1.12 Cell Division

Introduction

The continuity of life is based on the reproduction of cells or cell division. The dividing cell is called the parent cell and the two new cells formed are called daughter cells. The two main types of cell divisions are mitosis and meiosis. The somatic cells divide by mitosis and the germ line cells divide by meiosis.

Definition

Cell Division: Cell division is the process by which a parent cell divides to produce two or more daughter cells.

In simple words: Cell division is how cells reproduce and make more cells.

Core Concept

• The continuity of life is based on the reproduction of cells or cell division.
• The dividing cell is called the parent cell.
• The two new cells formed are called daughter cells.
• The two main types of cell divisions are:

Detailed Explanation

Cell Cycle

Definition: The cell cycle is a series of stages a cell passes through to divide and produce new cells.

• Mitosis – Division of the nucleus.
• Cytokinesis – Division of the cytoplasm.

Phases of Cell Cycle

• Mitosis is only a part of the cell cycle.
• The mitotic (M) phase includes both mitosis and cytokinesis.
• The M phase is the shortest part of the cell cycle.
• Mitotic cell division alternates with a much longer stage called interphase.
• Interphase can be divided into three sub-phases:

Interphase

During the sub-phases, the cell grows by producing proteins and cytoplasmic organelles.

Important: Chromosomes are duplicated only during the S phase.

• A cell grows (G₁)
• Continues to grow as it copies its chromosomes (S)
• Grows more as it completes preparation for cell division (G₂)
• Divides (M phase)
• The daughter cells may then repeat the cycle.

Cell Cycle Checkpoints

• Cell cycle checkpoints are control mechanisms that ensure the normal course of the eukaryotic cell cycle.
• There are three major checkpoints:

MITOSIS

Definition: Mitosis is the division of the cell in such a manner that the chromosomes are duplicated and distributed equally to the daughter cells.

• The word mitosis comes from the Greek word 'mitos' which means thread.
• It refers to the thread-like appearance of chromosomes during this period.

Stages of Mitosis

• Prophase
• Prometaphase
• Metaphase
• Anaphase
• Telophase

Interphase (Before Mitosis)

• A nuclear envelope surrounds the nucleus.
• The nucleus contains one or more nucleoli.
• By the replication of a single centrosome, two centrosomes are formed.
• Each centrosome has two centrioles.
• The chromosomes that duplicated during S phase are not visible under a light microscope as they have not yet condensed.

Prophase

• The chromatin fibers become more tightly coiled.
• They condense into discrete chromosomes which are observable with a light microscope.
• Each duplicated chromosome appears as two identical sister chromatids joined together.
• The nucleoli disappear.
• The mitotic spindle begins to form.
• It is composed of centrosomes and the microtubules that extend from them.
• The radial arrays of shorter microtubules that extend from the centrosomes are called asters (stars).
• The centrosomes move away from each other.
• It is due to the lengthening of the microtubules between them.

Prometaphase

• During the formation of spindle, the nuclear membrane breaks.
• The microtubules of the spindle can now invade the nuclear area and interact with the chromosomes, which have become even more condensed.
• Microtubules extend from each centrosome towards the middle of the cell.
• Each of the two chromatids of a chromosome now has a kinetochore.
• Some of the microtubules attach to the kinetochore becoming 'kinetochore microtubules'.
• Non-kinetochore microtubules interact with those from the opposite pole of the spindle.

Metaphase

• It is the longest stage of mitosis, lasting about twenty minutes.
• The centrosomes are now at opposite ends of the cell.
• The chromosomes arrange on the metaphase plate, an imaginary plane that is equidistant between two poles of the spindle.
• The centromeres of the chromosomes lie on the metaphase plate.
• For each chromosome, the kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles.
• The entire apparatus is called the spindle because of its shape.

Anaphase

• It is the shortest stage of mitosis, lasting only a few minutes.
• It begins when the two sister chromatids of each pair suddenly part.
• Each chromatid thus becomes a full-fledged chromosome.
• The two liberated chromosomes begin to start moving towards opposite ends of the cell, as their kinetochore microtubules shorten.
• Because these microtubules are attached at the centromere region, the chromosomes move centromere first.
• The cell elongates as the non-kinetochore microtubules lengthen.
• By the end of the anaphase, the two ends of the cell have equivalent and complete collection of chromosomes.

