Studyledge

The Mitotic Cell Cycle: Question 6

Syllabus 5.1

Multiple choice AS 1 mark

A biologist views a stained chromosome from a human cell under an electron microscope.

Which statement correctly describes how the DNA of a chromosome is organised?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Recall the components of a chromosome

A chromosome is not simply a bare strand of DNA. Instead, the DNA double helix is associated throughout its length with proteins called histones. The DNA winds around clusters of histone proteins, and this combined DNA-histone material is called chromatin. Chromatin itself is further coiled and folded, packaging an extremely long DNA molecule into the much shorter, thicker structure that can be seen as a chromosome.

Step 2: Relate this to the visibility of chromosomes

This DNA-histone packaging exists throughout the cell cycle, but it is not equally condensed at all times. During most of interphase, chromatin is more loosely coiled and cannot be seen as individual chromosomes with a light microscope. During prophase of mitosis, the chromatin condenses (supercoils) much further, which is why chromosomes only become clearly visible as distinct structures at this stage. This condensation is a change in how tightly the DNA-histone structure is coiled, not a change in what the structure is made of.

Step 3: Evaluate each option

  • A: correct. DNA is wound around histone proteins, and this DNA-histone chromatin is coiled and folded further to produce the compact structure of a chromosome.
  • B: incorrect. DNA is not “bare” within a chromosome; it is closely associated with histone proteins, and its packaging is not driven by hydrogen bonding between bases (which instead holds the two strands of the double helix together).
  • C: incorrect, this confuses transcription (making an RNA copy of a gene for protein synthesis) with the structural packaging of DNA into a chromosome; the chromosome itself is made of DNA and protein, not RNA.
  • D: incorrect. Cellulose is a structural carbohydrate found in plant cell walls, not a component of chromosomes, which are found within the nucleus in both plant and animal cells.

Final answer

  • A chromosome’s DNA is wound around histone proteins and this chromatin is further coiled into a compact structure, option A.