Nucleus structure and function

The nucleus is usually the largest organelle in a cell and acts as its control centre, because it holds the cell's genetic instructions and directs how the cell behaves.

Almost every eukaryotic cell has one nucleus. It is bounded by a double membrane and holds the DNA that carries the code for every protein the cell can make, which is why a cell that loses its nucleus soon stops working.

A bacterium, by contrast, has no true nucleus: its DNA lies free in the cytoplasm.

Parts and functions

How structure suits function

Each feature of the nucleus is matched to its job of storing the genetic material and controlling the cell.

• Double nuclear membrane, keeps the DNA separate from the cytoplasm, protecting it from the reactions and enzymes outside.
• Nuclear pores, allow selective exchange, letting messenger RNA carry the genetic code out to the ribosomes while keeping the DNA itself inside.
• Chromatin that can condense, the long threads coil tightly into chromosomes before division, so the genetic material can be shared out accurately between the new cells.
• Nucleolus, its dense grouping of the machinery for ribosome assembly lets the cell make the ribosomes it needs for protein synthesis.
• Large size and central position, the nucleus is easy to identify and lies where it can control the whole cell by determining which proteins are made.

Related processes

The nucleus is the starting point of protein synthesis: a gene in the DNA is copied (transcribed) into messenger RNA, which passes out through a nuclear pore to a ribosome, where the protein is assembled. By controlling which genes are transcribed, the nucleus controls which proteins the cell makes, and so controls the cell's structure and activity.

During mitosis the chromatin condenses into chromosomes that are copied and shared equally, producing two genetically identical daughter cells for growth and repair. During meiosis the chromosome number is halved to make gametes, which is the basis of variation and inheritance.

This links the nucleus directly to the chapters on cell division and genetics.

Common labelling errors

Worked question

Question (in the style of Paper 2 Section A): Diagram M shows the nucleus of a cell. Structure S is a double boundary with small gaps in it, and structure T is a dense body inside.

(a) Name S and T. [2 marks] (b) State the function of the gaps in S. [1 mark] (c) Explain why a cell whose nucleus is removed soon stops making enzymes and dies. [3 marks]

Model answer: (a) S is the nuclear membrane (nuclear envelope); T is the nucleolus. (b) The gaps are nuclear pores; they let messenger RNA pass out to the cytoplasm (and selected molecules in).

(c) The nucleus contains the DNA / genes that carry the code for making proteins (enzymes). Without the nucleus no messenger RNA can be made, so the ribosomes cannot assemble enzymes and the cell's reactions stop, so the cell dies.

Marking note: in part (c) the three marks are for genes/DNA carry the code, messenger RNA can no longer be made, and enzymes/proteins are no longer produced.

Source:SRC-DSKP-EN

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