Exercises
What is the average cell cycle span for a mammalian cell?
Answer
The average cell cycle span for a mammalian cell is approximately 24 hours. However, this varies greatly among different cell types:
• Some cells divide rapidly (e.g., epithelial cells of intestinal lining: every 8–12 hours)
• Some divide very slowly (e.g., liver cells: once a year; nerve cells: do not divide in adults)
The 24-hour cell cycle is divided as follows:
• Interphase (G₁ + S + G₂): ~23 hours (~95% of total cycle)
• M phase (Mitosis): ~1 hour (~5% of total cycle)
• G₁ phase: ~11 hours
• S phase (DNA synthesis): ~8 hours
• G₂ phase: ~4 hours
Distinguish cytokinesis from karyokinesis.
Answer
Feature | Karyokinesis | Cytokinesis |
Definition | Division of the nucleus | Division of the cytoplasm |
Result | Two daughter nuclei | Two daughter cells |
Timing | Occurs during M phase (prophase to telophase) | Occurs at the end of telophase/after M phase |
In plant cells | Normal spindle formation | Cell plate formation (phragmoplast) |
In animal cells | Normal spindle formation | Cleavage furrow (constriction) |
Without the other | Karyokinesis without cytokinesis forms multinucleate cells (coenocyte/syncytium) | Cytokinesis without karyokinesis is very rare |
Describe the events taking place during interphase.
Answer
Interphase is the phase of the cell cycle between two successive cell divisions. It is a period of active metabolic activity and cell growth. It occupies about 95% of the total cell cycle duration. Interphase is divided into three sub-phases:
1. G₁ Phase (First Gap / Growth Phase 1)
• Cell grows in size.
• Active synthesis of RNA and proteins.
• Organelles are duplicated.
• Cell prepares for DNA synthesis.
• A key checkpoint (G₁ checkpoint / restriction point) determines if cell should proceed to S phase.
2. S Phase (Synthesis Phase)
• DNA replication (synthesis) occurs — DNA content doubles (from 2C to 4C).
• Each chromosome is duplicated to form two sister chromatids joined at the centromere.
• Histone proteins are synthesized to package newly synthesized DNA.
• Chromosome number remains the same (2n) but DNA amount doubles.
3. G₂ Phase (Second Gap / Growth Phase 2)
• Cell continues to grow.
• Synthesis of proteins needed for mitosis (e.g., tubulin for spindle fiber formation).
• Mitochondria and other organelles are further duplicated.
• Cell checks for DNA damage and repairs it (G₂ checkpoint).
What is G₀ (quiescent phase) of cell cycle?
Answer
The G₀ phase (quiescent phase or resting phase) is a stage in the cell cycle where a cell exits the normal cycling process and enters a state of metabolic inactivity or specialized differentiation.
• Cells enter G₀ from the G₁ phase when they receive signals to stop dividing (or in the absence of growth signals).
• In G₀, cells are still metabolically active and perform their specialized functions but do not replicate their DNA or divide.
• G₀ can be reversible (cells can re-enter the cycle when stimulated, e.g., liver cells after partial hepatectomy) or irreversible (permanently differentiated cells, e.g., mature neurons, RBCs).
• Most adult cells in the body are in G₀ phase.
• Cells in G₀ have not committed to dividing and are not progressing through the cell cycle checkpoints.
Why is mitosis called equational division?
Answer
Mitosis is called equational division because it produces two daughter cells that are genetically identical to each other and to the parent cell, with the same chromosome number (2n = 2n).
• In mitosis, the chromosomal number of the parent cell is maintained in both daughter cells.
• If the parent cell has 2n = 46 chromosomes, each daughter cell also has 2n = 46 chromosomes.
• The DNA is first replicated during S phase (4C), and then during mitosis, chromosomes separate so each daughter cell receives exactly 2C DNA (2n chromosomes).
• There is no reduction in chromosome number — the equation is: 2n
⇒ 2n + 2n.
• This is in contrast to meiosis (reductional division), where the chromosome number is halved (2n
⇒ n).
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.
Answer
(i) Chromosomes are moved to the spindle equator (metaphase plate)
⇒ Metaphase (metaphase of mitosis or metaphase I of meiosis)
(ii) Centromere splits and chromatids separate
⇒ Anaphase (anaphase of mitosis); in meiosis I, the centromere does NOT split — it splits in anaphase II
(iii) Pairing between homologous chromosomes (synapsis/bivalent formation)
⇒ Zygotene (sub-stage of Prophase I of meiosis I)
(iv) Crossing over between homologous chromosomes
⇒ Pachytene (sub-stage of Prophase I of meiosis I)
Q7. Describe the following
(a) synapsis (b) bivalent (c) chiasmata
Draw a diagram to illustrate your answer.
