MPSC 2026 Prelims Paper-I · Question 58 of 100

Correct — option (2), "Mitochondria". Only two organelles in a eukaryotic cell carry their own genetic material and their own protein-synthesising machinery: the mitochondrion and the chloroplast. Both hold a small circular DNA molecule of their own, distinct from the nuclear DNA, and both contain 70S ribosomes rather than the 80S ribosomes found free in the cytoplasm of a eukaryotic cell. Since the chloroplast is not on offer here, the mitochondrion is the answer. The ribosome size is the decisive detail and it is worth being precise about. Prokaryotic cells — bacteria and archaea — have 70S ribosomes, made of a 50S large subunit and a 30S small subunit. Eukaryotic cytoplasmic ribosomes are larger, 80S, made of 60S and 40S subunits. The S is a Svedberg unit, a sedimentation coefficient measured in a centrifuge, and it depends on mass, shape and density together, which is why the numbers do not add arithmetically: 50S and 30S combine to give 70S, not 80S. That non-additivity is itself a favourite question. So a eukaryotic organelle containing 70S ribosomes is carrying prokaryote-type machinery inside a eukaryotic cell, and the explanation is the endosymbiotic theory, argued at length by Lynn Margulis. On this account mitochondria descend from free-living aerobic bacteria that were engulfed by an ancestral host cell and retained rather than digested, the two settling into a permanent partnership — the guest supplying efficient aerobic respiration, the host supplying shelter and substrate. Chloroplasts are held to have arrived the same way, from engulfed photosynthetic cyanobacteria. Four lines of evidence support it, and each of them is a fact about the mitochondrion worth knowing in its own right: it has its own circular DNA, like a bacterial chromosome; it has 70S ribosomes, the bacterial size; it is bounded by a double membrane, as expected if one membrane is the guest's own and the other the host's engulfing vesicle; and it divides by a fission-like process of its own rather than being built afresh by the cell. A fifth, more striking, is that mitochondrial protein synthesis is inhibited by several antibiotics that act on bacterial ribosomes and leave the cell's own cytoplasmic ribosomes untouched. Beyond its ancestry, the mitochondrion is the site of the Krebs cycle, which runs in the matrix, and of oxidative phosphorylation, which runs on the inner membrane — folded into cristae to multiply the surface area available for it. That is why it is called the powerhouse of the cell: it is where the great majority of a eukaryotic cell's ATP is made. In humans mitochondrial DNA is inherited maternally, since the egg contributes the cytoplasm of the zygote, which is the basis of mitochondrial disease inheritance patterns and of maternal-line ancestry tracing.

• (1)Lysosome — A lysosome is a single-membrane-bound sac filled with hydrolytic enzymes that digest worn-out organelles, engulfed material and, on rupture, the cell itself — hence its nickname, the suicide bag of the cell. It contains no DNA and no ribosomes of any kind. Its enzymes are made on ribosomes of the rough endoplasmic reticulum and packaged by the Golgi apparatus, which is exactly the point: a lysosome is a product of the cell's secretory pathway, not a semi-autonomous compartment with machinery of its own.
• (3)Golgi Apparatus — The Golgi apparatus is a stack of flattened membranous cisternae that receives proteins and lipids from the endoplasmic reticulum, modifies them — glycosylation is the characteristic step — and then sorts and packages them into vesicles for secretion, for the plasma membrane, or for lysosomes. It has neither DNA nor ribosomes. Like the lysosome it is part of the endomembrane system, all of whose components derive from the endoplasmic reticulum and none of which is of endosymbiotic origin.
• (4)Endoplasmic Reticulum — This is the thoughtful candidate's error, because the rough endoplasmic reticulum genuinely does have ribosomes — that is what makes it rough. But they are the cell's own 80S cytoplasmic ribosomes docked on the outer face of the membrane, not 70S ribosomes belonging to the organelle, and the endoplasmic reticulum contains no DNA at all. The stem asks for an organelle with its OWN DNA and 70S ribosomes, which is a claim about genetic autonomy; bearing borrowed ribosomes on its surface is a different thing entirely.

Cells fall into two grades of organisation. Prokaryotic cells — bacteria and archaea — have no nucleus, no membrane-bound organelles, a single circular chromosome lying in the nucleoid region, and 70S ribosomes. Eukaryotic cells have a true nucleus enclosed by a nuclear envelope, an elaborate set of membrane-bound organelles, linear chromosomes, and 80S ribosomes in the cytoplasm. Cutting across that division are the two organelles which sit inside eukaryotic cells but carry prokaryotic characteristics: mitochondria and, in plants and algae, chloroplasts. Each has a double membrane, its own circular DNA, its own 70S ribosomes and the ability to divide semi-autonomously. The endosymbiotic theory explains the anomaly historically — these organelles are the descendants of free-living prokaryotes taken up by an ancestral eukaryotic cell and never expelled. Their autonomy is only partial: most of the proteins a mitochondrion needs are now encoded in the nuclear genome, made on cytoplasmic ribosomes and imported, because over evolutionary time most of the endosymbiont's genes migrated to the host nucleus. What survives in the organelle's own genome is a small, essential core.

