Which cell organelles have their own DNA and Ribosomes?
- (a)Golgi body and Endoplasmic Reticulum
- (b)Mitochondria and Plastids
- (c)Lysosome and Golgi body
- (d)Vacuole and Plastids
Correct — B, Mitochondria and Plastids. These two organelles carry a small circular DNA of their own and their own ribosomes, so each can make some of its own proteins independently of the nucleus. That oddity is the strongest evidence for the endosymbiotic account of their origin: they descend from free-living bacteria that were engulfed by an early eukaryotic cell and kept, which also explains their double membrane and the fact that they arise only by division of existing ones rather than being built afresh. Their ribosomes are of the smaller 70S bacterial type, not the 80S type found in the surrounding cytoplasm. One consequence a student meets later is that mitochondrial disorders pass down the maternal line alone, because the sperm's mitochondria are destroyed after fertilisation.
- (a)Golgi body and Endoplasmic Reticulum — Both are membrane systems that process and ship proteins. The rough endoplasmic reticulum has ribosomes stuck on its outer face, but those are the cell's ordinary cytoplasmic ribosomes sitting on it, and neither organelle holds DNA.
- (c)Lysosome and Golgi body — Lysosomes are sacs of digestive enzymes budded off the Golgi. Neither has DNA or ribosomes; the lysosome's job is breaking material down, not making protein.
- (d)Vacuole and Plastids — The half-right option, and the tempting one. Plastids do qualify, but the vacuole is a fluid-filled store bounded by a single membrane, with no DNA and no ribosomes of its own.
Almost all of a eukaryotic cell's DNA sits in the nucleus, but two organelles keep their own separate genome — mitochondria, where respiration releases energy, and plastids, the family that includes the chloroplast. Each has a double membrane, a circular DNA molecule and 70S ribosomes, and each multiplies by dividing. These are the semi-autonomous organelles.
The four options are built as a pairs puzzle, and only one member of each wrong pair is a serious candidate. Working it by elimination is quicker than recall: Golgi, endoplasmic reticulum, lysosome and vacuole are all single-membrane compartments of the endomembrane system, none of which has ever been claimed to hold DNA. That knocks out (a) and (c) at once and leaves (d) as the only real competitor to (b). Option (d) fails on the vacuole. Note that the paper writes 'Plastids' rather than 'Chloroplast' — plastid is the wider family, taking in the chloroplast, the coloured chromoplast and the colourless leucoplast that stores starch, and all of them carry the genome.
- Mitochondria and plastids each contain their own circular DNA and their own 70S ribosomes.
- Both have a double membrane and arise only by division of pre-existing organelles.
- The endosymbiotic theory holds that both descend from bacteria engulfed by an ancestral eukaryotic cell.
- Plastids include chloroplasts, chromoplasts and leucoplasts; all belong to the same family.
- Human mitochondria are inherited from the mother alone, because sperm mitochondria are destroyed after fertilisation.
- Counting the rough endoplasmic reticulum as an organelle with its own ribosomes — the ribosomes on it belong to the cytoplasm.
- Picking the pair that contains plastids without checking the other half of the pair.
- Reading 'plastid' as 'chloroplast only' and rejecting the option in a non-green cell.
As a pick-the-pair item on semi-autonomous organelles, or as a statement item on endosymbiosis and the double membrane.
In the context of hereditary diseases, consider the following statements: 1. Passing on mitochondrial diseases from parent to child can be prevented by mitochondrial replacement therapy either before or after in vitro fertilization of egg. 2. A child inherits mitochondrial diseases entirely from mother and not from father. Which of the statements given above is/are correct?
- (a) 1 only
- (b) 2 only
- (c) Both 1 and 2
- (d) Neither 1 nor 2
Answer(c) Both 1 and 2
The consequence of this question's fact, tested at a higher level. Because the mitochondrion carries its own DNA and a child receives mitochondria only from the mother, mitochondrial disease follows the maternal line and can be headed off by replacing the organelles.
In a plant cell, which one of the following contains their own DNA ?
- (a) Nucleus and Endoplasmic Reticulum
- (b) Ribosome and Golgi apparatus
- (c) Mitochondria and Chloroplast
- (d) Chloroplast and Vacuoles
Answer(c) Mitochondria and Chloroplast
The same pair asked with 'own DNA' rather than 'own DNA and ribosomes'. Its option list also puts vacuoles beside chloroplast as a decoy, which is the trap this question repeats in its fourth option.
- practice — not a real PYQ
The presence of circular DNA and 70S ribosomes in mitochondria and chloroplasts is taken as evidence for
- (a)the fluid mosaic model
- (b)the endosymbiotic theory
- (c)the cell theory
- (d)the germ theory
Answer(b) the endosymbiotic theory — the bacterial-type genome and ribosomes point to an origin as free-living bacteria taken up by an early eukaryotic cell.
- practice — not a real PYQ
Which one of the following is not a plastid?
- (a)Chloroplast
- (b)Chromoplast
- (c)Leucoplast
- (d)Lysosome
Answer(d) Lysosome — a single-membrane sac of digestive enzymes budded from the Golgi; the other three are members of the plastid family.