Structure of Chromosome Class 10 ICSE — Selina Chapter 2

Structure of Chromosome Class 10 ICSE is the second chapter of the Selina Concise Biology book. It covers how chromosomes form, what DNA is made of, and how cells divide. You will meet mitosis, meiosis, the cell cycle, and the chromosome numbers of body and sex cells. This page explains every section in the book’s own order. See the Class 10 Biology collection for other chapters.

ICSE Class 10 Biology Selina Concise Chapter 2 Structure of Chromosomes Cell Cycle and Cell Division PDF

What is inside Chapter 2

13 pages · 8 sections · 11 exercise questions

• PROGRESS CHECK: 1 questions
• REVIEW QUESTIONS: 10 questions

2.1 WHAT ARE CHROMOSOMES?

When you observe a normal unstained living cell under a light microscope, its nucleus appears empty. Add a suitable dye and several structures become visible inside the nucleus. The cell may be in its non-dividing stage, called interphase. Depending on which part of the body the cell came from, you might also catch some division stages.

During interphase, the nucleus shows a network of very long, extremely thin, dark-staining fibres called chromatin fibres. As the cell begins prophase, the first stage of cell division, the chromatin fibres condense. They form the structures known as chromosomes.

Chromosomes readily pick up certain dyes and get coloured. Hence the name comes from Greek: chroma means coloured and soma means body. In simple terms, chromosomes are the highly condensed, coiled chromatin fibres. They only appear as distinct bodies during division.

2.2 DISCOVERY OF CHROMOSOMES

Walther Fleming, a German scientist, first discovered chromosomes in 1882. He noticed them in the rapidly dividing cells of salamander larvae. Through his old microscope, he saw minute threads that appeared to be dividing lengthwise. He called this division mitosis, a word that literally means thread.

Later, more powerful microscopes and special techniques allowed scientists to study chromosomes in detail. They examined the chromosomes themselves and their constituents. The discovery shows how better instruments expand biological knowledge. Each improvement in microscopy revealed more about the thread-like structures Fleming first saw. Modern understanding of chromosomes rests on these early observations.

This section is deliberately short in the chapter. Its job is to fix the discovery in your memory. The name mitosis will appear again when you study the phases of cell division later in this chapter. Remember the year 1882 and the salamander larvae, as these often appear in exam questions.

2.3 STRUCTURE OF CHROMOSOMES

In its condensed form, each chromosome has two chromatids joined at a point along its length. This point of attachment is called the centromere. It appears as a small constricted region. The centromere also attaches the chromosome to spindle fibres during cell division. Each chromosome’s centromere sits at a particular site.

As the spindle fibre contracts, the sister chromatids separate at the centromere. Each chromatid is pulled toward a pole of the dividing cell. After division finishes, the chromatids, now called chromosomes, decondense. They revert to long, fine chromatin fibres. The nucleus holds as many chromatin fibres as there were chromosomes during division.

Chromatin material is formed of two substances. DNA makes up about 40 percent. Histones, a type of protein, make up about 60 percent. In the chromatin fibre, DNA winds around a core of histone molecules. This core acts like a football around which a long rope is wound. Each such complex is called a nucleosome. A single human chromosome may hold about a million nucleosomes. The entire fibre is coiled and supercoiled, like a telephone cord.

DNA is a large macromolecule made of two complementary strands. They wind around each other in a double helix. Rosalind Franklin studied its shape in 1953. Watson and Crick worked out the structure that same year. Each strand is composed of repeating nucleotides. A nucleotide has three components: phosphate, a pentose sugar, and a nitrogenous base.

Four nitrogenous bases occur in DNA: Adenine (A), Guanine (G), Cytosine (C) and Thymine (T). A pairs with T using two hydrogen bonds. G pairs with C using three hydrogen bonds. The two strands form a ladder-like arrangement, with the bases as the rungs. The consistent pairing keeps the rungs of the DNA ladder uniform. If one strand has a run of bases A-T-G-C, the complementary strand pairs T-A-C-G with it.

During interphase, each DNA molecule duplicates. The double helix opens at one end, freeing the two strands. New complementary strands form along each one. This process continues for the whole length of the DNA.

