What went wrong when the king rode under the arch?

It was too low and knocked off his crown

How does the chain of blame unfold in the poem?

Each accused person blames another, ending with the king's own changes

What is the average cell cycle span for a mammalian cell?

A typical human cell in culture divides about once every 24 hours.

Distinguish cytokinesis from karyokinesis.

Karyokinesis is the division of the nucleus, while cytokinesis is the division of the cytoplasm that follows it.

What happens during the S phase of interphase?

DNA synthesis or replication takes place so that the amount of DNA per cell doubles, although the chromosome number remains the same.

What is the G0 or quiescent stage of the cell cycle?

It is an inactive stage that cells which do not divide further enter from G1; they remain metabolically active but stop proliferating.

Why is mitosis called equational division?

Because the number of chromosomes in the parent and daughter cells remains the same.

Name the stage at which crossing over and pairing of homologous chromosomes take place.

Pairing (synapsis) occurs in zygotene and crossing over occurs in pachytene of prophase I of meiosis.

How does cytokinesis in plant cells differ from that in animal cells?

In animal cells a furrow appears in the plasma membrane and deepens to divide the cell, while in plant cells a cell plate forms in the centre and grows outward to meet the lateral walls.

Describe synapsis, bivalent and chiasmata.

Synapsis is the pairing of homologous chromosomes, a bivalent or tetrad is the structure formed by a synapsed pair, and chiasmata are the X-shaped sites where crossing over has occurred.

What is the significance of meiosis?

Meiosis conserves the chromosome number of a species across generations by halving it before fertilisation, and it increases genetic variability, which is important for evolution.

List the main differences between mitosis and meiosis.

Mitosis is one division giving two diploid identical cells, while meiosis is two divisions with a single DNA replication, involving pairing and crossing over and giving four haploid genetically varied cells.

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Chapter 10: Cell Cycle and Cell Division — Class 11 Biology

Biology · 19 chapters

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Chapter 10: Cell Cycle and Cell Division

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DNA replication during the cell cycle takes place in the:

Summary

This chapter explains how cells reproduce through the cell cycle and the two kinds of division, mitosis and meiosis. The cell cycle is the orderly sequence by which a cell duplicates its genome, synthesises other constituents and divides; in a typical human cell it lasts about 24 hours. It has two basic phases: interphase, which occupies over 95 per cent of the cycle and is subdivided into G1 (growth without DNA replication), S (DNA replication, when DNA content doubles though chromosome number stays the same) and G2 (preparation for mitosis), and the M phase (mitosis). Cells that stop dividing enter a quiescent G0 stage. Mitosis, the equational division, keeps the chromosome number constant and proceeds through prophase (chromosome condensation, spindle formation), metaphase (chromosomes align on the metaphase plate), anaphase (centromeres split and chromatids move to poles) and telophase (nuclei reform), followed by cytokinesis (a furrow in animal cells, a cell plate in plant cells). Meiosis, the reductional division, occurs in diploid cells that form gametes; it has two divisions but one round of DNA replication. Meiosis I includes a long prophase I (leptotene, zygotene, pachytene with crossing over, diplotene and diakinesis), and meiosis II resembles mitosis, yielding four haploid cells. Mitosis enables growth and repair; meiosis conserves chromosome number across generations and generates variation.

Phases of the cell cycle: G1, S, G2M phase and the quiescent G0 stageMitosis: prophase to telophaseCytokinesis and significance of mitosisMeiosis I and prophase I sub-stagesMeiosis II and significance of meiosis

Key terms

Cell cycle

The sequence of events by which a cell duplicates its genome and divides into two daughter cells.

Interphase

The phase between two M phases, comprising G1, S and G2, during which the cell grows and replicates its DNA.

Mitosis

The equational division of a cell into two daughter cells with the same chromosome number as the parent.

Meiosis

The reductional division that halves the chromosome number and produces four haploid cells.

Crossing over

The exchange of genetic material between non-sister chromatids of homologous chromosomes during pachytene.

Cytokinesis

The division of the cytoplasm following nuclear division, completing cell division.

Important questions

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The sequence of events by which a cell duplicates its genome and divides into two daughter cells.

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Practice quiz · Cell Cycle and Cell Division

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