Bacteria have an undefined nuclear region which lacks a nuclear membrane and contains only DNA without proteins. Such a region is called :
- (a)Nucleosome
- (b)Nucleus
- (c)Nucleoprotein
- (d)Nucleoid
Correct — D, Nucleoid. Bacteria are prokaryotes, and the defining feature of a prokaryote is that its genetic material is not shut away behind a nuclear envelope. The DNA — usually a single circular molecule — sits in a region of the cytoplasm that has no membrane of its own and no sharp edge, and that region is called the nucleoid. The name is deliberately provisional: it means nucleus-like, marking it as the functional counterpart of a nucleus without being one. Everything else about the bacterial cell follows from the same absence. There is no nuclear membrane, so transcription and translation happen in the same space and can proceed at once; there is no spindle apparatus of the eukaryotic kind, and the cell divides by binary fission rather than by mitosis.
- (a)Nucleosome — A eukaryotic structure, and a very small one — about 147 base pairs of DNA wound around eight histone protein molecules, the repeating bead in the chromatin fibre. It is a unit of DNA packaging, not a region of a cell.
- (b)Nucleus — The word the stem is carefully avoiding. A nucleus is bounded by a double nuclear membrane with pores in it, and its presence is what makes a cell eukaryotic; the whole point about bacteria is that they lack one.
- (c)Nucleoprotein — Names a class of molecules — any complex of nucleic acid with protein, such as chromatin or a ribosome — and not a part of a cell. The suffix is doing the deceiving here.
The prokaryote–eukaryote divide is the deepest division among cells. A prokaryotic cell — bacteria and archaea — has no membrane-bound nucleus and no membrane-bound organelles at all: no mitochondria, no plastids, no endoplasmic reticulum, no Golgi apparatus. It does have a cell membrane, a cell wall, ribosomes of the smaller 70S type, and often plasmids, flagella and pili. A eukaryotic cell has a true nucleus with a double envelope, linear chromosomes wound around histones, and a full set of compartmented organelles.
Answer this by taking the words apart. Nucleo-some is a small body, nucleo-protein is a molecule, nucleus is the real thing the stem has just ruled out, and nucleo-id is the like-a-nucleus region that is left. One point of honesty about the stem: the paper says the region 'contains only DNA without proteins', which is the school-textbook simplification. A real bacterial nucleoid is roughly four-fifths DNA by mass but does carry nucleoid-associated proteins such as HU, IHF, H-NS and Fis, along with some RNA. Those proteins fold and organise the DNA, though they are not histones and do not build nucleosomes, which is why bacterial DNA is often described as naked. The exam is testing the name, and the name is not affected; the point is worth knowing before a more advanced question asks it.
- The nucleoid is the irregularly shaped region of a prokaryotic cell that holds its genetic material and has no membrane around it.
- Bacterial DNA is typically a single circular chromosome, often accompanied by smaller independent circles called plasmids.
- Prokaryotes have no membrane-bound organelles — no nucleus, mitochondria, plastids, endoplasmic reticulum or Golgi apparatus.
- A nucleosome, by contrast, is a eukaryotic structure: about 147 base pairs of DNA wound around eight histone proteins.
- Bacteria divide by binary fission rather than by mitosis, and their ribosomes are of the 70S type against the 80S of the eukaryotic cytoplasm.
- Choosing 'nucleus' because the stem describes a nuclear region; the absence of a membrane is exactly what rules the word out.
- Confusing nucleoid with nucleolus — the nucleolus is a dense body inside a eukaryotic nucleus where ribosomes are assembled.
- Taking 'no proteins' literally; the simplification is the textbook's, and a nucleoid does contain organising proteins, just not histones.
As a direct naming question, as a prokaryote-versus-eukaryote contrast, or as a which-structure-is-absent item on the bacterial cell.
In prokaryotic organisms, nuclear region is not surrounded by a membrane. This undefined nuclear region is known as
- (a) Nucleic acid
- (b) Nucleoid
- (c) Nucleolus
- (d) Nucleosome
Answer(b) Nucleoid
The same definition, almost the same sentence, three years earlier — an undefined nuclear region with no membrane around it. Even the decoys overlap, since both papers offer nucleosome as the near-miss.
Which one of the following structures is not present in a prokaryotic cell ?
- (a) Cell wall
- (b) Ribosomes
- (c) Nucleus
- (d) Plasma membrane
Answer(c) Nucleus
The same fact turned into an elimination question. Because there is no nucleus, the DNA has to sit somewhere else — and that somewhere is the nucleoid.
- practice — not a real PYQ
Which one of the following structures is absent in a bacterial cell?
- (a)Cell wall
- (b)Ribosome
- (c)Mitochondrion
- (d)Plasma membrane
Answer(c) Mitochondrion — bacteria have no membrane-bound organelles at all. They do have a cell wall, a plasma membrane and ribosomes of the 70S type.
- practice — not a real PYQ
Small circular DNA molecules found in bacteria, separate from the main chromosome and able to replicate independently, are called
- (a)plastids
- (b)plasmids
- (c)nucleosomes
- (d)lysosomes
Answer(b) plasmids — they often carry genes for antibiotic resistance and are the standard vectors used to move genes in recombinant DNA work.