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Cell: Structure and Function - Cell Cycle and Cell Division (Mitosis and Meiosis)

Grade 11ICSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The Cell Cycle is divided into two main phases: Interphase (the preparatory phase) and MM-Phase (the division phase).

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Interphase consists of three stages: G1G_1 phase (Gap 1), SS phase (Synthesis), and G2G_2 phase (Gap 2).

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During the SS phase, DNA replication occurs. The DNA content doubles from 2C2C to 4C4C, but the chromosome number remains the same (2n2n).

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Mitosis is called equational division because the chromosome number in daughter cells remains the same as the parent cell (2n→2n2n \rightarrow 2n).

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Mitosis stages include Prophase, Metaphase (chromosomes align at the equatorial plate), Anaphase (sister chromatids separate), and Telophase.

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Meiosis is a reductional division occurring in germ cells, where one diploid (2n2n) cell produces four haploid (nn) daughter cells.

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Meiosis I reduces the chromosome number (2n→n2n \rightarrow n), while Meiosis II is similar to mitosis (n→nn \rightarrow n).

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Prophase I of Meiosis is prolonged and divided into: Leptotene, Zygotene (synapsis of homologous chromosomes), Pachytene (crossing over), Diplotene (chiasmata formation), and Diakinesis.

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Crossing over occurs between non-sister chromatids of homologous chromosomes during the Pachytene stage, leading to genetic recombination.

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Cytokinesis in animal cells occurs via a cleavage furrow (centripetal), while in plant cells, it occurs via cell plate formation (centrifugal).

📐Formulae

Number of cells after n mitotic divisions=2n\text{Number of cells after } n \text{ mitotic divisions} = 2^n

Number of generations (n) for N cells=log⁡2N\text{Number of generations (n) for } N \text{ cells} = \log_2 N

Meiotic divisions for x seeds (plants)=x+x4\text{Meiotic divisions for } x \text{ seeds (plants)} = x + \frac{x}{4}

DNA content after S phase=4C (where 2C is initial content)\text{DNA content after } S \text{ phase} = 4C \text{ (where } 2C \text{ is initial content)}

Chromosome number after Meiosis I=n (where 2n is initial number)\text{Chromosome number after Meiosis I} = n \text{ (where } 2n \text{ is initial number)}

💡Examples

Problem 1:

A root tip cell of an onion has 2n=162n = 16 chromosomes. How many chromosomes will be present in the cell at (a) G1G_1 phase, (b) G2G_2 phase, and (c) daughter cells after mitosis?

Solution:

(a) 1616, (b) 1616, (c) 1616.

Explanation:

In mitosis, the chromosome number (2n2n) remains constant throughout the stages and in the resulting daughter cells. Although DNA doubles during the SS phase (between G1G_1 and G2G_2), the number of centromeres (chromosomes) does not change.

Problem 2:

Calculate the number of mitotic divisions required to produce 10241024 cells from a single parent cell.

Solution:

1010 divisions.

Explanation:

Using the formula 2n=N2^n = N, where N=1024N = 1024. 210=10242^{10} = 1024, therefore n=10n = 10.

Problem 3:

In a pea plant, how many meiotic divisions are required to produce 100100 seeds?

Solution:

125125 meiotic divisions.

Explanation:

To produce 100100 seeds, 100100 male gametes (pollen) and 100100 female gametes (ovules) are needed. 100100 pollen grains require 100/4=25100/4 = 25 meiotic divisions. 100100 ovules require 100100 meiotic divisions (since 3 out of 4 megaspores degenerate). Total = 100+25=125100 + 25 = 125 divisions.