During which phase(s) of mitosis are chromosomes composed of two sister chromatids?
a. prophase, prometaphase, metaphase, and anaphase
b. prophase, prometaphase, and metaphase
c. anaphase and telophase only
d. all phases ofmitosis
e. prophase and prometaphase only

b. prophase, prometaphase, and metaphase

Explanation:

DNA replication during S phase replicates chromosomes and each of the chromosomes has two sister chromatids held together at the centromere. Chromosomes with two sister chromatids become visible in prophase which is marked by condensation of chromatin and beginning of the formation of the mitotic spindle.

Prophase is followed by prometaphase. Breakdown of nuclear envelop during prometaphase is followed by the joining of microtubules to kinetochores of chromosomes.

Metaphase is marked by the assembly of chromosomes, each with two sister chromatids, at the cell's equator. This is followed by anaphase wherein splitting of centromere separates the sister chromatids. The separated sister chromatids are now called daughter chromosomes and move towards opposite poles.