA Content (C): 4C, chromosomes align at the metaphase plate.
Anaphase:
Number of Chromosomes (N): The number of chromatids is temporarily double, but chromosomal count is considered 2N.
Amount of DNA Content (C): 4C, chromatids (now chromosomes) move towards opposite poles.
Telophase:
Number of Chromosomes (N): 2N, as chromatids reach poles and start forming new nuclei.
Amount of DNA Content (C): 4C initially, but will reduce after cytokinesis.
Cytokinesis:
Number of Chromosomes (N): Each daughter cell receives diploid number of chromosomes (2N).
Amount of DNA Content (C): Each daughter cell receives half the DNA content post-division (2C).
Summary
Interphase:
Gā: Chromosomes = 2N, DNA content = 2C
S: Chromosomes = 2N, DNA content = 2C to 4C
Gā: Chromosomes = 2N, DNA content = 4C
M Phase:
Prophase: Chromosomes = 2N, DNA content = 4C
Metaphase: Chromosomes = 2N, DNA content = 4C
Anaphase: Chromosomes = 4N temporarily, DNA content = 4C
Telophase: Chromosomes = 2N, DNA content = 4C
Cytokinesis: Chromosomes = 2N, DNA content = 2C (in each daughter cell)
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Comprehensive Class 11 Notes on Cell Cycle and Cell Division: Key Concepts Explained
Understanding the cell cycle and cell division is crucial for comprehending how organisms grow, repair themselves, and reproduce. This article offers a detailed explanation of these fundamental biological processes.
The Cell Cycle
The cell cycle is a series of events that cells go through as they grow and divide. It consists of two main phases: Interphase and the M Phase (Mitosis phase).
Phases of the Cell Cycle
Interphase
G1 Phase (Gap 1): Cell growth and metabolic activity
S Phase (Synthesis): DNA synthesis and replication
G2 Phase (Gap 2): Preparation for mitosis
M Phase (Mitosis)
Prophase
Metaphase
Anaphase
Telophase
Interphase
Interphase is the phase where the cell grows and prepares for division. It is subdivided into three stages:
G1 Phase (Gap 1)
During the G1 phase, the cell is metabolically active and continuously grows but does not replicate its DNA. This phase comes between the end of mitosis and the beginning of DNA replication.
S Phase (Synthesis)
The S phase marks the period during which DNA replication occurs. During this time, the amount of DNA in the cell doubles.
G2 Phase (Gap 2)
In the G2 phase, the cell continues to grow and prepares for mitosis. Proteins necessary for cell division are synthesised during this phase.
M Phase (Mitosis)
Mitosis is the phase where actual cell division happens. It involves a series of stages that ensure the chromosomes are accurately divided and distributed to the daughter cells.
Prophase
In prophase, the chromatin condenses into distinct chromosomes. Each chromosome is composed of two sister chromatids connected at the centromere. The nuclear envelope breaks down, and the spindle apparatus begins to form.
Metaphase
During metaphase, the chromosomes align at the metaphase plate (equator of the cell). Spindle fibres attach to the centromere of each chromosome.
Anaphase
Anaphase is characterised by the splitting of centromeres and the movement of sister chromatids to opposite poles of the cell.
Telophase
In telophase, the chromatids reach the opposite poles, and the nuclear envelope re-forms around each set of chromosomes, now called daughter chromosomes. The chromosomes begin to decondense.
Cytokinesis
Cytokinesis is the process where the cytoplasm divides, creating two daughter cells. In animal cells, cytokinesis is achieved by a contractile ring that pinches the cell in two. In plant cells, a cell plate forms along the centre line of the cell, ultimately becoming the cell wall that separates the two new cells.
Significance of Mitosis
Mitosis is vital for the growth and repair of tissues. It ensures that each daughter cell receives an identical set of chromosomes, thereby maintaining genetic consistency across cells.
Meiosis
Meiosis is a specialised type of cell division that reduces the chromosome number by half, producing four haploid cells. This process is essential for sexual reproduction.
Overview of Meiosis
Meiosis consists of two sequential cell divisions:
Meiosis I
Meiosis II
Meiosis I
Prophase I
Prophase I of meiosis is lengthy and complex, involving homologous chromosomes pairing and exchanging segments (crossing over).
Metaphase I
Homologous chromosomes align at the metaphase plate.
Anaphase I
Homologous chromosomes, each still composed of two chromatids, are pulled to opposite poles.
Telophase I
The chromosomes reach opposite poles, and the cell divides to form two haploid cells.
Meiosis II
Meiosis II resembles a normal mitosis, where sister chromatids are separated, ultimately resulting in four haploid daughter cells.
Prophase II
Chromosomes condense again, and the nuclear envelope breaks down.
Metaphase II
Chromosomes align at the metaphase plate.
Anaphase II
Centromeres split, and sister chromatids move to opposite poles.
Telophase II
Chromatids reach the poles, and nuclear membranes re-form, followed by cytokinesis.
Significance of Meiosis
Meiosis generates genetic diversity through crossing over and independent assortment of chromosomes. This genetic variability is crucial for evolution and adaptation. Meiosis ensures that offspring have the same chromosome number as their parents, maintaining species stability.
Conclusion
The cell cycle and cell division are fundamental biological processes essential for the growth, reproduction, and maintenance of all living organisms. Understanding these processes provides insights into how organisms develop and how genetic information is replicated and passed on.
flowchart TD
A[Cell Cycle] -->|Interphase| B(G1 Phase)
A -->|Interphase| C(S Phase)
A -->|Interphase| D(G2 Phase)
A -->|"M Phase (Mitosis)"| E(Prophase)
A -->|"M Phase (Mitosis)"| F(Metaphase)
A -->|"M Phase (Mitosis)"| G(Anaphase)
A -->|"M Phase (Mitosis)"| H(Telophase)
A -->|"M Phase (Mitosis)"| I(Cytokinesis)
subgraph Interphase
B
C
D
end
Feel free to use this detailed guide to better understand the cell cycle and cell division and excel in your Class 11 biology studies.
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