Telophase

• The two daughter nuclei begin to form in the cell.
• Nuclear envelopes arise from the fragments of the parent cell's nuclear envelope and other portions of the endomembrane system.
• The chromosome becomes less condensed.
• Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete.

Cytokinesis

• The division of the cytoplasm is usually well underway by the late telophase.
• The two daughter cells appear shortly after the end of mitosis.
• Commonly the nuclear division is followed by separation of the cytoplasm into two parts.
• This separation is accomplished by pinching of the cell membrane.
• The pinching near the middle of an animal cell forms a cleavage furrow.
• The process of cytoplasmic division is called cytokinesis (Gk: cyto; cell, kinesis; movement).
• Cell organelles are distributed to the two daughter cells.

Significance of Mitosis

• It is important for growth and multiplication of cells.
• It ensures that the two daughter cells inherit the same number of chromosomes, and hence the same characteristics as the parent cells.
• Maintains the continuity of metabolism by transmitting to the daughter cells exactly the same information as is coded in the DNA of the parent cell.
• Plays a significant role in wound healing, regeneration of damaged parts, and replacement of cells lost during normal wear and tear.
• May give rise to tumour or cancerous growth if the process goes out of control.

MEIOSIS

Definition: The process where a single cell divides twice to produce four cells containing half the original amount of genetic information is called meiosis.

• Sperm and eggs are produced by meiosis in animals.
• Spores are produced by meiosis in plants.

Stages of Meiosis

• Meiosis I (First Meiotic Division)
• Meiosis II (Second Meiotic Division)

The First Meiotic Division (Meiosis I)

• Interphase I
• Prophase I
• Metaphase I
• Anaphase I
• Telophase I

Interphase I

• The DNA in the cell is copied as a result two identical full sets of chromosomes are formed.
• There are two centrosomes outside the nucleus, each containing a pair of centrioles.
• During interphase, microtubules extend from these centrosomes.

Prophase I

• It is a lengthy process of meiosis.
• Each chromosome has two chromatids.
• Chromosomes look like interweaving threads.
• Due to condensation of chromatin material, chromosomes become apparent and distinct, nucleus increases in size.
• Initially the thin chromosomes become shorter and thicker.
• The chromosome number of the cell is seen.
• Homologous chromosomes start getting closer to each other.
• In the cell there are two of each type of chromosome.
• Identical chromosomes are called homologous chromosomes.
• The homologous chromosomes begin to pair lengthwise with its homologue.
• The process of pairing is called synapsis (si-nap-sis).
• The synapsis may start from any point with the corresponding complementary DNA strand of the other.
• Each pair of synapsed chromosome consists of four chromatids, two centromeres and is called a tetrad or bivalent.
• The chromatids of the homologues may cross each other and the point of crossing is an X-shaped figure under the light microscope.
• It is called chiasma (plural: chiasmata).
• Chromosome segment is exchanged between the two homologous chromosomes at the chiasma and is called crossing over.
• The paired homologous chromosomes begin to separate by repelling.
• The tetrad are more evenly distributed in the nucleus.
• The nucleoli disappear.
• The nuclear membrane is still present.
• The diad (the two chromatids attached to a single centromere is called diad) behaves as a single unit because they are held together by a common centromere, throughout the meiosis.

Metaphase I

• The nuclear membrane disintegrates.
• The microtubules form the spindle.
• The chromosomes line up in double row i.e., pair on the equator.
• Microtubules bind only to one kinetochore of each centromere.
• The centromere of one homologue becomes attached to microtubules extending to one pole, whereas the centromere of the other homologue becomes attached to microtubules extending to the other pole.

Anaphase I

• The attachment of chromosomes to the spindle is complete.
• The microtubules that are attached to the homologous chromosomes begin to slide over one another, as a result the spindle poles move apart.
• The spindle fibers become shorter as the spindle fibers are dragged to the poles.
• They are broken by the action of enzymes.
• With the shortening of the spindle fibers, the chromosomes that are attached to the spindle are also pulled towards each pole.
• When the shortening of the spindle fibers is completed, each pole has a single set of chromosomes, consisting of one member of each homologous pair.

Telophase I and Cytokinesis

• The chromosomes complete their movement to the opposite poles of the cell.
• At each pole of the cell a full set of chromosomes gather together.
• A membrane forms around each set of chromosomes to create two new nuclei.
• The single cell through the process of cytokinesis forms two separate daughter cells, which are haploid.

