A single human cell has a DNA thread about 2 m long in all, shared among its 46 chromosomes (23…

(a) Per chromosome. $\dfrac{2\ \text{m}}{46} = \dfrac{200\ \text{cm}}{46} = 4.35\ \text{cm}$. A thread about this long is packed into a nucleus only a few micrometres across.

(b) At metaphase. During S phase the amount of DNA per cell doubles, from 2C to 4C. So the cell now holds $2 \times 2\ \text{m} = 4\ \text{m}$ of DNA. The number of chromosomes stays $46$.

(c) Chromatids and kinetochores. Each metaphase chromosome is made of two sister chromatids held together at the centromere: $46 \times 2 = 92$ chromatids. Kinetochores are disc-shaped structures on the sides of the centromere, one for each chromatid: one chromatid is joined by its kinetochore to spindle fibres from one pole, and its sister by its own kinetochore to the opposite pole. So there are $92$ kinetochores.

(d) After cytokinesis. At anaphase the sister chromatids separate and go to opposite poles, so each daughter cell gets one chromatid of every chromosome: $46$ chromosomes and $2\ \text{m}$ of DNA, as in the parent cell at G1.