Question

Human chromosomes undergo structural changes during the cell cycle. Chromosomal structure can be best visualized if a chromosome is isolated from a cell at
A. G1 phase
B. S phase
C. G2 phase
D. M phase

Answer 1

Hint:(i) The sequence of events by which a cell duplicates its genome and synthesizes other cell contents and eventually divides into two daughter cells is formed as cell cycle.
(ii) The regular sequence of G1, S, G2 (…. Interphase) and M phase (u mitotic phase) is called the cell cycle.

Complete answer
A. G1 phase: It is the stage of initial growth of a newly formed cell. It corresponds to the interval between mitosis and initiation of DNA replication. During the G1 phase the cell is metabolically active and continuously grows but does not replicate its DNA.
B. S phase: It is the synthesis phase during which DNA synthesis or replication takes place. During this phase the amount of DNA per cell doubles.
C. G2 phase: During G2 phase proteins are synthesis in preparation for mitosis while cell growth continues.
D. M phase: M phase follows the interphase. It starts with the nuclear division, corresponding to the separation of daughter chromosomes is karyokinesis and usually ends with the divisions of cytoplasm in cytokinesis. During the metaphase of M phase, the condensation of chromosomes if completed and they can be clearly observed under microscope.

The correct option is D, M phase.

Note: M phase includes: Karyokinesis, cytokinesis
(i) Karyokinesis is divided into four stages: Prophase, Metaphase, Anaphase, Telophase
(ii) Metaphase is the most suitable time to determine the number of chromosomes and study then morphology.