Ans.
(a) Synapsis:
Synapsis is the pairing of homologous chromosomes during the zygotene stage of prophase I of meiosis I. The two homologous chromosomes align precisely along their entire lengths in a zipper-like fashion. This pairing is facilitated by a protein framework called the synaptonemal complex (SC), which holds the homologs together. Synapsis is essential for subsequent events like crossing over.
(b) Bivalent (Tetrad):
A bivalent (also called a tetrad) is the structure formed when two homologous chromosomes come together during synapsis. Since each chromosome already consists of two sister chromatids (joined at centromere), a bivalent consists of four chromatids (two pairs of sister chromatids). The number of bivalents equals the haploid number (n). For example, in a cell with 2n = 46, there are 23 bivalents in prophase I.
(c) Chiasmata (singular: chiasma):
Chiasmata are the X-shaped structures visible during diplotene and diakinesis of prophase I of meiosis I. They represent the points where crossing over (exchange of genetic material between non-sister chromatids of homologous chromosomes) has occurred during pachytene. Chiasmata hold the homologous chromosomes together until anaphase I, when they "terminalize" and the homologs separate.
Diagram description
A bivalent (tetrad) showing two homologous chromosomes, each with two sister chromatids (four chromatids total), with a chiasma visible as an X-crossing between non-sister chromatids of the two homologs. The centromeres of each homolog are labeled, and the point of crossing-over (chiasma) is indicated.
How does cytokinesis in plant cells differ from that in animal cells?
Answer
Feature | Cytokinesis in Plant Cells | Cytokinesis in Animal Cells |
Mechanism | Cell plate formation | Cleavage furrow (constriction) |
Direction | Inside-out (center to periphery) | Outside-in (periphery to center) |
Structure formed | Cell plate (phragmoplast) — new cell wall | Cleavage furrow formed by actin-myosin ring |
Organelles involved | Golgi vesicles and phragmoplast (microtubules) | Contractile ring of actin and myosin filaments |
Centrioles | Usually absent (higher plants) | Present; help organize the cleavage process |
Result | New cell wall between daughter cells | Two daughter cells separated by plasma membrane |
Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size.
Answer
Equal-sized four daughter cells from meiosis:
• Spermatogenesis in males: Meiosis of spermatocytes produces four equal-sized spermatids (which then differentiate into spermatozoa).
• Microspore formation (male gametogenesis in plants): Pollen mother cell undergoes meiosis to form four equal-sized microspores (pollen grains).
Unequal-sized four daughter cells from meiosis
• Oogenesis in females: Meiosis of oocytes produces one large egg (ovum) and three small polar bodies. The unequal division is due to unequal distribution of cytoplasm. The polar bodies eventually degenerate.
• Megasporogenesis (female gametogenesis in flowering plants): Meiosis of megaspore mother cell produces four megaspores, but only one functional megaspore survives (the one with most cytoplasm); the rest degenerate. The divisions can be unequal.
Distinguish anaphase of mitosis from anaphase I of meiosis.
Answer
Feature | Anaphase of Mitosis | Anaphase I of Meiosis |
What separates | Sister chromatids separate (centromere splits) | Homologous chromosomes separate (centromere does NOT split; sister chromatids stay together) |
Number moving to each pole | 2n chromatids (one from each chromosome) | n chromosomes (each still consisting of two chromatids) |
Centromere | Splits during anaphase | Does NOT split; remains intact |
Genetic content at poles | Each group = 2n (diploid) | Each group = n (haploid, but each chromosome has 2 chromatids) |
Chiasmata | Not present (already terminalized) | Chiasmata terminalize and disappear as homologs separate |
List the main differences between mitosis and meiosis.
Answer
Feature | Mitosis | Meiosis |
Type of cells | Somatic (body) cells | Sex cells (gonads) — germinal cells |
Number of divisions | One (single division) | Two successive divisions (meiosis I and II) |
Daughter cells produced | Two | Four |
Chromosome number | Same as parent (2n → 2n) | Half of parent (2n → n) |
Synapsis | Does not occur | Occurs in prophase I (zygotene) |
Crossing over | Does not occur | Occurs during pachytene of prophase I |
Genetic variation | No variation (genetically identical cells) | Creates genetic variation in daughter cells |
Purpose | Growth, repair, asexual reproduction | Sexual reproduction — formation of gametes |
Duration | Shorter | Longer (especially prophase I) |
What is the significance of meiosis?
