NDA (II) 2018 General Ability Test · Question 57 of 146

Correct — A, Mitochondria, nucleus, chloroplast. The nucleus carries the cell's main store of DNA, packed with protein into chromosomes. Mitochondria and chloroplasts are the two organelles that carry DNA of their own in addition — a small circular molecule, unbound by histones, together with their own ribosomes — and they grow and divide by fission rather than being built afresh by the cell. That is the evidence behind the endosymbiotic account, which holds that both descend from free-living bacteria taken up by an early eukaryotic cell. Option (a) is the only one that names all three of these and nothing else.

• (b)Mitochondria, Golgi bodies, nucleus — Golgi bodies are stacks of flattened membrane sacs that modify, sort and package material arriving from the endoplasmic reticulum, and dispatch it in vesicles. They carry no genetic material of any kind.
• (c)Mitochondria, plasma membrane, nucleus — The plasma membrane is a lipid bilayer with proteins embedded in it, controlling what enters and leaves the cell. It has no DNA, and it is a boundary rather than an organelle in the usual sense.
• (d)Chloroplast, nucleus, ribosomes — Ribosomes are built of RNA and protein and read messenger RNA to assemble polypeptides. They contain RNA, never DNA — and this option also drops mitochondria, which do have their own DNA.

A eukaryotic cell keeps genetic material in three places. The nucleus holds the bulk of it as chromosomes inside a double nuclear envelope. Mitochondria, the sites of aerobic respiration and ATP production, hold a small circular genome and their own ribosomes. Plastids, of which chloroplasts are the photosynthetic kind, do the same. Because mitochondria and chloroplasts have their own DNA, their own ribosomes and a double membrane, and because they divide independently of the cell, they are described as semi-autonomous organelles.

The examiner's method with cell biology is to build option sets that each contain one or two DNA-bearing structures and then spoil them with a third that has none. So test each name in the group rather than the group as a whole, and reject the option the moment one member fails. It also helps to fix in advance the short list of what is not a candidate — Golgi bodies, lysosomes, ribosomes, vacuoles, the endoplasmic reticulum, the plasma membrane and centrosomes hold no DNA. That leaves the nucleus, mitochondria and plastids, and no exam question on this topic has any other answer.

• The nucleus, mitochondria and plastids such as chloroplasts are the DNA-bearing structures of a eukaryotic cell.
• Mitochondrial and chloroplast DNA is circular, like bacterial DNA, and both organelles carry their own ribosomes.
• Both are bounded by a double membrane and divide by fission independently of the cell — the endosymbiotic evidence.
• Human mitochondrial DNA is inherited only from the mother, since sperm mitochondria are destroyed after fertilisation.
• Ribosomes contain RNA and protein but no DNA; Golgi bodies, lysosomes and the plasma membrane contain neither.

• Adding ribosomes to the DNA list because they take part in protein synthesis; they hold RNA, not DNA.
• Forgetting chloroplasts and answering with the nucleus and mitochondria alone.
• Calling the plasma membrane an organelle with genetic material.

NDA sets this in almost every session — sometimes as 'which two organelles have their own genetic material', sometimes as a group of three with one spoiler planted in each option.

• practice — not a real PYQ

  Mitochondria and chloroplasts are called semi-autonomous organelles because they

  • (a)are found only in plant cells
  • (b)have their own DNA and ribosomes and divide by fission
  • (c)are bounded by a single membrane
  • (d)can survive outside the cell indefinitely

  Answer(b) have their own DNA and ribosomes and divide by fission — they make some of their own proteins and multiply independently of the cell's own division.

• practice — not a real PYQ

  Which one of the following cell organelles does NOT contain any DNA?

  • (a)Nucleus
  • (b)Mitochondrion
  • (c)Chloroplast
  • (d)Golgi apparatus

  Answer(d) Golgi apparatus — it modifies, sorts and packages material from the endoplasmic reticulum but carries no genetic material.