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

Structure Of Chromosomes, Cell Cycle And Cell Division

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

Scope Of Syllabus: Cell cycle - Interphase (G1, S, G2) 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.

Note: 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 cocoanut tree. Some of the details of cell division are slightly difficult to understand, but we have tried to make it as simple as possible.

2A. 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 a 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.

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, page 13).

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.

The DNA strand winds around a core of eight histone molecules. This core can be imagined like a football, around which a long rope is wound with one or two loops. Each such complex is called nucleosome.

A single human chromosome may have about a million nucleosomes!

The entire chromatin fibre is coiled and supercoiled something like the coils and supercoils we see in a typical telephone cord (Fig. 2.4).

Structure Of DNA

Note: The shape of the DNA molecule was studied by Rosalind Franklin in 1953 and the structure was finally worked out by Watson and Crick in the same year.

The DNA is a very large single molecule, and hence it is described as a macromolecule. It is composed of two complementary strands wound around each other in a double helix (Fig. 2.5).

Each single DNA strand is composed of repeating nucleotides which are made of three components, phosphate, sugar (pentose) arranged lengthwise and a nitrogenous base attached to the sugar inwards.

The DNA strand winds around a core of eight histone molecules (Fig. 2.6A) which extends to join (by a hydrogen bond) the complementary nitrogenous base from the other strand (Fig. 2.6B). Thus, the two strands together make a ladder-like arrangement, with the nitrogenous bases forming the "rungs" of the ladder. The bases are - Adenine (A), Guanine (G), Cytosine (C) and Thymine (T). Adenine pairs with Thymine with two hydrogen bonds. Guanine pairs with Cytosine with three hydrogen bonds (Fig. 2.6 reveals the same).

Formation Of The New DNA. During the interphase of cell cycle, each DNA molecule (i.e. the chromosome) duplicates in readiness for their equitable distribution in the two daughter cells during mitosis. For replication, the DNA double helix opens at one end, making the two strands free to which new strands begin to form and the process continues in a sequence for the whole length of the DNA (Fig. 2.7). The entire replication is a very complicated process which you will study in detail in higher classes.

2.4 What Are Genes?

Genes are specific sequences of nucleotides on a chromosome, that encode particular proteins which express in the form of some particular feature of the body.

Teacher's Note

Understanding chromosome structure helps explain why inherited traits pass from parents to children. Just as instructions in a recipe control how a cake turns out, genes control how our body develops.

Progress Check

1. Fill in the blanks:

(i) Chromatin fibre is made up of DNA and .........................................................................

(ii) The chromatids are attached to each other at ......................................................................

(iii) DNA replicates in the .............................................. of the cell cycle.

(iv) Who first discovered the structure of DNA?

........................................................................................................

........................................................................................................

Review Questions

A. Multiple Choice Type

Choose the best option out of the four alternatives a, b, c and d.

1. The chromatin material is formed of

(a) DNA only (b) DNA and Histones (c) Histones only (d) Nucleotides

2. The term "chromosomes" literally means

(a) Inherited bodies (b) Twisted threads (c) Coloured bodies (d) Shining threads

B. Very Short Answer Type

1. Name the following:

(a) The repeating components of each DNA strand lengthwise.

(b) The complex structure consisting of DNA strand and a core of histones.

(c) The type of bond which joins the complementary nitrogenous bases.

(d) The three components of a nucleotide.

C. Short Answer Type

1. What is the difference between chromatin fibre and chromosome?

2. What are the rungs of the "DNA ladder" made of?

3. Correct the following statements if there is any mistake.

(a) The four nitrogenous bases in the DNA are Guanine, Thiamine, Adrenaline and Cytosine.

(b) Genes are specific sequences of bases on a chromosome.

(c) A nucleotide is composed of a sulphate, a sugar (pentose) and a nitrogenous base.

(d) Nucleosomes are groups of cysteine molecules surrounded by DNA strands.

(e) If there are 46 chromosomes in a cell, there will be 23 chromatin fibres inside the nucleus during interphase.

D. Long Answer Type

1. What are nucleosomes?

2. What are genes?

E. Structured/Application/Skill Type

1. Given below is a schematic diagram of a portion of DNA.

(a) How many strands are shown in the diagram? with the first and ending with the last?

(b) How many nucleotides have been shown in each strand?

(c) Name the parts numbered 1, 2, 3, 4 and 5 respectively.

(d) Name the DNA unit constituted by the parts 1, 2 and 3 collectively.

2. The three sketches given below (A, B and C) are intended to represent the replication of DNA. What should be their correct sequence starting ...

Teacher's Note

DNA replication is like making a photocopy of an instruction manual - each new copy is identical to the original, ensuring that daughter cells receive complete genetic information.

This is a preview of the first 3 pages. To get the complete book, click below.