CDS (I) 2020 General Knowledge · Question 29 of 120

Correct — B, Mitochondrion. Most of a cell's DNA is in the nucleus, but the mitochondrion carries a small genome of its own — in humans a circular molecule of 16,569 base pairs coding for 13 proteins, all of them subunits of the oxidative phosphorylation machinery. It also has its own ribosomes and makes some of its own protein. The reason is evolutionary: mitochondria descend from bacteria engulfed by the ancestors of modern cells, and the endosymbiotic theory explains the retained circular genome as a relic of that ancestry. In plant cells the plastids, including chloroplasts, hold DNA for the same reason. The Golgi apparatus, lysosomes and endoplasmic reticulum have none.

• (a)Golgi apparatus — A stack of membrane sacs that modifies, packages and dispatches material made in the endoplasmic reticulum. It carries no genetic material of its own.
• (c)Lysosome — A membrane sac of digestive enzymes that breaks down worn-out organelles and foreign material. No DNA.
• (d)Endoplasmic reticulum — A network of membranes running through the cytoplasm, involved in the synthesis and transport of proteins and lipids. The ribosomes on its rough regions make protein from messages sent out of the nucleus; the organelle itself holds no DNA.

Two organelles in a eukaryotic cell carry DNA besides the nucleus — mitochondria in all such cells, and plastids in plants and algae. Both are double-membraned and both are thought to be descended from free-living bacteria taken in by an early host cell. Human mitochondrial DNA was the first substantial piece of the human genome to be sequenced, and because it evolves faster than nuclear markers it has become a standard tool in tracing population relationships.

The four options split cleanly by ancestry. Golgi, lysosome and endoplasmic reticulum are all products of the cell's own internal membrane system; the mitochondrion is the outsider that was once a separate organism, and that is the origin of its DNA. Framing it that way also answers the plant-cell variant of this question, where chloroplast is the expected additional answer. One nuance worth carrying: the great majority of mitochondrial proteins — around 1,500 kinds in mammals — are actually coded by nuclear DNA and imported, so the mitochondrion's own genome is small and specialised rather than self-sufficient.

• Human mitochondrial DNA is a circular molecule of 16,569 base pairs coding for 13 proteins.
• Mitochondria also carry their own ribosomes and synthesise some of their own protein.
• Plastids, including chloroplasts, carry DNA in plants and algae for the same reason.
• The endosymbiotic theory traces mitochondrial DNA to the circular genomes of bacteria engulfed by the ancestors of modern cells.
• Most mitochondrial proteins, about 1,500 kinds in mammals, are coded by nuclear DNA and imported into the organelle.

• Answering nucleus when the stem has already set it aside as the obvious case.
• Forgetting plastids, which are the expected second answer in a plant-cell version of this item.
• Assuming ribosomes carry DNA; they are made of RNA and protein.

As which organelle other than the nucleus holds DNA, as an organelle-to-function match, or through the endosymbiotic origin of mitochondria.

• practice — not a real PYQ

  Mitochondria and chloroplasts are thought to have originated from free-living bacteria taken up by an ancestral cell. This idea is called the

  • (a)cell theory
  • (b)endosymbiotic theory
  • (c)germ theory
  • (d)fluid mosaic model

  Answer(b) endosymbiotic theory — it accounts for the double membranes, the circular DNA and the ribosomes of both organelles.

• practice — not a real PYQ

  Which one of the following organelles is known as the suicide bag of the cell?

  • (a)Mitochondrion
  • (b)Ribosome
  • (c)Lysosome
  • (d)Golgi apparatus

  Answer(c) Lysosome — it holds powerful digestive enzymes that can break down the cell's own material.