Which of the following combinations correctly represents the genetic materials in prokaryotes and eukaryotes respectively?

Correct — A, Nucleoid and Chromatin. A prokaryotic cell has no nuclear membrane, so its DNA lies directly in the cytoplasm in an irregular, ill-defined region called the nucleoid — usually one circular molecule with very little protein bound to it. A eukaryotic cell keeps its DNA inside a membrane-bound nucleus, wound around histone proteins into a fibre called chromatin, which coils tighter into visible chromosomes when the cell prepares to divide. The order in the stem is prokaryotes first and eukaryotes second, and option (a) is the only one that matches that order.

• (b)Chromatin and Nucleoid — Both terms are right but the order is reversed. Chromatin needs histone proteins and a nucleus to sit in, neither of which a prokaryote has.
• (c)Nucleoid and Nucleolus — Gets the prokaryote right and then names the wrong eukaryotic structure. The nucleolus is a dense body inside the nucleus where ribosomal RNA is made and ribosome subunits are assembled; it is not the cell's genetic material.
• (d)Nucleolus and Chromatin — Places the nucleolus in the prokaryote, which has no nucleus at all and therefore nothing for a nucleolus to sit inside.

The dividing line between prokaryotic and eukaryotic cells is the nuclear membrane. Bacteria and archaea have none, so their genetic material occupies an unbounded region of the cytoplasm known as the nucleoid, typically a single circular DNA molecule with small extra circles called plasmids alongside it. Eukaryotes enclose their DNA in a nucleus and package it with histones as chromatin, which condenses into chromosomes at cell division.

Three terms starting with 'nucle-' are set against each other here, and telling them apart is the whole exercise. The nucleoid is the DNA-containing region of a cell that has no nucleus. Chromatin is the DNA-plus-protein material of a cell that has one. The nucleolus is a structure inside that nucleus with a manufacturing job — making ribosomal RNA — and it is not genetic material in the sense the question means. Notice also that the stem says 'respectively', which converts an easy pairing into an order test; option (b) contains exactly the same two right words in the wrong sequence and will catch a fast reader.

• The nucleoid is the region of a prokaryotic cell holding its DNA; it has no surrounding membrane.
• Bacterial DNA is usually a single circular chromosome, often accompanied by smaller circles called plasmids.
• Chromatin is DNA wound around histone proteins, and it condenses into chromosomes before cell division.
• The nucleolus lies within the nucleus and is the site of ribosomal RNA synthesis and ribosome subunit assembly.
• Prokaryotes lack all membrane-bound organelles — no nucleus, no mitochondria, no Golgi — though they do have ribosomes.

• Reversing the pair because the stem's 'respectively' was skipped.
• Confusing the nucleolus with the nucleoid on the strength of the similar spelling.
• Assuming prokaryotes have no DNA-associated proteins at all; they have some, just not the histone-based chromatin of eukaryotes.

Asked as an ordered pairing, so both the terms and their sequence have to be right.

• practice — not a real PYQ

  The nucleolus in a eukaryotic cell is chiefly concerned with

  • (a)storage of the cell's DNA
  • (b)synthesis of ribosomal RNA
  • (c)digestion of worn-out organelles
  • (d)synthesis of lipids

  Answer(b) synthesis of ribosomal RNA — and the assembly of ribosome subunits.

• practice — not a real PYQ

  Small circular DNA molecules found in bacteria, apart from the main chromosome, are called

  • (a)plasmids
  • (b)nucleosomes
  • (c)centromeres
  • (d)chromatids

  Answer(a) plasmids — widely used as vectors in genetic engineering.