The organelle table is one of the highest-yield pages in general science, and MPSC works through it steadily. The mitochondrion runs the Krebs cycle in its matrix and oxidative phosphorylation on the cristae of its inner membrane, and it produces the bulk of the cell's ATP. The chloroplast performs photosynthesis and shares the mitochondrion's double membrane, circular DNA and 70S ribosomes. The rough endoplasmic reticulum, studded with cytoplasmic ribosomes, synthesises proteins destined for secretion or for membranes, while the smooth endoplasmic reticulum makes lipids and steroids, stores calcium and detoxifies drugs and poisons. The Golgi apparatus modifies, sorts and packages what the endoplasmic reticulum sends it, and forms lysosomes. Lysosomes digest. Ribosomes themselves are non-membranous and are found in every cell, prokaryotic or eukaryotic. Peroxisomes handle oxidative reactions and break down hydrogen peroxide. Two additional facts about mitochondria that examiners like: their DNA is inherited maternally in humans, since the egg supplies the zygote's cytoplasm, and their protein synthesis is sensitive to antibiotics that target bacterial ribosomes — a pharmacological echo of their bacterial ancestry.

• Mitochondria and chloroplasts are the only eukaryotic organelles with their own DNA and their own 70S ribosomes. Both are bounded by a double membrane and divide semi-autonomously.
• Prokaryotes have 70S ribosomes (50S + 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S + 40S). The S is a Svedberg sedimentation coefficient, which is why the subunit values do not add arithmetically.
• The endosymbiotic theory, argued by Lynn Margulis, holds that mitochondria descend from engulfed aerobic bacteria and chloroplasts from engulfed cyanobacteria. Evidence: circular DNA, 70S ribosomes, double membranes, independent division, and sensitivity of organelle protein synthesis to antibacterial antibiotics.
• The mitochondrion carries out the Krebs cycle in its matrix and oxidative phosphorylation on the inner membrane, whose folds — cristae — increase the available surface area. It generates most of the cell's ATP, hence 'powerhouse of the cell'.
• Lysosomes are single-membrane sacs of hydrolytic enzymes with no DNA or ribosomes; the Golgi apparatus modifies, sorts and packages proteins and forms lysosomes; the rough endoplasmic reticulum bears the cell's own 80S ribosomes on its outer surface but has no DNA of its own.

• Choosing the endoplasmic reticulum because the rough ER has ribosomes. Those are the cell's own 80S cytoplasmic ribosomes bound to its surface; the ER has no DNA and no ribosomes of its own.
• Expecting ribosomal subunit values to add up. 50S plus 30S gives 70S because the Svedberg unit measures sedimentation behaviour, which depends on shape and density as well as mass.
• Forgetting the chloroplast. It shares every feature named in this stem, and when both are offered the question must be narrowed by some other clue such as the cell type or the function named.
• Treating mitochondrial autonomy as complete. Most mitochondrial proteins are encoded in the nucleus and imported; only a small core is encoded by the organelle's own DNA.

Cell biology is among the most predictable sections of MPSC general science, because the material is a finite table of organelles with fixed properties. Three shapes recur. The identification question, as here, describes an organelle by one or two distinctive properties and asks which it is; the properties chosen are almost always double membrane, own DNA, 70S ribosomes, hydrolytic enzymes or a named metabolic pathway. The function question runs the other way, naming an organelle and asking what it does or what nickname it carries. The comparison question sets prokaryotic against eukaryotic cells, or plant against animal cells. All three are answered from the same table, so the efficient preparation is to build it once with four columns — organelle, membrane arrangement, contents, function — and to note in it the two organelles that break the eukaryotic pattern.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which of the following is NOT evidence supporting the endosymbiotic origin of mitochondria ?

  • (a)They contain circular DNA of their own
  • (b)They contain 70S ribosomes
  • (c)They are bounded by a double membrane
  • (d)They are the site of the Krebs cycle

  Answer(d) They are the site of the Krebs cycle. That is a statement about what the mitochondrion does, not about where it came from — a compartment of any origin could house a metabolic pathway. The other three are precisely the structural parallels with free-living bacteria that the endosymbiotic theory explains: a circular genome like a bacterial chromosome, bacterial-sized 70S ribosomes, and two membranes as expected if one belonged to the engulfed cell and the other to the host's vesicle.

• practice — not a real PYQ

  The ribosomes found in the cytoplasm of a eukaryotic cell are of which type ?

  • (a)70S, composed of 50S and 30S subunits
  • (b)80S, composed of 60S and 40S subunits
  • (c)70S, composed of 40S and 30S subunits
  • (d)80S, composed of 50S and 30S subunits

  Answer(b) 80S, composed of 60S and 40S subunits. Prokaryotes and the mitochondria and chloroplasts of eukaryotic cells carry the smaller 70S ribosomes made of 50S and 30S subunits. Note that neither pair of subunit values adds arithmetically to the whole, because the Svedberg unit records sedimentation behaviour, which depends on the shape and density of a particle as well as on its mass.