Answer
Significance of meiosis:
1. Maintains chromosome number: Meiosis halves the chromosome number in gametes (n), so when fertilization occurs (n + n = 2n), the species-specific chromosome number is maintained across generations.
Genetic variation: Meiosis introduces genetic variation through:
(a) Crossing over (recombination) — exchange of genetic material between homologous chromosomes during pachytene creates new combinations of alleles.
(b) Independent assortment of homologous chromosomes — random segregation of maternal and paternal chromosomes creates 2ⁿ possible combinations.
Answer
3. Formation of gametes: Meiosis produces four haploid cells (gametes or spores) needed for sexual reproduction.
4. Evolution: Genetic variation produced by meiosis provides the raw material for evolution by natural selection.
5. Repair: The recombination process allows exchange of DNA, which can also repair damaged DNA segments.
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.
Answer
This is a discussion-based activity. Key points:
(i) Haploid organisms/cells that undergo cell division:
• Male bees (drones): Develop from unfertilized eggs (parthenogenesis); they are haploid and their cells divide by mitosis to produce a haploid adult.
• Gametophyte generation in lower plants (mosses, ferns): The haploid gametophyte plant is the dominant generation. Cells of the gametophyte divide mitotically for growth. Gametes are produced by mitosis (not meiosis) from gametophyte cells.
• Algae like Chlamydomonas: The vegetative (haploid) cells divide by mitosis for vegetative reproduction.
(ii) Haploid cells in higher plants that do not divide:
• Pollen grains (mature): The haploid generative cell may not divide further in some plants.
• Egg cell: The haploid egg cell in the embryo sac does not divide unless fertilized.
• Synergids and antipodal cells: These haploid accessory cells of the embryo sac do not divide — they degenerate after fertilization.
Can there be mitosis without DNA replication in 'S' phase?
Answer
No, normally mitosis cannot occur without prior DNA replication in the S phase.
• DNA replication during S phase is essential to ensure that each daughter cell receives a complete and identical copy of the genome.
• Without S phase replication, the chromosomes would not be duplicated, and during mitosis, chromatids would not be available for separation.
• However, experimentally, if DNA synthesis is inhibited, cells may still attempt to divide (aberrant mitosis), resulting in daughter cells with incomplete genetic material, which typically leads to cell death or genetic instability (aneuploidy).
• Checkpoint mechanisms (G₁/S checkpoint and G₂/M checkpoint) normally prevent a cell from entering mitosis without completing DNA replication. If DNA is damaged or unreplicated, the S-phase checkpoint halts progression.
Therefore, in normal physiological conditions, mitosis is always preceded by DNA replication.
Can there be DNA replication without cell division?
Answer
Yes, DNA replication can occur without subsequent cell division.
Examples:
1. Polytene chromosomes: In salivary gland cells of Drosophila, DNA replicates many times (endoreduplication) without cell division or mitosis, resulting in giant polytene chromosomes with hundreds of copies of DNA.
2. Endopolyploidy: In some plant cells and mammalian liver cells, the nucleus undergoes repeated rounds of DNA replication without cytokinesis, forming polyploid cells.
3. Abortive cell cycle: A cell may replicate DNA but fail to divide due to checkpoint failure or lack of growth signals.
4. Bacteria under stress: Initiation of chromosome replication can occur without subsequent cell division.
Thus, DNA replication (S phase) is a prerequisite for cell division, but cell division does not always follow DNA replication.
Analyse the events during every stage of cell cycle and notice how the following two parameters change:
(i) number of chromosomes (N) per cell
(ii) amount of DNA content (C) per cell
Answer
Changes in chromosome number (N) and DNA content (C) through cell cycle (for a diploid cell, 2n = 46, starting at G₁ with 2C DNA):
Stage | Chromosome Number (N) | DNA Content (C) |
G₁ phase | 2n = 46 | 2C |
S phase (during) | 2n = 46 | Gradually increases from 2C to 4C |
G₂ phase | 2n = 46 | 4C (DNA doubled, each chromosome = 2 chromatids) |
Prophase | 2n = 46 | 4C (chromosomes condense) |
Metaphase | 2n = 46 | 4C |
Anaphase | 4n = 92 (chromatids at poles) | 4C (same total, just separated) |
Telophase/After cytokinesis | 2n = 46 in each daughter cell | 2C in each daughter cell |
Key observations: Chromosome number stays constant (2n) during G₁, S, G₂, and most of M phase. It appears to "double" in anaphase when chromatids separate but then returns to 2n in each daughter cell. DNA content doubles during S phase (2C
⇒ 4C) and then halves after cell division (4C
⇒ 2C in each daughter).
