Hello, and welcome to your Class Ten Biology lesson. Today, we are stepping into one of the most fundamental processes of life itself — how cells divide and perpetuate. Our chapter is Structure of Chromosomes, Cell Cycle and Cell Division. We will explore what chromosomes are made of, how cells prepare to divide, the intricate steps of mitosis, and the special reduction division called meiosis that creates sex cells.
Let us begin with the chromosome itself. In a living cell that is not dividing, the nucleus appears relatively quiet under a microscope. But stain it with suitable dyes, and you will notice long, thread-like structures. These are chromosomes — the name comes from Greek words meaning "coloured body."
Chromosomes are not always visible. Between divisions, they exist as a diffuse network called chromatin. Think of chromatin as a very long, extremely thin fibre made of DNA wrapped around proteins. When a cell prepares to divide, this chromatin coils and condenses dramatically, becoming thick enough to see as distinct chromosomes.
What exactly is chromatin made of? Two main components: DNA, making up about forty percent, and special proteins called histones, making up about sixty percent.
The structure of DNA is one of the most elegant discoveries in biology. It is a double-stranded helix — imagine a twisted ladder. The backbone of this ladder is made of alternating sugar and phosphate groups. The rungs are pairs of nitrogenous bases. Four types exist: adenine, guanine, cytosine, and thymine. Adenine always pairs with thymine, and guanine always pairs with cytosine. This precise pairing is the foundation of genetic inheritance.
Now, how does two metres of DNA fit into a nucleus only six micrometres across? The answer is remarkable packaging. DNA winds around a core of eight histone proteins, forming a structure called a nucleosome. Picture a football with a rope wrapped around it — that is roughly a nucleosome. These nucleosomes coil and supercoil, like a telephone cord, eventually forming the compact chromosome.
When we examine a chromosome during cell division, we see it consists of two identical halves called chromatids — specifically, sister chromatids. These sister chromatids are joined at a narrow region called the centromere. The centromere has a crucial job — it attaches to spindle fibres that will eventually pull the chromatids apart.
Embedded within the DNA are genes — specific sequences of nucleotides that encode particular proteins. Genes are the units of heredity, passed from parents to offspring, determining everything from eye colour to enzyme production.
Why do cells need to divide at all? Four essential reasons: growth, replacement, repair, and reproduction.
Every organism begins as a single cell. Through repeated divisions, that cell multiplies into trillions, forming tissues and organs. Meanwhile, millions of your red blood cells die every second — and are replaced through division. Injuries heal because cells divide to fill gaps and mend broken tissue. And life perpetuates because certain cells divide to create offspring.
Before a cell can divide, it must complete an orderly sequence of events called the cell cycle. The cell cycle has two main phases: interphase, when the cell prepares for division, and the mitotic phase, when actual division occurs.
Interphase itself has three sub-phases. First, G₁ — the first growth phase — where the cell synthesises RNA and proteins, and organelles like mitochondria and chloroplasts divide. In late G₁, cells either enter a resting phase or proceed to the next phase. Then comes S — the synthesis phase — where the cell replicates its DNA, duplicating every chromosome. Finally, G₂ — the second growth phase — where the cell produces proteins needed for division. Only then is the cell ready to enter mitosis.
Mitosis is the division of a parent cell into two identical daughter cells, each with the same chromosome number. It occurs in two coordinated processes: karyokinesis — division of the nucleus — and cytokinesis — division of the cytoplasm.
Karyokinesis proceeds through four stages. In prophase, chromatin condenses into visible chromosomes, each already duplicated into two sister chromatids joined at the centromere. The nuclear membrane and nucleolus disappear, and in animal cells, the centrosome splits with simultaneous duplication of centrioles, forming two star-like structures called asters at opposite poles, with spindle fibres stretching between them.
In metaphase, chromosomes line up precisely at the cell's equator, each attached to spindle fibres at its centromere. This alignment ensures equal distribution.
Anaphase is dramatic — centromeres split, and sister chromatids separate, becoming individual chromosomes. Spindle fibres contract, pulling these daughter chromosomes to opposite poles.
Finally, in telophase, chromosomes reach the poles, decondense into chromatin threads, and nucleoli reappear in each daughter nucleus. New nuclear membranes form around each set. Then cytokinesis completes the process — in animal cells, a cleavage furrow deepens from the cell membrane inward to separate two daughter cells; in plant cells, a cell plate forms at the centre and grows outward to the periphery.
There is a second, fundamentally different type of cell division: meiosis. Meiosis occurs only in reproductive organs — testes and ovaries in animals, anthers and ovaries in plants. Its purpose is to produce sex cells: sperms and eggs, or pollen grains and ovules.
The defining feature of meiosis is reduction. Where mitosis maintains the diploid chromosome number — 2n — meiosis halves it, producing haploid cells with only n chromosomes. This reduction is essential for sexual reproduction. When sperm and egg fuse at fertilisation, the diploid number is restored, maintaining species chromosome counts across generations.
Meiosis involves two successive divisions. The first is the true reduction division — homologous chromosomes pair up, and maternal and paternal chromosomes separate, reducing the chromosome number to half. During this pairing, a remarkable event called crossing over occurs: non-sister chromatids of homologous chromosomes exchange segments at points called chiasmata, creating new genetic combinations. The second meiotic division resembles mitosis, separating sister chromatids. The result: four genetically unique haploid cells from one diploid parent.
Let us recap the essential takeaways from this chapter.
First, chromosomes are condensed structures of chromatin, composed of DNA and histone proteins, organised into nucleosomes. Second, the cell cycle comprises interphase — with G₁, S, and G₂ phases — followed by the mitotic phase. Third, mitosis ensures equal chromosome distribution for growth, repair, and asexual reproduction, maintaining the diploid number. Fourth, meiosis is reduction division producing four haploid gametes from one diploid cell, enabling sexual reproduction with restored chromosome numbers at fertilisation. Fifth, crossing over during meiosis creates genetic variation through recombination, making siblings genetically unique. And sixth, cytokinesis differs between animal and plant cells — furrowing versus cell plate formation.
You have now journeyed through the machinery of life at its most fundamental level — how genetic information is packaged, replicated, and distributed with extraordinary precision. Cell division is not merely a biological process; it is the thread connecting every living generation to the next. Keep exploring, keep questioning, and remember: every cell in your body carries the legacy of billions of years of perfected division. Until next time, stay curious.