The Second Meiotic Division (Meiosis II)

• Meiosis II is simply a mitotic division.
• It occurs to separate the sister chromatids as in mitosis.

Interphase II

• DNA does not duplicate in interphase II.
• It is a very brief phase.
• Each of the two cells resulting from meiosis I progresses into meiosis II very quickly.

Prophase II

- The complicated nuclear events of prophase do not take place.

Metaphase II and Anaphase II

• The chromosomes line up in the same fashion as they did in metaphase I.
• Here again a random distribution of chromosomes takes place.
• The centromere divides, as a result the two chromatids are separated.
• As each chromatid is now a separate structure they are called chromosomes.
• Chromosomes are distributed in equal number, to each pole.

Telophase II

• The nuclei are reconstructed in the typical manner.
• Each nucleus now contains a haploid set of chromosomes, because DNA has duplicated only once during the cell division.

Significance of Meiosis

• The number of chromosomes becomes half in the gametes due to which the number of chromosomes remains constant from generation to generation.
• It provides a mechanism for the transmission of genetic material from one generation to the next.
• It provides genetic variability.
• It offers a chance for the production of best adapted individual and better chance for survival in the changed environment.
• It provides raw material for evolution.

Intermediate Level

Comparison of Mitosis and Meiosis

Cell Cycle Checkpoints

Mitosis vs Meiosis: Chromosome Number

Significance of Mitosis

• Growth: Increases cell number in multicellular organisms.
• Repair: Replaces damaged or dead cells.
• Regeneration: Some organisms can regenerate lost body parts.
• Asexual reproduction: Some organisms reproduce by mitosis.
• Genetic stability: Daughter cells are genetically identical to the parent cell.

Significance of Meiosis

• Gamete formation: Produces sperm and egg cells.
• Chromosome number: Reduces chromosome number by half.
• Genetic variation: Crossing over and independent assortment create new gene combinations.
• Evolution: Provides raw material for natural selection.
• Species continuity: Ensures the chromosome number remains constant from generation to generation.

Important Points

• 1. Cell division is the process by which a parent cell divides to produce daughter cells.
• 2. The two main types of cell division are:
• - Mitosis – Somatic cells divide by mitosis.
• - Meiosis – Germ line cells divide by meiosis.
• 3. Cell cycle is a series of stages a cell passes through to divide and produce new cells.
• 4. Cell division can be divided into:
• - Mitosis – Division of the nucleus.
• - Cytokinesis – Division of the cytoplasm.
• 5. Mitotic (M) phase includes both mitosis and cytokinesis.
• 6. Mitotic cell division alternates with a much longer stage called interphase.
• 7. Interphase has three sub-phases:
• - G₁ phase – First gap; cell grows.
• - S phase – Synthesis; DNA replication.
• - G₂ phase – Second gap; cell prepares for division.
• 8. Chromosomes are duplicated only during the S phase.
• 9. Cell cycle checkpoints are control mechanisms:
• - G₁ checkpoint
• - G₂ checkpoint
• - Spindle checkpoint
• 10. Mitosis is the division of the cell in such a manner that the chromosomes are duplicated and distributed equally to the daughter cells.
• 11. Stages of mitosis:
• - Prophase
• - Prometaphase
• - Metaphase
• - Anaphase
• - Telophase
• 12. Prophase: Chromatin condenses; nucleoli disappear; mitotic spindle forms; centrosomes move apart.
• 13. Prometaphase: Nuclear membrane breaks; microtubules invade nuclear area; kinetochores form; kinetochore microtubules attach.
• 14. Metaphase: Chromosomes align on metaphase plate; longest stage (~20 minutes).
• 15. Anaphase: Sister chromatids separate; shortest stage (few minutes); chromosomes move centromere-first.
• 16. Telophase: Two daughter nuclei form; nuclear envelopes arise; chromosomes decondense.
• 17. Cytokinesis: Division of cytoplasm; cleavage furrow in animal cells.
• 18. Significance of mitosis:
• - Growth and multiplication of cells
• - Same number of chromosomes in daughter cells
• - Continuity of metabolism
• - Wound healing, regeneration, replacement of cells
• - May give rise to tumour/cancer if uncontrolled
• 19. Meiosis is the process where a single cell divides twice to produce four cells containing half the original amount of genetic information.
• 20. Meiosis I:
• - Interphase I
• - Prophase I (synapsis, tetrad, chiasma, crossing over)
• - Metaphase I (double row)
• - Anaphase I (homologous chromosomes separate)
• - Telophase I and cytokinesis (haploid daughter cells)
• 21. Meiosis II:
• - Interphase II (no DNA duplication)
• - Prophase II
• - Metaphase II
• - Anaphase II (sister chromatids separate)
• - Telophase II (four haploid cells)
• 22. Significance of meiosis:
• - Chromosome number becomes half
• - Transmission of genetic material
• - Genetic variability
• - Best adapted individual
• - Raw material for evolution