2.4 WHAT ARE GENES ?

Genes are specific sequences of nucleotides on a chromosome. They encode particular proteins. These proteins express as particular features of the body. So a gene carries the instruction for one trait, such as eye colour or height.

Most of the DNA between genes is non-functional. This region, about 99 percent of the total DNA, shows tremendous variation from person to person. It helps in identification by DNA fingerprinting, better called DNA profiling. This technique is useful in establishing paternity and maternity in criminal cases. Blood, semen, hair, or clothing from a crime scene provide biological evidence. The chapter marks this topic as not in the syllabus.

A progress check in this section asks recall questions. First, chromatin fibre is made up of DNA and histones. Second, chromatids are attached to each other at the centromere. Third, DNA replicates during the synthesis phase of the cell cycle. These three facts anchor the whole chapter. Genes return in more detail in the genetics fundamentals chapter.

2.5 NEW CELLS NEED TO BE PRODUCED

Cells divide for four reasons: growth, replacement, repair, and reproduction.

• Growth: Every organism begins as a single fertilised egg. This cell divides repeatedly to form a cluster of cells. The cells then specialise into tissues and organs.
• Replacement: Cells wear out during normal body functions. About 20 million red blood cells are destroyed every minute. New ones form by division in the bone marrow. Plants grow new leaves to replace old dried ones.
• Repair: Cuts in the skin and fractures in bone need new cells. Dividing cells cover the gaps and join broken ends.
• Reproduction: Amoeba and bacteria divide into two independent cells. Higher forms use special cells in reproductive organs to make sperms and eggs.

Human chromosome numbers follow a clear pattern:

The symbol 2n means diploid, the paired chromosome set. The symbol n means haploid, one chromosome from each pair. A fertilised egg receives 23 from the sperm and 23 from the egg, restoring 46. The reduction in meiosis is significant because it keeps this balance.

Consider a fertilised egg that divides three times. The cell count doubles each time: 1, then 2, then 4, then 8 cells. Every new cell carries the full 46 chromosomes. Growth depends on this repeated doubling.

2.6 TYPES OF CELL DIVISION

There are two types of cell division. Mitosis leads to growth and development. Meiosis leads to the production of gametes, the sex cells.

Mitosis is the division in which one parent cell produces two identical daughter cells. The normal chromosome number is maintained at each division. Before division, the nucleus doubles the quantity of DNA. This happens during interphase, which looks resting but actively synthesises DNA.

Karyokinesis, the nuclear division, has four phases. In prophase, chromosomes become short, thick and visible, each already duplicated into two chromatids joined at the centromere. The nuclear membrane and nucleolus disappear. In metaphase, chromosomes line up on the equatorial plane, each attached to a spindle fibre by its centromere. In anaphase, the centromere divides and sister chromatids separate toward opposite poles. In telophase, chromatids lengthen into chromatin threads and the nuclear membrane and nucleolus reappear.

Cytokinesis divides the cytoplasm. Animal cells form a furrow that deepens and splits the cell. Plant cells lay down a cell plate at the equatorial plane.

Mitosis has five points of significance. It causes growth, repairs damaged tissues, replaces old and dead cells, enables asexual reproduction, and maintains the same chromosome number in daughter cells. The terms “parent” and “daughter” cells are conventional, not literal.

Body cells have different life spans. The chapter gives several examples:

Worked example: a cell with 8 chromosomes undergoes mitosis. Each daughter cell receives a complete set of 8 chromosomes. The number stays constant through every mitotic division.

2.7 CELL CYCLE — “Divide, grow and redivide”

New cells at the end of mitosis are small, with a full-sized nucleus and little cytoplasm. They enter interphase, during which they prepare for the next division. Interphase has three phases.

• First growth phase (G₁): RNA and proteins are synthesised. The volume of cytoplasm increases. Mitochondria and chloroplasts divide, as they have their own DNA. In late G₁, cells either withdraw into a resting phase or enter the next phase.
• Synthesis phase (S): More DNA is synthesised. The chromosomes are duplicated.
• Second growth phase (G₂): A shorter phase in which RNA and proteins needed for division continue to be made. The cell is now ready for the next division.

