CDS (II) 2023 General Knowledge · Question 12 of 120

Correct — C, proteins. A chromosome is not bare DNA. Inside a eukaryotic nucleus the DNA double helix is wound about two turns around a core of eight histone protein molecules to form a bead called a nucleosome; the string of beads coils and folds again and again, and at cell division the whole assembly condenses into the rod-shaped bodies visible under a microscope. The DNA-plus-protein material is called chromatin, and a chromosome is simply chromatin in its most tightly packed state. The proteins are not packing material only — histones carry chemical marks that help decide which genes are read, and a second group of non-histone proteins handles replication, repair and the machinery that pulls the chromosomes apart during division. Without them a human cell could not fit about two metres of DNA into a nucleus a few micrometres across.

• (a)lipids — Lipids build membranes, including the nuclear envelope that surrounds the chromosomes, and they store energy. They are not part of the chromosome itself.
• (b)vitamins — Vitamins are micronutrients needed in small amounts, most of them working as coenzymes in metabolism. They are not structural components of any chromosome.
• (d)carbohydrates — The tempting answer, because a sugar does appear in the story — deoxyribose is part of the DNA backbone. But that sugar is inside the DNA molecule, not a separate component alongside it, and free carbohydrates are not chromosomal material.

DNA is the hereditary material, but in eukaryotes it never exists naked in the nucleus. It is packaged with basic proteins called histones — H2A, H2B, H3 and H4 forming the octamer core, with H1 sealing the DNA on to it — into nucleosomes, and the nucleosome fibre is coiled further into chromatin. Loosely packed euchromatin is the transcriptionally active form; densely packed heterochromatin is largely silent. During cell division chromatin condenses into individual chromosomes, each with a centromere by which the spindle grips it.

One word answers this item, and the word is protein, but it is worth knowing why the exam keeps returning to it. The contrast is with bacteria, where the circular chromosome is often called naked precisely because it is not wrapped in histones, and where the whole DNA-bearing region has no membrane around it and is called the nucleoid — which is the very next question in this paper. Every human body cell carries 46 chromosomes in 23 pairs; the number is a species character and has nothing to do with how complex the organism is. Note that the strict statement is that a chromosome is DNA plus protein, with a small amount of RNA also present; the exam wants the major partner, which is protein.

• A chromosome consists of DNA together with proteins, chiefly the histones; the combined material is called chromatin.
• About 147 base pairs of DNA wrap around an octamer of histone proteins to form one nucleosome, the basic unit of packaging.
• A human somatic cell has 46 chromosomes, arranged in 23 pairs, of which one pair determines sex.
• Bacterial DNA is described as naked because it is not wound around histone proteins in this way.
• Chromatin exists as loosely packed euchromatin, where genes can be read, and densely packed heterochromatin, where they largely cannot.

• Answering carbohydrates because deoxyribose is a sugar; that sugar sits inside the DNA molecule and is not a separate chromosomal component.
• Assuming every organism packages DNA with histones — bacterial DNA is not wound around histones and is called naked for that reason.
• Treating chromosome number as a measure of an organism's complexity; it is simply a species character.

As a one-line composition question like this, as a prokaryote-against-eukaryote contrast, or through the nucleosome as the unit of DNA packaging.

• practice — not a real PYQ

  The complex of DNA and histone proteins found in the nucleus of a eukaryotic cell is called

  • (a)chromatin
  • (b)chromoplast
  • (c)nucleolus
  • (d)nucleoid

  Answer(a) chromatin — DNA wound around histones. It condenses into visible chromosomes at cell division; the nucleoid is the corresponding, membrane-free region in bacteria.

• practice — not a real PYQ

  How many chromosomes are present in a normal human somatic cell?

  • (a)23
  • (b)44
  • (c)46
  • (d)48

  Answer(c) 46 — twenty-three pairs, of which twenty-two are autosomes and one is the sex-chromosome pair. A gamete carries 23, half the somatic number.