Remember

• - Mitosis = growth and repair; 2 daughter cells; same chromosome number
• - Meiosis = gamete production; 4 daughter cells; half chromosome number
• - Cell cycle = Interphase + M phase
• - **Interphase** = G₁ + S + G₂
• - **S phase** = DNA replication
• - Mitosis stages = Prophase, Prometaphase, Metaphase, Anaphase, Telophase
• - Cytokinesis = division of cytoplasm
• - Meiosis I = homologous chromosomes separate
• - Meiosis II = sister chromatids separate
• - Crossing over = exchange of segments between homologous chromosomes
• - Synapsis = pairing of homologous chromosomes
• - Tetrad/Bivalent = four chromatids, two centromeres
• - Chiasma = point of crossing over
• - Significance of mitosis = growth, repair, replacement
• - Significance of meiosis = gamete production, genetic variation, evolution

Common Mistakes

• 1. Confusing mitosis and meiosis – Mitosis produces 2 identical diploid cells; meiosis produces 4 genetically different haploid cells.
• 2. Thinking DNA replicates in both meiosis I and meiosis II – DNA replicates only once, before meiosis I. There is no DNA replication in meiosis II.
• 3. Confusing homologous chromosomes and sister chromatids – Homologous chromosomes are pairs of chromosomes (one from each parent); sister chromatids are identical copies of a chromosome.
• 4. Thinking crossing over occurs in mitosis – Crossing over occurs only in Prophase I of meiosis.
• 5. Forgetting the stages of mitosis – Prophase, Prometaphase, Metaphase, Anaphase, Telophase.
• 6. Confusing metaphase and anaphase – Metaphase = chromosomes align on metaphase plate; Anaphase = sister chromatids separate and move to opposite poles.
• 7. Thinking cytokinesis is the same in plant and animal cells – Animal cells form a cleavage furrow; plant cells form a cell plate.
• 8. Forgetting the significance of meiosis – Reduces chromosome number, creates genetic variation, provides raw material for evolution.
• 9. Mixing up the phases of interphase – G₁ (growth), S (DNA synthesis), G₂ (preparation for division).
• 10. Thinking the cell cycle is only mitosis – The cell cycle includes interphase (G₁, S, G₂) and M phase (mitosis + cytokinesis).
• 11. Confusing tetrad and chiasma – Tetrad = four chromatids, two centromeres; Chiasma = point of crossing over.
• 12. Forgetting that meiosis I is reductional division – Homologous chromosomes separate, reducing the chromosome number by half.
• 13. Thinking meiosis II is the same as mitosis – Meiosis II is similar to mitosis but occurs in haploid cells and produces four haploid cells total.
• 14. Forgetting the checkpoints – G₁, G₂, and spindle checkpoints.