Can the cell cycle go on endlessly? No. It stops permanently in some places, temporarily in others, and continues where needed. Nerve cells do not divide once formed. Liver cells may divide once every one to two years. Skin cells are continuously lost and replaced. Plant meristems divide rapidly to produce new leaves, buds and flowers. Uncontrolled non-stop cycles may lead to tumours.

Cell production and cell death balance differently across life. In childhood, new cells outnumber dying cells, which is why we grow. In adulthood, the cell population stays constant. In old age, new cells fall short of those dying. This section is covered within the broader Class 10 books section of the site.

2.8 MEIOSIS (Reduction division producing gametes)

Meiosis produces the sex cells or gametes. In humans, it takes place in the testis and ovary to produce sperms and ova. In flowering plants, it takes place in the anthers and the ovary to produce pollen grains and ovules.

The most significant aspect of meiosis is that the chromosome number in the sex cells is halved. Humans have 23 pairs of chromosomes in body cells. Only one member of each pair, the haploid (n) number, passes to each sex cell. When male and female gametes fuse at fertilisation, the normal diploid (2n) number is restored.

As per the scope of the ICSE syllabus, the stages of meiosis are not required. You do not need to memorise the phase names of meiosis. You do need to know that meiosis is completed in two divisions. Meiosis I is the reduction division. Meiosis II is mitotic in nature.

Meiosis provides for mixing up of genes in two ways. First, maternal and paternal chromosomes get mixed up during the first division as they separate from homologous pairs. Second, cross joining, called crossing over, exchanges chromatid material between the two members of a homologous pair. This exchange happens at the chiasma and results in genetic recombination. These permutations provide the innumerable variations seen in the progeny.

Worked example: a cell with 8 chromosomes undergoes meiosis. It produces four gametes. Each gamete carries 4 chromosomes, one from each of the 4 pairs. Fertilisation of two gametes restores 8. Compare this with mitosis, where a cell with 8 chromosomes gives two daughter cells, each still carrying 8.

You can verify the Class 10 Biology syllabus requirements on the official CISCE website to confirm which parts of this chapter are examinable.

What to practise from this chapter

• PROGRESS CHECK — 1 fill-in-the-blanks question on chromatin composition, centromere attachment, and DNA replication.
• REVIEW QUESTIONS — 10 questions spanning multiple choice, very short answer, short answer, long answer, and structured application types.

Attempt the review questions after reading every section. The structured questions ask you to identify stages from diagrams and draw stages of mitosis. One question requires a labelled diagram of metaphase in an animal cell with 6 chromosomes.

Frequently asked questions

What is the difference between a chromatid and a chromosome?

A chromosome in its condensed form has two chromatids joined at the centromere. After the centromere divides in anaphase, each separated chromatid is called a chromosome.

Why must gametes be produced by meiosis and not by mitosis?

Gametes must carry half the chromosome number so that fertilisation restores the full set. If gametes were produced by mitosis, they would carry 46 chromosomes, and the fertilised egg would have 92.

What happens during the synthesis phase of the cell cycle?

During the S phase, more DNA is synthesised and the chromosomes are duplicated. Each chromosome now consists of two chromatids ready for separation in mitosis.

In which body cells does mitosis occur, and where does meiosis take place?

Mitosis occurs in somatic or body cells throughout life. Meiosis takes place only in reproductive organs: the testis and ovary in humans, and the anthers and ovary in flowering plants.

Why is the chromosome number halved in meiosis if it is restored at fertilisation?

The halving ensures that the diploid number is restored when gametes fuse. If gametes kept the full number, the zygote would carry twice the normal chromosomes.

Reference: ICSE Class 10 Biology textbook material, chapter 2: Structure of Chromosomes Cell Cycle and Cell Division — CISCE-aligned.

More from this book

• Chapter 3: Genetics Some Basic Fundamentals
• Chapter 4: Absorption By Roots the Processes Involved
• Chapter 5: Transpiration
• Chapter 6: Photosynthesis
• Chapter 7: The Circulatory System
• Chapter 8: The Excretory System
• Chapter 9: The Nervous System
• Chapter 10: Endocrine Glands
• Chapter 11: The Reproductive System
• Chapter 12: Population
• Chapter 13: Aids to Health
• Chapter 14: Health Organisations
• Chapter 15: Pollution