Question
What is the nucleus of a cell? Explain its structure and describe its main functions in the cell.
Solution — Step by Step
The nucleus is the largest organelle in most eukaryotic cells. It is often called the “control centre” or “brain of the cell” because it directs all cellular activities.
Not all cells have a nucleus. Prokaryotic cells (bacteria, archaea) have no membrane-bound nucleus — their DNA floats freely in the cytoplasm in a region called the nucleoid. Eukaryotic cells (plant, animal, fungal cells) have a well-defined nucleus bounded by a double membrane.
Red blood cells (mature) are an exception among eukaryotes — they lose their nucleus to make more room for haemoglobin.
The nucleus has four main components:
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Nuclear envelope (nuclear membrane): A double membrane surrounding the nucleus, with tiny pores (nuclear pores) that allow selective passage of molecules between nucleus and cytoplasm.
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Nucleoplasm: The jelly-like fluid inside the nucleus, analogous to cytoplasm in the cell body.
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Chromatin: Long, thin threads of DNA wrapped around proteins called histones. During cell division, chromatin condenses into visible chromosomes.
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Nucleolus: A dense, dark-staining region inside the nucleus (not bounded by a membrane). The nucleolus manufactures ribosomal RNA (rRNA) and assembles ribosomes.
The nucleus contains the cell’s DNA — the genetic blueprint. DNA carries instructions (genes) for making proteins. Proteins are the workhorses of the cell: they form structures, act as enzymes, and carry out every cellular function.
By controlling which genes are “switched on” at any time, the nucleus controls what the cell does, when it does it, and how.
The nucleus stores the cell’s genetic information. When a cell divides (mitosis or meiosis), the nucleus is replicated and distributed to daughter cells, ensuring each new cell receives the full set of instructions.
In sexual reproduction, the nucleus of sperm and egg cells each contain half the genetic information; their fusion restores the full set in the offspring. This is how traits are inherited from parents.
The nucleus directs protein synthesis even though ribosomes (where proteins are made) are in the cytoplasm. The sequence is:
- DNA in the nucleus is transcribed into messenger RNA (mRNA)
- mRNA leaves the nucleus through nuclear pores
- mRNA is translated by ribosomes in the cytoplasm into protein
The nucleus thus serves as the information source; the cytoplasm carries out production.
Why This Works
The nucleus is isolated from the rest of the cell by its double membrane. This separation protects the DNA from damaging chemicals in the cytoplasm and allows the cell to regulate precisely what information gets expressed at any time.
The nuclear pores are not simple holes — they are gated channels with complex proteins (nucleoporins) that selectively allow molecules to pass. Large proteins (histones, RNA polymerases) enter the nucleus; mRNA and ribosomes exit. This selective permeability is crucial for gene regulation.
Alternative Method
For CBSE Class 8, a simple three-function answer is sufficient:
- The nucleus is the control centre of the cell — it directs all activities.
- It contains DNA/chromosomes that carry hereditary information.
- It controls cell division and inheritance of traits.
A labelled diagram showing nuclear membrane, nucleoplasm, nucleolus, and chromatin earns full marks in board exams.
Common Mistake
Students often say “the nucleus makes proteins.” The nucleus does NOT make proteins — it contains the instructions (DNA) for making proteins, and makes mRNA. The actual protein synthesis happens at ribosomes in the cytoplasm. In questions asking “where are proteins made?” the answer is ribosomes, not nucleus.
Remember: prokaryotes have no nuclear membrane, but they still have DNA. “No nucleus” means no membrane-bound nucleus — not no genetic material. In CBSE Class 8 and 9, this distinction about prokaryotes vs eukaryotes often appears as a 1-mark question.