Which cells in human body have no nucleus?
- (a)Muscle cells
- (b)Phagocytic cells
- (c)Red blood cells
- (d)Ciliated cells
Correct — C, Red blood cells. NCERT puts it plainly: 'RBCs are devoid of nucleus in most of the mammals and are biconcave in shape.' A human red cell is made in the red bone marrow, and in the last stage of its maturation it expels the nucleus, along with its mitochondria and endoplasmic reticulum. What is left is a flexible biconcave disc packed with haemoglobin — more room for oxygen, and a shape that squeezes through capillaries narrower than the cell itself. The cost is that the cell can no longer repair or replace its proteins, which is why it lasts about 120 days and is then broken down. Every other cell offered here keeps its nucleus.
- (a)Muscle cells — Muscle cells are nucleated, and a skeletal muscle fibre is in fact multinucleate — many nuclei pushed to the edge of a single long fibre.
- (b)Phagocytic cells — Phagocytes such as neutrophils and macrophages are white blood cells with prominent nuclei; the neutrophil's is famously many-lobed, which is how it is identified under a microscope.
- (d)Ciliated cells — Ciliated cells line the respiratory tract and the oviduct; they are ordinary nucleated epithelial cells whose distinguishing feature is the cilia on the free surface, not any loss of a nucleus.
Blood is a fluid connective tissue with three formed elements — erythrocytes or red blood cells, leucocytes or white blood cells, and platelets. Red cells are the most abundant: a healthy adult man carries about 5 to 5.5 million of them per cubic millimetre. They are produced in the red bone marrow, carry the iron-containing protein haemoglobin, live roughly 120 days, and are then destroyed. Their defining structural quirk is the absence of a nucleus in the mature cell.
Read the option list as a sorting exercise rather than a recall test. Three of the four options name cells defined by a function — contraction, engulfing microbes, beating cilia — and none of those functions requires giving up a nucleus. Only the red cell is defined by what it carries, and packing in the maximum haemoglobin is exactly the pressure that made the mammalian red cell shed its nucleus. Worth noticing NCERT's hedge: it says 'in most of the mammals', not 'in all cells everywhere', because the enucleate red cell is a mammalian specialisation rather than a universal rule of vertebrate blood. The exam is asking about the human body, where the statement holds without qualification.
- NCERT: 'RBCs are devoid of nucleus in most of the mammals and are biconcave in shape.'
- A healthy adult man has about 5 to 5.5 million RBCs per cubic millimetre of blood.
- RBCs are formed in the red bone marrow in adults and have an average life span of about 120 days.
- A healthy individual has 12 to 16 grams of haemoglobin in every 100 ml of blood.
- White blood cells, including the phagocytic neutrophils and macrophages, retain their nuclei.
- Choosing muscle cells because a skeletal muscle fibre looks unusual under the microscope — it is unusual for having many nuclei, not none.
- Reading 'phagocytic cells' as a synonym for red cells; phagocytes are white cells and are firmly nucleated.
- Generalising the rule beyond mammals; a bird's or a frog's red cells are not built the same way.
Asked either as a one-line 'which cell has no nucleus' item or, in sibling papers, as a list of what the red cell lacks — nucleus, mitochondria and endoplasmic reticulum together.
Red blood cells (RBCs) have
- (a) no nucleus, no mitochondria and no endoplasmic reticulum
- (b) nucleus, mitochondria and endoplasmic reticulum
- (c) nucleus, mitochondria but no endoplasmic reticulum
- (d) no mitochondria but endoplasmic reticulum is present
Answer(a) no nucleus, no mitochondria and no endoplasmic reticulum
The same fact asked in fuller form. The mature red cell loses the nucleus, the mitochondria and the endoplasmic reticulum together, which is why it relies on anaerobic glycolysis and cannot renew its own proteins.
CDS_GK_2023_I_Q32023If the red blood cells (RBCs) of human blood are isolated and are diluted in normal saline (an isotonic solution to blood), what will happen to the RBCs?
- (a) The RBCs will swell
- (b) The RBCs will swell and burst
- (c) The RBCs will shrink
- (d) No change in the diameters of the RBCs
Answer(d) No change in the diameters of the RBCs
The same cell, tested on how it behaves rather than on what it contains. CDS has now asked about the red cell in consecutive keyed papers, once on its structure and once on its osmotic response, so it is worth learning the cell as a package.
- practice — not a real PYQ
The biconcave shape of a mammalian red blood cell chiefly helps it to
- (a)store genetic material more compactly
- (b)increase surface area for gas exchange and bend through narrow capillaries
- (c)produce antibodies faster
- (d)engulf bacteria more efficiently
Answer(b) increase surface area for gas exchange and bend through narrow capillaries — the shape follows from the cell's job of carrying oxygen.
- practice — not a real PYQ
In an adult human, red blood cells are formed mainly in the
- (a)liver
- (b)spleen
- (c)red bone marrow
- (d)thymus
Answer(c) red bone marrow — the adult site of erythropoiesis, with an average red cell life span of about 120 days.