ICSE Class 10 Biology Chapter 02 Structure of Chromosomes Cell Cycle and Cell Division

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Syllabus: Cell Cycle and Cell Division. Structure of chromosome.

Scope of Syllabus: Cell cycle - Interphase ($G_1, S, G_2$) and M phase.

Cell Division: Mitosis and its stages. A basic understanding of Meiosis as a reduction division (stages not required). Significance and major differences between mitotic and meiotic division.

Basic structure of chromosome with elementary understanding of terms such as chromatin, chromatid, gene, structure of DNA and centromere.

(Structure of chromosomes has been discussed before cell cycle and cell division for better understanding of cell division)

Cell division is one of the most fundamental characteristics of life. This is the method which enables life to perpetuate generation after generation. This is equally true in the case of the simplest organisms like amoeba as is in the highly complex ones such as ourselves, the giant sized elephants or the tall coconut tree. Some of the details of cell division are slightly difficult to understand, but we have tried to make it as simple as possible.

2 A. Structure Of Chromosomes

You have learnt earlier that the most conspicuous events occurring during cell divisions are all related to chromosomes. The duplicated chromosomes get evenly distributed into the daughter cells during mitosis. Thus, all body cells that are the result of a long chain of repeated mitotic divisions, have the same type of chromosomes in the same number. This ensures the normal functioning of the cells and through that, the life of the organism.

2.1 What Are Chromosomes?

When a normal unstained living cell is observed under a light microscope, its nucleus may not appear to contain any particular thing inside. But when the same cell is stained with suitable dyes, several structures become noticeable in the nucleus. In all probability, you may be looking at a non-dividing stage (interphase) of the cell, although depending upon the part of the body from where the cell has been taken, it may as well show some division stages too.

In the interphase, the nucleus shows a network of very long extremely thin dark-staining fibres called chromatin fibres. As the cell begins to enter the first stage (prophase) of cell division, the chromatin fibres condense to form chromosomes (Fig. 2.1). The chromosomes readily pick up certain dyes and get coloured, hence the name chromosomes (chroma: coloured, soma: body).

Chromosomes are the highly condensed coiled chromatin fibres.

2.2 Discovery Of Chromosomes

Chromosomes were first discovered in 1882 by a German Scientist, Walther Fleming. He noticed these in the rapidly dividing cells of the larvae of salamander (an amphibian). The microscope he used was of old type and through it, he saw minute threads that appeared to be dividing lengthwise. Fleming called their division mitosis (literally meaning "thread"). Subsequently, with the help of more powerful microscopes and by using special techniques, the chromosomes and their constituents have been studied in great detail.

Fig. 2.1: Condensation of chromatin fibres into chromosomes during mitotic prophase

2.3 Structure Of Chromosomes

Each chromosome in its condensed form as visible during the start of cell division, consists of two chromatids joined at some point along the length. This point of attachment is called centromere, and it appears as a small constricted region (Fig. 2.2). The centromere also serves to attach to the spindle fibre during cell division. Each chromosome's centromere is located at a particular site. As the spindle fibre contracts, the sister chromatids are separated at the centromere, and each is pulled away from the other towards the two poles of the dividing cell (see Fig. 2.9, stages D & E).

Fig. 2.2: A human chromosome showing the chromatids and the centromere (constricted part)

After the completion of cell division, the chromatids (now called chromosomes) decondense and revert to their very long and fine thread-like chromatin fibres. There would be as many chromatin fibres inside the nucleus as the number of chromosomes that appear during cell division.

Chromatin: The chromatin material that constitutes the fibre is formed of two substances:

1. DNA (deoxyribonucleic acid) - about $40\%$.

2. Histones (a particular type of proteins) - about $60\%$.

Figure 2.3 given below is a highly diagrammatic representation of the structure of chromatin fibres showing the sequentially enlarged view of the DNA strand.

Fig. 2.3: A highly diagrammatic representation of the structure of a chromosome, the chromatin fibre and DNA

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