Important Extra Questions
What are the stages of prophase I of meiosis I? Describe each.
Answer
Prophase I is the longest phase of meiosis and is divided into five sub-stages (LZPDD):
1. Leptotene: Chromosomes become visible as long, thin threads; chromatin condenses; chromosomes are already duplicated (two chromatids).
2. Zygotene: Homologous chromosomes begin to pair (synapsis) with help of synaptonemal complex; bivalents form.
3. Pachytene: Synapsis is complete; each bivalent = 4 chromatids; crossing over occurs between non-sister chromatids at points called chiasmata; recombination nodules are visible.
4. Diplotene: Synaptonemal complex dissolves; homologs begin to separate but remain held at chiasmata; chiasmata are visible.
5. Diakinesis: Further condensation; chiasmata "terminalize" (move to chromosome ends); nucleolus and nuclear envelope disappear; spindle formation begins.
Explain the significance of crossing over in meiosis.
Answer
Crossing over is the exchange of segments between non-sister chromatids of homologous chromosomes during pachytene of prophase I.
Significance:
1. Creates new combinations of alleles (recombinant chromosomes) not present in either parent
⇒ increases genetic variation.
2. Is the basis for genetic mapping — distance between genes correlates with frequency of crossing over.
3. Provides the physical mechanism holding homologs together (chiasmata) until anaphase I separation.
4. Enables repair of double-strand DNA breaks (recombinational repair).
5. Is the primary source of heritable variation in sexually reproducing organisms, which drives evolution.
What is the spindle assembly checkpoint?
Answer
The spindle assembly checkpoint (SAC), also called the metaphase checkpoint or M checkpoint, is a quality-control mechanism that ensures all chromosomes are properly attached to spindle fibers before the cell proceeds from metaphase to anaphase.
• Each kinetochore must be attached to spindle microtubules from the opposite poles (amphitelic/bioriented attachment).
• If even one kinetochore is unattached, the checkpoint sends a "wait" signal (via proteins like Mad2, BubR1) that inhibits the Anaphase Promoting Complex (APC/C).
• APC/C normally degrades securin (releasing separase, which cleaves cohesin) and cyclin B, triggering anaphase.
• This checkpoint prevents chromosome non-disjunction (aneuploidy), which can lead to cancer or genetic disorders (e.g., Down syndrome).
What is the role of MPF (Maturation Promoting Factor) in the cell cycle?
Answer
MPF (Maturation Promoting Factor), now known as Cyclin B-CDK1 complex, is the key regulatory protein that drives the cell from G₂ phase into mitosis (M phase).
• MPF is composed of Cyclin B (regulatory subunit, levels fluctuate) + CDK1 (catalytic subunit, Cyclin-Dependent Kinase 1, constitutively present).
• As Cyclin B accumulates during G₂, it activates CDK1, forming active MPF.
• Active MPF phosphorylates target proteins to:
— Cause nuclear envelope breakdown
— Initiate chromosome condensation
— Promote spindle formation
• After chromosomes align, MPF activity drops as Cyclin B is degraded (by APC/C), allowing exit from mitosis.
• MPF was originally discovered by Masui and Markert in Xenopus eggs.
Explain the difference between somatic cell division and germ cell division.
Answer
Somatic cell division (Mitosis):
• Occurs in body (somatic) cells — skin, liver, muscle, bone cells, etc.
• Results in two genetically identical diploid (2n) cells.
• One division; no crossing over.
• Purpose: Growth, repair, and replacement of worn-out cells; also asexual reproduction in lower organisms.
Germ cell division (Meiosis)
• Occurs in reproductive organs (gonads) — testes and ovaries in animals; anthers and ovules in plants.
• Results in four genetically varied haploid (n) cells.
• Two successive divisions; crossing over occurs in prophase I.
• Purpose: Production of gametes (sperm/eggs) for sexual reproduction; maintains chromosome number across generations; generates genetic variation.