Q.With the help of a neat labelled diagram, describe the structure of a chromosome.

Step 1. Overall structure. A typical chromosome, best seen at metaphase when it is maximally condensed, consists of two identical sister chromatids held together at a single point called the centromere or primary constriction.

Step 2. The centromere/kinetochore. At the primary constriction sits a disc-shaped structure called the kinetochore, which is where spindle fibres attach to pull the chromatids apart during cell division.

Step 3. Chromatid internal structure. Each chromatid contains a long, unbranched, highly coiled DNA thread called the chromonema, running the full length of the chromatid; the chromonema is studded with bead-like chromomeres and is, chemically, a double-stranded DNA molecule extending end to end.

Step 4. Telomeres and secondary constrictions. The free ends of the chromosome are called telomeres. A few chromosomes carry one or two additional secondary constrictions: at secondary constriction I, the nucleolus reorganises during interphase (this is the nucleolar organiser region); a small satellite (SAT) body is occasionally attached beyond secondary constriction II.

Step 5. Shape classification. Depending on where the centromere sits along its length, a chromosome is classed as metacentric (centromere central, V-shaped), submetacentric (off-centre, L-shaped), acrocentric (near one end, J-shaped), or telocentric (at the very tip, rod-shaped). …

Describe the chromosome from the outside in: two chromatids, centromere/kinetochore, internal chromonema, telom …

• Omitting the kinetochore's function (spindle-fibre attachment) when describing the centromere. …