Q1.
Mature Red Blood Cells (RBCs) in humans do not have a nucleus (enucleate). Do you know any other cells without nucleus?
Answer
Yes. Several cells lose or never form a nucleus, and in each case the loss buys something useful.
| Cell | Found in | What the loss of the nucleus achieves |
|---|---|---|
| Mature red blood cell | Human blood | More room for haemoglobin, so more oxygen is carried |
| Platelets | Human blood | Small cell fragments that plug a wound quickly |
| Sieve tube cells of phloem | Plant vascular tissue | An open channel for food to flow through; a neighbouring companion cell keeps its nucleus and controls it |
| Mature xylem vessel and tracheid | Plant vascular tissue | Dead, hollow tubes — a clear pipe for water |
| Mature lens fibre cells of the eye | Eye lens | Perfectly transparent, since a nucleus would scatter light |
Why it happens: the nucleus is the control centre, so giving it up has a price — a cell without a nucleus cannot repair itself or divide. A human RBC therefore survives only about 120 days and must be replaced from bone marrow. The trade is worth it because the job is purely mechanical: carrying oxygen, plugging a leak, or acting as a pipe needs no fresh instructions. Wherever a cell has one simple, fixed job and can be replaced from elsewhere, evolution has often thrown the nucleus away.
Did you know? A red blood cell's biconcave, nucleus-free shape also makes it flexible enough to squeeze single file through capillaries narrower than the cell itself.