Select the organelle that contains its own DNA and 70S ribosomes but is found in eukaryotic cells.

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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.

Why the other options are wrong — (a) 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.  (c) 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.  (d) 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.

Concept: 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.

State PCS 2026 Q58 — Science & Technology PYQ with Answer

State PCS 2026 Q58 — Science & Technology PYQ with Answer