Cell Cycle and Cell Division
Fundamentals of Cell Cycle
1. What is the average cell cycle span for a mammalian cell?
The average cell cycle span for a typical mammalian cell (e.g., human cell in culture) is approximately 24 hours.
- Interphase: Lasts for more than 95% of the duration (approx. 23 hours).
- M Phase (Mitosis): Lasts for only about 1 hour.
2. Distinguish cytokinesis from karyokinesis.
| Karyokinesis | Cytokinesis |
|---|---|
| It is the division of the nucleus. | It is the division of the cytoplasm. |
| It involves the separation of daughter chromosomes. | It involves the partitioning of the cell into two daughter cells. |
| It occurs before cytokinesis. | It occurs after karyokinesis. |
3. Describe the events taking place during interphase.
Interphase is the resting phase where the cell prepares for division. It is divided into three sub-phases:
– Cell is metabolically active and grows continuously.
– Synthesizes proteins and RNA.
– No DNA replication occurs.
2. S Phase (Synthesis):
– DNA Replication: Amount of DNA per cell doubles ($2C \to 4C$).
– Chromosome number remains the same ($2N$).
– Centriole duplication begins in the cytoplasm (animal cells).
3. $G_2$ Phase (Gap 2):
– Proteins needed for mitosis (like tubulin) are synthesized.
– Cell growth continues.
4. What is $G_0$ (quiescent phase) of cell cycle?
The $G_0$ Phase (Quiescent Stage) is an inactive stage where cells exit the cell cycle. Cells in this stage remain metabolically active but do not proliferate unless called on to do so depending on the requirement of the organism.
Examples: Heart cells, Nerve cells (neurons) typically enter this phase permanently.
Mitosis & Cytokinesis
5. Why is mitosis called equational division?
Mitosis is called equational division because the number of chromosomes in the parent cell is exactly conserved in the daughter cells.
If the parent cell is diploid ($2N$), the daughter cells formed after mitosis are also diploid ($2N$) with identical genetic complement.
6. Name the stage of cell cycle for specific events.
- (i) Chromosomes moved to spindle equator: Metaphase.
- (ii) Centromere splits and chromatids separate: Anaphase.
- (iii) Pairing between homologous chromosomes: Zygotene (Prophase I of Meiosis).
- (iv) Crossing over between homologous chromosomes: Pachytene (Prophase I of Meiosis).
7. Describe Synapsis, Bivalent, Chiasmata.
- (a) Synapsis: The pairing of homologous chromosomes during the Zygotene stage of Prophase I. It is accompanied by the formation of the synaptonemal complex.
- (b) Bivalent: The complex formed by a pair of synapsed homologous chromosomes. It is clearly visible as a Tetrad (four chromatids) in the Pachytene stage.
- (c) Chiasmata: The X-shaped structures formed at the sites of crossing over. They are seen during the Diplotene stage when the homologous chromosomes start separating, except at the points of crossing over.
8. Cytokinesis: Plant vs Animal cells.
| Animal Cytokinesis | Plant Cytokinesis |
|---|---|
| Occurs by the appearance of a furrow in the plasma membrane. | Occurs by the formation of a cell plate. |
| The furrow deepens centripetally (from outside to inside). | The cell plate grows centrifugally (from center to outside). |
| Spindle fibres degenerate. | Spindle fibres persist (phragmoplast). |
Meiosis & Comparisons
9. Examples of equal vs unequal daughter cells in meiosis.
Equal Size:
– Spermatogenesis: Formation of sperms in human males. One primary spermatocyte gives four equal-sized spermatids.
Unequal Size:
– Oogenesis: Formation of ova (eggs) in human females. One primary oocyte gives one large ovum and three small polar bodies.
10. Distinguish Anaphase (Mitosis) from Anaphase I (Meiosis).
| Anaphase (Mitosis) | Anaphase I (Meiosis) |
|---|---|
| Centromeres split. | Centromeres do not split. |
| Chromatids separate and move to opposite poles. | Homologous chromosomes separate and move to opposite poles. |
| Resulting cells get same chromosome number as parent. | Resulting cells get half the chromosome number. |
11. Main differences between Mitosis and Meiosis.
| Mitosis | Meiosis |
|---|---|
| Occurs in somatic cells. | Occurs in germ cells. |
| Single division. | Two successive divisions (I and II). |
| Produces 2 diploid daughter cells. | Produces 4 haploid daughter cells. |
| Prophase is short; no crossing over. | Prophase I is long; crossing over occurs. |
| Identical to parent. | Genetically different (Variation). |
12. What is the significance of meiosis?
2. Genetic Variation: Crossing over during Prophase I introduces new combinations of genes, leading to genetic variability in the population, which is essential for evolution.
Critical Thinking & Analysis
13. Discuss haploid insects/plants where cell division occurs.
(i) Haploid Insects/Lower Plants with Cell Division:
– Male Honey Bees (Drones): They develop from unfertilized eggs (haploid) and undergo mitosis to produce sperms.
– Lower Plants (Algae, Bryophytes, Fungi): Their dominant phase is haploid (gametophyte), and they divide by mitosis.
(ii) Haploid Cells in Higher Plants without Division:
– Synergids and Antipodals: Present in the embryo sac of angiosperms. They are haploid but do not divide further and eventually degenerate.
14. Can there be mitosis without DNA replication in ‘S’ phase?
No.
Mitosis is the separation of duplicated chromosomes. Without DNA replication in the S phase, there are no sister chromatids to separate. Therefore, DNA replication is a prerequisite for cell division.
15. Can there be DNA replication without cell division?
Yes.
If DNA replication occurs (karyokinesis) but is not followed by cell division (cytokinesis), it leads to a condition called Polyploidy (multiple sets of chromosomes). This is common in plants (e.g., endosperm formation in liquid endosperm of coconut).
16. Analysis of Chromosome Number (N) and DNA Content (C).
Assuming a diploid cell ($2N$) with DNA content $2C$:
| Stage | No. of Chromosomes (N) | Amount of DNA (C) |
|---|---|---|
| $G_1$ | $2N$ | $2C$ |
| S (Synthesis) | $2N$ | $2C \to 4C$ (Doubles) |
| $G_2$ | $2N$ | $4C$ |
| M (Metaphase) | $2N$ | $4C$ |
| M (Anaphase) | $4N$ (Sister chromatids separate) | $4C$ |
| Daughter Cells | $2N$ | $2C$ |