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Q1. Name the stage during which chromatids start moving to the opposite poles.

Solution

Anaphase

Q2. Define bivalents.

Solution

Bivalents are the homologous chromosomes which are paired during the zygotene stage of prophase I in meiosis.

Q3. State the events which take place during prophase.

Solution

Events which occur during prophase:

Chromosomal material condenses into chromosomes. Chromatids and centromeres are visible.

The mitotic spindle starts forming in the cytoplasmic microtubules and proteinaceous components of the cytoplasm.

Q4. Which is the longest phase of the cell cycle?  

Solution

Interphase is the longest phase of the cell cycle.  

Q5. Sanjay observes a cell in the metaphase stage under a microscope. Describe the structure or appearance of chromosomes which he may have observed.

Solution

Chromosomes are thick and clearly visible. Their two sister chromatids are visible. The sister chromatids are held together by the centromere. Small disc-shaped structures are visible at the surface of the centromeres. These structures are called kinetochores, and they provide the surface of attachment of spindle fibres to chromosomes.

Q6. Distinguish between the cytokinesis of plant cell and animal cell.

Solution

Cytokinesis of Animal Cell Cytokinesis of Plant Cell

A furrow appears in the plasma membrane.

Cytokinesis begins with the formation of the cell plate in the middle of the cell.

The furrow deepens and joins the centre dividing the cell into two daughter cells.

The cell plate grows from the centre outwards, dividing the cell into two daughter cells.

Q7. State the events which occur during interphase.

Solution

Events which occur during the interphase are as follows:

The cells grow.

DNA replication takes place.

Q8. What does the cell plate represent?

Solution

The cell plate represents the middle lamella between the walls of the adjacent cells.

Q9. State the duration of the cell cycle in yeast.  

Solution

The duration of the cell cycle in yeast is 90 minutes.  

Q10. Identify the stages during which the following events occur in M phase:

Spindle fibres attach to the kinetochores of chromosomes.

Golgi complex and endoplasmic reticulum disappear.

Daughter chromosomes begin their migration towards the opposite poles.

Centromere splits.

Solution

Spindle fibres attach to the kinetochores of chromosomes - Metaphase

Golgi complex and endoplasmic reticulum disappear - Prophase

Daughter chromosomes begin their migration towards opposite poles - Anaphase

Centromere split - Anaphase

Q11. Name the following:

Complex structure formed during synapsis

The stage between two meiotic divisions

The sites at which the homologous chromosomes remain unseparated during prophase I

Enzyme involved in crossing over

Solution

Complex structure formed during synapsis - Synaptonemal complex

The stage between the two meiotic divisions - Interkinesis

The sites at which the homologous chromosomes remain unseparated during prophase I - Chiasmata

Enzyme involved in crossing over - Recombinase

Q12. State the event which marks the beginning of prophase during mitosis.

Solution

Initiation of condensation of chromatin material marks the beginning of prophase during mitosis.

Q13. Define the following:    

M phase  

Interphase  

Solution

M phase - It is the phase of the cell cycle during which actual cell division occurs.  

Interphase - It is the phase of the cell cycle which occurs between the two successive M phases.  

Q14. Define meiosis.

Solution

Meiosis is a specialised kind of cell division which reduces the chromosome number by half resulting in the formation of haploid daughter cells.

Q15. Define crossing over.

Solution

Crossing over is the exchange of genetic material between two homologous chromosomes.

Q16. Draw a diagram of the cell cycle indicating the formation of two new daughter cells from the parent cell.

Solution

 

Q17. Define cell cycle.

Solution

The sequence of events during which a cell duplicates its genome, synthesises its other constituents and eventually divides into two daughter cells is called the cell cycle.

Q18. Label the diagram and also determine the stage at which the contents of a cell appear like the following diagram.

Solution

Metaphase:  

Q19. Name the stage at which the morphology of chromosomes can be studied.

Solution

The morphology of chromosomes can be studied at metaphase.

Q20. In how many hours do human cells in a culture divide?

Solution

Human cells in a culture divide every 24 hours.

Q21. Explain the events which occur during a pachytene stage.  

Solution

The following events occur during the pachytene stage:    

Recombination nodules appear.  

Homologous chromosomes cross over at the recombination nodules for the exchange of genetic material.

Q22. Why is mitosis also called equational division?

Solution

During mitosis, the number of chromosomes in the parent and progeny cells is the same. Hence, it is also called equational division.

Q23. Distinguish between interphase and M phase.

Solution

Interphase M phase

It the phase between two successive M phases.

It is the phase during which actual cell division takes place.

It is the longest phase of the cell cycle.

It is the short phase of the cell cycle.

Chromosomes are in the form of chromatin.

Chromosomes are thick and rod-like.

Q24. State the two phases of the cell cycle.

Solution

The two phases of the cell cycle are

Interphase

Mitosis phases

Q25. Write the entities which will be missing if a cell at the end of prophase is observed under the microscope.  

Solution

If the cell at the end of prophase is observed under a microscope, the Golgi complex, endoplasmic reticulum, nucleolus and nuclear envelope would be missing.  

Q26. Describe the process of cytokinesis in plant cells.

Solution

In plant cells, the precursors of the cell wall, i.e. the cell plate, are formed in the centre of the cell. The cell plate grows from the middle of the cell outwards, dividing the cell into two daughter cells. The cell plate represents the middle lamella between the walls of two adjacent cells.

Q27. Name the enzyme involved in crossing over.

Solution

Recombinase

Q28. Draw the diagrams of    

Transition to metaphase  

Anaphase  

Solution

Transition to Metaphase:  

     

Anaphase:  

   

Q29. What are recombination nodules?

Solution

Recombination nodules are the sites at which crossing over occurs between two non-sister chromatids of the homologous chromosomes during the pachytene stage.

Q30. Name the cell in which the diplotene phase lasts for years.  

Solution

Oocytes of some vertebrates  

Q31. State the five phases of meiosis I.

Solution

The five phases of meiosis I are

Leptotene

Zygotene

Pachytene

Diplotene

Diakinesis

Q32. State the significance of fertilisation.

Solution

Fertilisation restores the diploid phase in the life cycle.

Q33. Name the following:

Division of cytoplasm

Division of nucleus

Solution

Division of cytoplasm - Karyokinesis

Division of nucleus - Cytokinesis

Q34. Describe the quiescent stage of the cell cycle.

Solution

Some cells do not exhibit cell division or some cells divide only when the body has lost cells during an injury. Such cells enter the quiescent stage. In this stage, the cells are metabolically active. However, the cells do not divide. They undergo division only when there is a requirement by the body under certain conditions.

Q35. Identify the phase shown in the following diagram.

Solution

Late Prophase:  

Q36. Write the three phases which are collectively known as interphase.

Solution

Three phases which are collectively known as the interphase are

G₁ phase

S phase

G₂ phase

Q37. State the events which occur during the S phase in animal cells.  

Solution

During the S phase in animal cells, DNA replicates in the chromosome and centrioles duplicate in the cytoplasm.  

Q38. How many chromosomes are present in an onion cell?  

Solution

14 chromosomes are present in an onion cell.  

Q39. What is the function of kinetochores?

Solution

Kinetochores provide the surface for the attachment of spindle fibres to the chromosomes.

Q40. State the event which marks the beginning of diakinesis.  

Solution

Terminalisation of chiasmata marks the beginning of diakinesis.  

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