Quick Revision

• Cell division is the process by which a parent cell divides to produce daughter cells.
• The two main types of cell divisions are mitosis and meiosis.
• Somatic cells divide by mitosis.
• Germ line cells divide by meiosis.
• Cell cycle is a series of stages a cell passes through to divide and produce new cells.
• Cell division can be divided into mitosis (division of nucleus) and cytokinesis (division of cytoplasm).
• Mitotic (M) phase includes both mitosis and cytokinesis.
• Mitotic cell division alternates with a much longer stage called interphase.
• Interphase can be divided into three sub-phases:
• G₁ phase (first gap)
• S phase (synthesis)
• G₂ phase (second gap)
• During the sub-phases, the cell grows by producing proteins and cytoplasmic organelles.
• Chromosomes are duplicated only during the S phase.
• Cell cycle checkpoints are control mechanisms that ensure the normal course of the eukaryotic cell cycle.
• There are three major checkpoints:
• **G₁ checkpoint** – At the G₁S transition.
• **G₂ checkpoint** – At the G₂M transition.
• Spindle checkpoint – During metaphase.
• Mitosis is the division of the cell in such a manner that the chromosomes are duplicated and distributed equally to the daughter cells.
• The word mitosis comes from the Greek word 'mitos' which means thread.
• Mitosis is a continuous process that is arbitrarily divided into five phases:
• Prophase: Chromatin fibers become more tightly coiled; condense into discrete chromosomes; nucleoli disappear; mitotic spindle begins to form; centrosomes move away from each other.
• Prometaphase: Nuclear membrane breaks; microtubules invade nuclear area; kinetochores form; kinetochore microtubules attach to kinetochores; non-kinetochore microtubules interact with opposite pole microtubules.
• Metaphase: Longest stage (~20 minutes); chromosomes arrange on the metaphase plate; centromeres lie on the metaphase plate; kinetochores of sister chromatids attached to kinetochore microtubules from opposite poles.
• Anaphase: Shortest stage (few minutes); sister chromatids separate; each chromatid becomes a full-fledged chromosome; chromosomes move centromere-first toward opposite poles; cell elongates.
• Telophase: Two daughter nuclei begin to form; nuclear envelopes arise from fragments of parent cell's nuclear envelope and other portions of the endomembrane system; chromosome becomes less condensed; mitosis is complete.
• Cytokinesis: Division of cytoplasm; cleavage furrow forms in animal cells; cell organelles are distributed to the two daughter cells.
• Significance of mitosis:
• Important for growth and multiplication of cells.
• Ensures that the two daughter cells inherit the same number of chromosomes.
• Maintains the continuity of metabolism.
• Plays a significant role in wound healing, regeneration of damaged parts, and replacement of cells.
• May give rise to tumour or cancerous growth if the process goes out of control.
• Meiosis is the process where a single cell divides twice to produce four cells containing half the original amount of genetic information.
• Sperm and eggs are produced by meiosis in animals.
• Spores are produced by meiosis in plants.
• Meiosis I (First Meiotic Division):
• Interphase I: DNA is copied; two centrosomes outside nucleus.
• Prophase I: Lengthy process; chromosomes have two chromatids; homologous chromosomes pair (synapsis); tetrad/bivalent forms; chiasma forms; crossing over occurs; nucleoli disappear; nuclear membrane still present.
• Metaphase I: Nuclear membrane disintegrates; spindle forms; chromosomes line up in double row on equator; microtubules bind only to one kinetochore of each centromere.
• Anaphase I: Homologous chromosomes separate; each pole has a single set of chromosomes.
• Telophase I and Cytokinesis: Chromosomes complete movement to opposite poles; full set of chromosomes at each pole; membrane forms around each set; two separate haploid daughter cells formed.
• Meiosis II (Second Meiotic Division):
• Interphase II: DNA does not duplicate; very brief phase.
• Prophase II: Complicated nuclear events of prophase do not take place.
• Metaphase II and Anaphase II: Chromosomes line up in the same fashion as in metaphase I; random distribution of chromosomes takes place; centromere divides; chromatids separate.
• Telophase II: Nuclei are reconstructed; each nucleus contains a haploid set of chromosomes; DNA has duplicated only once during the cell division.
• Significance of meiosis:
• The number of chromosomes becomes half in the gametes.
• Provides a mechanism for the transmission of genetic material from one generation to the next.
• Provides genetic variability.
• Offers a chance for the production of best adapted individual and better chance for survival in the changed environment.
• Provides raw material for evolution.

60-Second Revision

• Mitosis = growth and repair; 2 daughter cells; same chromosome number
• Meiosis = gamete production; 4 daughter cells; half chromosome number
• Cell cycle = Interphase + M phase
• **Interphase** = G₁ + S + G₂
• **S phase** = DNA replication
• Mitosis stages = Prophase, Prometaphase, Metaphase, Anaphase, Telophase
• Cytokinesis = division of cytoplasm
• Meiosis I = homologous chromosomes separate
• Meiosis II = sister chromatids separate
• Crossing over = exchange of segments between homologous chromosomes
• Synapsis = pairing of homologous chromosomes
• Tetrad/Bivalent = four chromatids, two centromeres
• Chiasma = point of crossing over
• Significance of mitosis = growth, repair, replacement
• Significance of meiosis = gamete production, genetic variation, evolution
• Warning: DNA replicates only once before meiosis. Crossing over occurs only in Prophase I. Mitosis produces identical cells; meiosis produces genetically different cells. Cytokinesis differs in plant and animal cells.