NDA (II) 2016 General Ability Test · Question 99 of 150

Correct — B, the cytoplasm. Bacteria are prokaryotes, and the defining feature of a prokaryote is that it has no nuclear membrane. Its DNA — a single, large, circular molecule — lies directly in the cytoplasm, coiled into an irregular region called the nucleoid that is not separated from the rest of the cell by any envelope. Smaller circular pieces of DNA called plasmids, which often carry genes for antibiotic resistance, float in the cytoplasm as well. Because nothing encloses the genetic material, transcription and translation can go on side by side in the same compartment, which is one reason bacteria can multiply so fast.

• (a)nucleus — The whole point of calling a bacterium a prokaryote is that it has no true nucleus. The DNA-containing region is called the nucleoid precisely to mark the difference — it looks like a nucleus under low magnification but has no nuclear membrane, no nucleolus and no chromosomes in the eukaryotic sense. This is the trap the question is built around.
• (c)cell membrane — There is a grain of truth here that makes it tempting: the bacterial chromosome is anchored to the plasma membrane at one point, and infoldings of that membrane are involved in DNA replication and separation during division. But the molecule itself is not housed in the membrane; it lies in the cytoplasm.
• (d)ribosome — Ribosomes are made of RNA and protein and are the machinery on which genetic instructions are translated into proteins. They are the readers of the message, not the store of it, and no cell keeps its genome inside its ribosomes.

Cells fall into two great groups. Prokaryotes — bacteria and archaea — have no nuclear membrane and no membrane-bound organelles at all, so no mitochondria, no chloroplasts, no endoplasmic reticulum and no Golgi apparatus; their genetic material lies free in the cytoplasm in the nucleoid, and they carry small 70S ribosomes. Eukaryotes — protists, fungi, plants and animals — enclose their DNA in a double-membraned nucleus, organise it into several linear chromosomes wound on histone proteins, and carry a full set of membrane-bound organelles along with larger 80S ribosomes.

This is a definitional item, and the way to be safe on it is to hold the prokaryote-eukaryote contrast as a package rather than as one fact. Every property in that package follows from the same absence: no nuclear envelope means the DNA is cytoplasmic, and no internal membranes means functions that a eukaryote gives to organelles are handled at the plasma membrane instead, which is why bacterial respiration happens there rather than in mitochondria. Two refinements worth knowing. First, the nucleoid is a real, observable structure with the DNA compacted by its own set of proteins; it is simply not a compartment. Second, mitochondria and chloroplasts inside eukaryotic cells carry their own small circular DNA and their own 70S ribosomes, a relic of their bacterial ancestry, which is why a question about DNA outside the nucleus so often points at them.

• The bacterial chromosome is a single circular DNA molecule lying in the cytoplasm in a region called the nucleoid, with no membrane around it.
• Plasmids are small extra circles of DNA in the bacterial cytoplasm and often carry antibiotic-resistance genes.
• Prokaryotes lack all membrane-bound organelles, including mitochondria, and carry 70S ribosomes against the eukaryote's 80S.
• Mitochondria and chloroplasts in eukaryotic cells contain their own circular DNA, a legacy of their prokaryotic origin.

• Marking nucleus because the region is called the nucleoid; the name is a deliberate reminder that it is nucleus-like and not a nucleus.
• Choosing the cell membrane because the chromosome is anchored to it; anchoring is not the same as being housed there.

The prokaryote-eukaryote contrast is one of NDA's most repeated biology topics, asked as which-structure-is-absent, where-is-the-DNA or which-cell-has-what, so learn the two cell types as one comparison table.

• practice — not a real PYQ

  The region of a prokaryotic cell in which the DNA is concentrated, without any surrounding membrane, is called the

  • (a)nucleolus
  • (b)nucleoid
  • (c)nucleosome
  • (d)mesosome

  Answer(b) nucleoid — it holds the circular chromosome but is not a membrane-bound compartment.

• practice — not a real PYQ

  Which one of the following is absent in a bacterial cell?

  • (a)Cell wall
  • (b)Ribosome
  • (c)Mitochondrion
  • (d)Plasma membrane

  Answer(c) Mitochondrion — prokaryotes have no membrane-bound organelles, and respiration takes place at the plasma membrane instead.